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The molecular analysis of leaf senescence-A genomics approach
Plant Biotechnology Journal
Abstract
Senescence in green plants is a complex and highly regulated process that occurs as part of plant development or can be prematurely induced by stress. In the last decade, the main focus of research has been on the identification of senescence mutants, as well as on genes that show enhanced expression during senescence. Analysis of these is beginning to expand our understanding of the processes by which senescence functions. Recent rapid advances in genomics resources, especially for the model plant species Arabidopsis, are providing scientists with a dazzling array of tools for the identification and functional analysis of the genes and pathways involved in senescence. In this review, we present the current understanding of the mechanisms by which plants control senescence and the processes that are involved.
... Under nutritional deficiency, where senescence is prematurely triggered, the increase of soluble sugars can lead to a loss in both functional and structural integrity in cell membranes, increasing the lipid catabolism of the membrane and the production of sugar by gluconeogenesis [17]. During senescent leaf metabolism, the assimilation of carbon and nitrogen is replaced by a catabolism of chlorophyll and macromolecules such as proteins, RNA and membrane lipids, whose degradation marks the phase of senescence [18]. ...
... ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco)], expression of senescence-downregulated genes (SDGs) often related to photosynthesis (e.g. Cab gene encoding a chlorophyll a/b-binding protein) and upregulation of senescence-associated genes (SAGs), as well as de novo synthesis of proteins [17], being on of the most suitable molecular indicators of senescence progression [17,24]. ...
... ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco)], expression of senescence-downregulated genes (SDGs) often related to photosynthesis (e.g. Cab gene encoding a chlorophyll a/b-binding protein) and upregulation of senescence-associated genes (SAGs), as well as de novo synthesis of proteins [17], being on of the most suitable molecular indicators of senescence progression [17,24]. ...
ChatGPT, a state-of-the-art AI model created by OpenAI, is renowned for its ability to produce text responses that closely mimic human communication. Recently, it has broadened its scope to encompass multimodal interactions, rendering it particularly advantageous for medical applications. In this paper, we explore into the potential of ChatGPT in the realm of Ultrasound (US) imaging, Our investigation encompasses a series of case studies and examples, each tailored to demonstrate ChatGPT’s application in various aspects of US imaging. These include emphasizing its capacity for intricate analyses, including grayscale texture examination and perfusion metrics in contrast-enhanced ultrasound (CEUS). Moreover, its involvement in real-time imaging guidance, its role in streamlining workflow efficiencies, and its utility in the simplification of medical report generation to facilitate better patient communication. The case studies, while successful, unveil certain limitations and challenges, prominently featuring concerns regarding privacy and technical constraints. Despite these challenges, the evidence gathered from our examples indicates that ChatGPT's innovative features and its ability to operate across multiple modes significantly enhance the efficiency of quantitative image analysis in US imaging. By improving diagnostic precision and potentially elevating patient care outcomes, ChatGPT marks a significant stride forward in healthcare technology, though it necessitates careful consideration of its associated ethical and technical challenges.
... Under nutritional deficiency, where senescence is prematurely triggered, the increase of soluble sugars can lead to a loss in both functional and structural integrity in cell membranes, increasing the lipid catabolism of the membrane and the production of sugar by gluconeogenesis [17]. During senescent leaf metabolism, the assimilation of carbon and nitrogen is replaced by a catabolism of chlorophyll and macromolecules such as proteins, RNA and membrane lipids, whose degradation marks the phase of senescence [18]. ...
... ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco)], expression of senescence-downregulated genes (SDGs) often related to photosynthesis (e.g. Cab gene encoding a chlorophyll a/b-binding protein) and upregulation of senescence-associated genes (SAGs), as well as de novo synthesis of proteins [17], being on of the most suitable molecular indicators of senescence progression [17,24]. ...
... ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco)], expression of senescence-downregulated genes (SDGs) often related to photosynthesis (e.g. Cab gene encoding a chlorophyll a/b-binding protein) and upregulation of senescence-associated genes (SAGs), as well as de novo synthesis of proteins [17], being on of the most suitable molecular indicators of senescence progression [17,24]. ...
Leaf senescence in plants is the last stage of leaf development that is characterized by a decline in photosynthetic activity, an active degeneration of cellular structures and the recycling of accumulated nutrients to areas of active growth such as buds, young leaves, flowers, fruits and seeds. This process holds economic significance as it can impact yield, influencing the plant´s ability to maintain an active photosynthetic system during prolonged periods, especially during the grain filling stage, which affects plant weight and oil content. It can be associated with different stresses or environmental conditions, manifesting itself widely in the context of climate change and limiting yield, especially in crops of agronomic relevance. In this work we study the stability of two widely described sunflower genotypes belonging to the INTA Breeding Program against differential N conditions, to verify their yield stability in control conditions and under N supply. Two inbred lines were utilized, namely R453 (early senescence) and B481–6 (late senescence), with contrasting nitrogen availability in the soil but sharing the same ontogeny cycle length. It was observed that, starting from R5.5, the B481-6 genotype not only delayed senescence but also exhibited a positive response to increased nitrogen availability in the soil. This response included an increase in intercepted radiation, resulting in a statistically significant enhancement in grain yield. Conversely, the R453 genotype did not show significant differences under varying nitrogen availability and exhibited a tendency to decrease grain yield when nitrogen availability was increased. The response to nitrogen can vary depending on the specific genotype.
... The premature senescence causes postharvest deterioration of many horticultural products such as detached whole and fresh-cut leafy vegetables, perishable fruit, cut flowers and foliage. Early senescence might be due to natural physiological reasons or promoted by stressful circumstances such as insufficient light, mechanical stress, water stress, high/low temperature, nutritional stress and pathogen infection, arising during harvesting, processing and handling (Buchanan-Wollaston et al., 2003;Cantwell and Suslow, 2004;Wagstaff et al., 2009). The onset and progression of senescence are programmed and genetically coded and require active cell metabolism.The process is finalized by the actual death of cells (Jones, 2001; van Doorn and Woltering, 2004). ...
... During the 'senescence phase' various other processes in common with PCD components also take place. Among them are nuclear condensation, breakdown of nuclear DNA, chlorophyll breakdown, activation of oxidative stress-associated and other destructive pathways, changes in cytoskeleton, increase in the permeability of tonoplast and plasma membrane and electrolyte leakage, proteolysis, changes in phosphorylation/dephosphorylation balance and others (Buchanan-Wollaston et al., 2003;Coupe et al., 2004;van Doorn and Woltering, 2004, 2005, 2008. The occurrence of shared gene-coded and metabolic processes in the two phases indicates that the death program is already operative in the 'senescence phase' during which the (still living) cells partially degrade their content (presumably for nutrient recycling) and develop competence for committing suicide. ...
... By contrast, APL PCD shares similarities to animal apoptosis; it involves rapid cytoplasm shrinkage, activation of CLPs and results in dead cells that still contain remnants of the living cell. In difference to petal and leaf senescence in intact plants, to which a larger volume of work is devoted (Yen and Yang, 1998;d, Quirino et al., 2000;Rubinstein, 2000;Lee and Chen, 2002;Buchanan-Wollaston et al., 2003;Rogers, 2006;Lim et al., 2007;Price et al., 2008;Wagstaff et al., 2009;Wang et al., 2013Wang et al., , 2021Woo et al., 2013), the possible contribution of PCD in postharvest deterioration of perishable commodities has not been studied in detail. The process was investigated in few products only, such as leafy and other green vegetables, fruit and root vegetables that are suggested as appropriate model systems for elucidating the involvement of PCD in storage-induced disorders. ...
A R T I C L E I N F O Keywords: senescence programmed cell death postharvest disorders storage-induced stress A B S T R A C T Postharvest performance of fresh horticultural products is largely affected by processing and storage conditions. Various practices such as chemical and physical treatments and controlled or modified atmosphere storage can delay the senescence and reduce deterioration. However, the treatments and storage environment may also be stressful and induce detrimental physiological, metabolic and molecular changes resulting in quality loss. The macroscopic symptoms of quality decline have gotten an appropriate attention, but the processes underlying the defects at the cellular level are not well understood. It is suggested that some of the postharvest disorders may involve programmed cell death (PCD): a genetically determined process of cellular suicide indispensable for normal plant development and an important mechanism for survival in response to stressful environmental factors of biotic and abiotic origin. In this review the contribution of PCD to postharvest senescence and storage-related deterioration of perishable horticultural products is discussed. For better comprehension of plant PCD, the major concepts are outlined. Senescence is considered as a specific form of PCD. Examples of recent and earlier findings demonstrating the incidence of storage-related PCD are presented. It is suggested that revealing the implication of PCD in postharvest disorders may trigger the development of new or optimized preservation strategies addressing cell death. Identification of PCD related markers can be a promising tool for predicting the shelf life of harvested products. The control over postharvest stress-induced PCD may be beneficial for the postharvest industry in sustaining the quality in the supply chain as well as in breeding programs for obtaining products with improved tolerance to storage-induced stress.
... Leaf senescence is a complex mechanism controlled by multiple genetic and environmental variables that influence the yield of crops [6]. A delay or reduction in the senescence Under nutritional deficiency, where senescence is prematurely triggered, the increase in soluble sugars can lead to a loss in both functional and structural integrity in cell membranes, increasing the lipid catabolism of the membrane and the production of sugar by gluconeogenesis [18]. During senescent leaf metabolism, the assimilation of carbon and nitrogen is replaced by a catabolism of chlorophyll and macromolecules, such as proteins, RNA, and membrane lipids, whose degradation marks the phase of senescence [19]. ...
... Here, we observed this differential expression, both because of the genotype and the effect of the N1 treatment. This is why, as with other types of programmed cell death, plant senescence is accompanied by a decrease in protein synthesis (e.g., ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco)], expression of senescence-downregulated genes (SDGs) often related to photosynthesis (e.g., CAB gene encoding a chlorophyll a/b-binding protein), and upregulation of senescence-associated genes (SAGs), as well as de novo synthesis of proteins [18], being one of the most suitable molecular indicators of senescence progression [17,25]. ...
Leaf senescence in plants is the last stage of leaf development and is characterized by a decline in photosynthetic activity, an active degeneration of cellular structures, and the recycling of accumulated nutrients to areas of active growth, such as buds, young leaves, flowers, fruits, and seeds. This process holds economic significance as it can impact yield, influencing the plant’s ability to maintain an active photosynthetic system during prolonged periods, especially during the grain filling stage, which affects plant weight and oil content. It can be associated with different stresses or environmental conditions, manifesting itself widely in the context of climate change and limiting yield, especially in crops of agronomic relevance. In this work, we study the stability of two widely described sunflower (Helianthus annuus L.) genotypes belonging to the INTA Breeding Program against differential N conditions, to verify their yield stability in control conditions and under N supply. Two inbred lines were utilized, namely R453 (early senescence) and B481-6 (late senescence), with contrasting nitrogen availability in the soil but sharing the same ontogeny cycle length. It was observed that, starting from R5.5, the B481-6 genotype not only delayed senescence but also exhibited a positive response to increased nitrogen availability in the soil. This response included an increase in intercepted radiation, resulting in a statistically significant enhancement in grain yield. Conversely, the R453 genotype did not show significant differences under varying nitrogen availability and exhibited a tendency to decrease grain yield when nitrogen availability was increased. The response to nitrogen can vary depending on the specific genotype.
... In recent years, genetic and genomic studies have revealed many of the molecular processes and some of the regulatory genes behind leaf senescence. In particular, transcriptome analyses of Arabidopsis revealed thousands of SAGs that are up-regulated during normal, developmentally programed or stress-induced senescence in Arabidopsis (Buchanan-Wollaston et al., 2003;Lin & Wu, 2004;van der Graaff et al., 2006;Breeze et al., 2011). Loss of function of some SAGs results in delayed leaf or wholeplant senescence and altered nutrient translocation (Uauy et al., 2006;Lim et al., 2007). ...
... Senescence is a highly regulated process, involving up-and down-regulation of many genes (Buchanan-Wollaston et al., 2003), by which the nutrients accumulated in aging tissues are remobilized to growing vegetative or reproductive organs (Lim et al., 2007). Senescence-associated TFs play important roles in orchestrating controlled shoot senescence and effective N remobilization. ...
... hormone balance, plant growth) and external (environmental conditions, biotic interactions) signals leads to the execution of a distinct gene expression program. A decline in expression of photosynthetic genes and particularly an up-regulation of hundreds to thousands of senescence associated genes (SAGs) define the developmental transition from maturity to senescence (Buchanan-Wollaston et al., 2003;Ahmad and Guo, 2019). The leaf senescence syndrome, observed macroscopically as leaf yellowing, is characterized by a metabolic shift from nutrient assimilation to nutrient remobilization. ...
... Excess or limiting availability of water, nutrients, and light and/or extreme temperatures exerts a strong influence on the onset and the progression of leaf senescence. Indeed, most transcriptome studies reveal an overlap between natural (developmental, age-related) and stress-induced leaf senescence (Buchanan-Wollaston et al., 2003;Ahmad and Guo, 2019;Tan et al., 2023). A meta-analysis comprising 28 microarrays of gene expression profiles associated to developmental and induced senescence (provoked by a variety of biotic and abiotic stresses, such as pathogen, wounding, salt or sugar excess, and others) revealed distinctive signaling pathways triggering senescence, which coalesce in common execution programs. ...
Exposure to abiotic stresses accelerates leaf senescence in most crop plant species, thereby reducing photosynthesis and other assimilatory processes. In some cases, genotypes with delayed leaf senescence (i.e., “stay-greens”) show stress resistance, particularly in cases of water deficit, and this has led to the proposal that senescence delay improves crop performance under some abiotic stresses. In this review, we summarize the evidence for increased resistance to abiotic stress, mostly water deficit, in genotypes with delayed senescence, and specifically focus on the physiological mechanisms and agronomic conditions under which the stay-green trait may ameliorate grain yield under stress.
... Leaf senescence is an important process of plant development because it enables plants to efficiently transfer nutrients from older leaves to other new growth reservoirs by breaking down organelles and degrading macromolecules (Buchanan-Wollaston et al., 2003;Himelblau & Amasino, 2001;Lim et al., 2007). The onset of leaf senescence mainly depends on age, but can also be regulated by endogenous and exogenous factors, such as phytohormones and environmental stresses (Breeze et al., 2011;Lim et al., 2007;Quirino et al., 2000). ...
... Many phytohormones have been reported to play a role in leaf senescence. Ethylene, jasmonic acid, abscisic acid (ABA), and salicylic acid can accelerate senescence, while auxin, gibberellic acid, and cytokinin can delay senescence (Buchanan-Wollaston et al., 2003;Jibran et al., 2013;Lim et al., 2007). Accumulating evidence suggests that ABA is involved in regulating developmental leaf senescence in response to environmental conditions. ...
Abscisic acid (ABA) is a phytohormone that promotes leaf senescence in response to environmental stress. We previously identified methyl CpG‐binding domain 10 (MBD10) as a phosphoprotein that becomes differentially phosphorylated after ABA treatment in Arabidopsis. ABA‐induced leaf senescence was delayed in mbd10 knockout plants but accelerated in MBD10 ‐overexpressing plants, suggesting that MBD10 positively regulates ABA‐induced leaf senescence. ABA‐induced phosphorylation of MBD10 occurs in planta on Thr‐89, and our results demonstrated that Thr‐89 phosphorylation is essential for MBD10's function in leaf senescence. The in vivo phosphorylation of Thr‐89 in MBD10 was significantly downregulated in a quadruple mutant of group C MAPKs ( mpk1/2/7/14 ), and group C MAPKs directly phosphorylated MBD10 in vitro . Furthermore, mpk1/2/7/14 showed a similar phenotype as seen in mbd10 for ABA‐induced leaf senescence, suggesting that group C MAPKs are the cognate kinases of MBD10 for Thr‐89. Because group C MAPKs have been reported to function downstream of SnRK2s, our results indicate that group C MAPKs and MBD10 constitute a regulatory pathway for ABA‐induced leaf senescence.
... However, leaves will eventually age, which is an active process of death. Apart from enhancing plant adaptation to biotic and abiotic stresses, another important role of leaf senescence is to serve as a nutrient reservoir, transferring nutrients to developing organs or grains [14][15][16][17][18]. This process involves the regulation of multiple genes [14,[19][20][21][22]. ...
... Apart from enhancing plant adaptation to biotic and abiotic stresses, another important role of leaf senescence is to serve as a nutrient reservoir, transferring nutrients to developing organs or grains [14][15][16][17][18]. This process involves the regulation of multiple genes [14,[19][20][21][22]. Many molecular factors involved in zinc uptake and transport to different plant parts have been identified [23,24]. ...
Rice lacks sufficient amounts of zinc despite its vitality for human health. Leaf senescence enables redistribution of nutrients to other organs, yet Zn retransfer during deficiency is often overlooked. In this hydroponic experiment, we studied the effect of Zn deficiency on rice seedlings, focusing on the fourth leaf under control and deficient conditions. Growth phenotype analysis showed that the growth of rice nodal roots was inhibited in Zn deficiency, and the fourth leaf exhibited accelerated senescence and increased Zn ion transfer. Analyzing differentially expressed genes showed that Zn deficiency regulates more ZIP family genes involved in Zn ion retransfer. OsZIP3 upregulation under Zn-deficient conditions may not be induced by Zn deficiency, whereas OsZIP4 is only induced during Zn deficiency. Gene ontology enrichment analysis showed that Zn-deficient leaves mobilized more biological pathways (BPs) during aging, and the enrichment function differed from that of normal aging leaves. The most apparent “zinc ion transport” BP was stronger than that of normal senescence, possibly due to Zn-deficient leaves mobilizing large amounts of BP related to lipid metabolism during senescence. These results provide a basis for further functional analyses of genes and the study of trace element transfer during rice leaf senescence.
... A number of these stimuli and signals are recognised by cell surface receptor proteins, which subsequently transport the information across the plasma membrane to start signal transduction inside the cell and turn on the senescence gene regulatory network. Senescence's complicated regulatory network cannot be fully understood from the little information available on the age-and environment-mediated signal pathways involved in the process [14,15]. Nevertheless, a sizable number of genes that could be involved in signal transduction have been discovered by transcriptome research. ...
... g) Jasmonic acid (JA) and methyl jasmonates (MeJA): Are engaged in several abiotic and biotic stress responses as well as numerous senescence-related processes in plants. The first indication of jasmonate's role in controlling senescence was the accelerated senescence of MeJAtreated detached oat leaves [14]. Senescence-associated enhancer trap lines in 14 of the 125 Arabidopsis plants had reporter gene expression brought on by Jasmonic acid. ...
The word "senescence in plants" refers to the natural ageing process that takes place in plant tissues, organs, and cells and ultimately causes plant death. It is an intricate, tightly controlled process that is affected by both external and internal variables. The internal biological clock of the plant, which is controlled by many external conditions like temperature, light, and nutrition availability, is one of the primary causes of senescence. The internal clock alerts the plant to begin senescing when it reaches a specific age or developmental stage. Several biochemical and physiological changes that the plant experiences during senescence cause the disintegration of cellular elements such as nucleic acids, proteins, chlorophyll and other macromolecules. The plant may continue to grow and develop because these breakdown products are recycled and utilised to create new tissues and organs. In addition, the plant generates and stores a variety of signalling chemicals, including reactive oxygen species (ROS) and hormones, that control the senescence process. The different physiological and biochemical changes that take place during senescence are coordinated by these signalling molecules, which serve as messengers. Senescence is an ordinary and necessary component of the life cycle of plants, but it can also be brought on early by many environmental challenges, including disease, nutrient inadequacy, and drought. The relevance of comprehending the processes that regulate the senescence procedure is emphasized by the fact that premature senescence can have a major influence on plant growth and productivity. The genetically controlled degenerative process of leaf/plant senescence includes nutrient remobilization before the death of leaf/plant tissues. Together with other senescence-causing elements, age plays a significant developmental role in the process. Senescence is regulated at the cellular level by a variety of signalling molecules, hormones, and transcription factors. The complexity of the senescence process, as well as the perception and transmission of senescence signals, as well as subsequent regulatory events, are briefly reviewed in this overview, which also covers current advancements in this area. It will be explored where this field is headed in the future and how related techniques might be used to improve crops.
... Genetic studies have revealed the acceleration or delay of leaf senescence by phytohormones (Buchanan-Wollaston et al., 2003;Lim et al., 2007), and the promotive role of ethylene (ET) in leaf senescence has long been elucidated (Aharoni & Lieberman, 1979;Kao & Yang, 1983). Exogenous application of ET accelerates leaf senescence while inhibition of ET biosynthesis or perception delays leaf senescence (Wang et al., 2001;van der Graaff et al., 2006). ...
... Leaf senescence is a complex process of tissue degeneration that occurs in an age-dependent manner. The senescence leads to massive physiological and biochemical changes upon environmental cues and endogenous signals (Guo & Gan, 2005), which involve changes of plant hormone (Jiang et al., 2014;Chen et al., 2017) and alterations of expression profiles of massive TFs (Buchanan-Wollaston et al., 2003;Besseau et al., 2012). However, little is known regarding the roles of TFs in hormone signalling during leaf senescence. ...
The onset of leaf de‐greening and senescence is governed by a complex regulatory network including environmental cues and internal factors such as transcription factors (TFs) and phytohormones, in which ethylene (ET) is one key inducer. However, the detailed mechanism of ET signalling for senescence regulation is still largely unknown.
Here, we found that the WRKY TF SbWRKY50 from Sorghum bicolor L., a direct target of the key component ETHYLENE INSENSITIVE 3 in ET signalling, functioned for leaf senescence repression. The clustered regularly interspaced short palindromic repeats/CRISPR‐associated protein9‐edited SbWRKY50 mutant (SbWRKY5O‐KO) of sorghum displayed precocious senescent phenotypes, while SbWRKY50 overexpression delayed age‐dependent and dark‐induced senescence in sorghum.
SbWRKY50 negatively regulated chlorophyll degradation through direct binding to the promoters of several chlorophyll catabolic genes. In addition, SbWRKY50 recruited the Polycomb repressive complex 1 through direct interaction with SbBMI1A, to induce histone 2A mono‐ubiquitination accumulation on the chlorophyll catabolic genes for epigenetic silencing and thus delayed leaf senescence. Especially, SbWRKY50 can suppress early steps of chlorophyll catabolic pathway via directly repressing SbNYC1 (NON‐YELLOW COLORING 1). Other senescence‐related hormones could also influence leaf senescence through repression of SbWRKY50.
Hence, our work shows that SbWRKY50 is an essential regulator downstream of ET and SbWRKY50 also responds to other phytohormones for senescence regulation in sorghum.
... Leaf senescence is a crucial phase in leaf development, governed by age or external environmental signals (Buchanan-Wollaston et al. 2003). Darkness is often used to simulate leaf senescence due to its ability to induce leaf yellowing (Zheng et al. 2005;Wada et al. 2009;Weaver et al. 1998;Guo et al. 2004). ...
Melatonin (MT) is a key regulator in plants’ response to leaf senescence induced by aging or various abiotic stresses. Here, we demonstrated that darkness and leaf aging enhance endogenous MT levels by upregulating key genes involved in MT synthesis. Additionally, exogenous MT application significantly mitigated leaf senescence induced by darkness and leaf aging in tomato plants, leading to higher chlorophyll content and maximum photochemical efficiency (Fv/Fm) compared to the control. Using Solanum lycopersicum L. tryptophan decarboxylase (SlTDC)-overexpressed and -knockout transgenic tomato seedlings, we found that SlTDC overexpression increased endogenous MT content and suppressed mRNA levels of chlorophyll degradation-related genes: pheophorbide a oxygenase (PAO), pheophytinase (PPH), and non-yellow coloring1 like (NOL). Furthermore, SlTDC overexpression alleviated photosystem II complex (PSII) photoinhibition, increased ribulose-1,5-bisphosphate carboxylase (Rubisco) and Rubisco activase (RCA) activities and mRNA levels, and maintained higher photosynthetic efficiency in leaves under darkness and in leaves aged over 35 d compared to wild-type plants. Conversely, SlTDC knockout accelerated darkness- or leaf aging-induced leaf senescence in tomatoes. Our findings suggest that MT application or SlTDC overexpression can effectively alleviate leaf senescence by regulating photosynthesis in tomato plants.
... As the primary producers in many ecosystems, plants rely on leaves, the major site of photosynthesis, which provide the fundamental basis of their survival. Throughout their lifespan, leaves undergo a series of developmental and physiological shifts accompanied by orderly changes in metabolism and gene expression (Buchanan-Wollaston et al., 2003). Leaf aging is a crucial developmental strategy in both annual and perennial plants, as the cellular materials accumulated during the leaf's growth stage are converted into exportable nutrients for relocation to newly developing or reproductive organs, or storage in roots or stems (Lim et al., 2007). ...
The circadian clock regulates the daily pattern of temporal gene expression. In Arabidopsis, aging is associated with a shortening of the endogenous period of circadian rhythms under circadian conditions. However, the functional link between the circadian clock and aging under diurnal conditions and its physiological relevance remain elusive. In this study, we investigate and characterize the effect of aging on the waveforms of rhythmic gene expression patterns under light/dark cycles. Our analysis revealed that the diurnal rhythmic patterns of core clock genes undergo significant rhythmic alteration with phase shift and change of waveforms in aged plants compared to younger plants. Transcriptomic analysis indicated that this age-dependent rhythmic alteration occurs not only in core clock genes but also globally. Due to the rhythmic alteration patterns of the diurnal rhythmic gene expression, aged plants experience subjectively a shorter day and longer night. We also observed that genetic mutants of core clock component genes exhibited broadly yet distinctively altered changes in diurnal rhythmic gene expression patterns as aging progresses. Collectively, our findings support that age-dependent rhythmic alteration of diurnal gene expression rhythms reprograms the timetable of daily gene expression, leading to the physiological changes required for plant senescence.
... To validate the status and regulations of cell vitality and starch synthesis in regions of the endosperm, we calculated a physiological developmental index (PDI) to quantify development progress or cellular aging that is associated with cell dysfunction and cell vitality loss ( Figure 5). Considering that physiological development and aging are not solely controlled by a single gene or metabolic pathway, but are comprehensively regulated by multiple organelles and metabolic pathways in the organism, with complex transcriptional regulation mechanisms (Buchanan-Wollaston et al., 2003;Quirino et al., 2000), the current study screened these development progress marker genes and then integrated the expressions of these genes to quantify the PDI of different endosperm regions. Notably, the PDI in the BE was significantly lower than that in the AE and ME throughout the filling stage ( Figure 5C), validating the physiological developmental progress is postponed and slower in the BE ( Figure 5C). ...
Starch synthesis in maize endosperm adheres to the basipetal sequence from the apex downwards. However, the mechanism underlying nonuniformity among regions of the endosperm in starch accumulation and its significance is poorly understood. Here, we examined the spatiotemporal transcriptomes and starch accumulation dynamics in apical (AE), middle (ME), and basal (BE) regions of endosperm throughout the filling stage. Results demonstrated that the BE had lower levels of gene transcripts and enzymes facilitating starch synthesis, corresponding to incomplete starch storage at maturity, compared with AE and ME. Contrarily, the BE showed abundant gene expression for genetic processing and slow progress in physiological development (quantified by an index calculated from the expression values of development progress marker genes), revealing a sustained cell vitality of the BE. Further analysis demonstrated a significant parabolic correlation between starch synthesis and physiological development. An in‐depth examination showed that the BE had more active signaling pathways of IAA and ABA than the AE throughout the filling stage, while ethylene showed the opposite pattern. Besides, SNF1‐related protein kinase1 (SnRK1) activity, a regulator for starch synthesis modulated by trehalose‐6‐phosphate (T6P) signaling, was kept at a lower level in the BE than the AE and ME, corresponding to the distinct gene expression in the T6P pathway in starch synthesis regulation. Collectively, the findings support an improved understanding of the timing of starch synthesis and cell vitality in regions of the endosperm during development, and potential regulation from hormone signaling and T6P/SnRK1 signaling.
... As the course of the senescence process is related to plant species, plant developmental stage, and plant environmental conditions, these treatments cannot be considered the same. The genome resources available for Arabidopsis have made it a useful model of identification and functional analysis of genes regulated by senescence [9][10][11]. However, in many plants, such as barley, the removal of developing flowers and pods significantly extends the life of their leaves, while in Arabidopsis, male-sterile mutants from which developing bolts have been removed do not extend the life of leaves. ...
Background
Developmental leaf senescence (DLS) is an irreversible process followed by cell death. Dark-induced leaf senescence (DILS) is a reversible process that allows adaptations to changing environmental conditions. As a result of exposure to adverse environmental changes, plants have developed mechanisms that enable them to survive. One of these is the redirection of metabolism into the senescence pathway. The plant seeks to optimise resource allocation. Our research aims to demonstrate how epigenetic machinery regulates leaf senescence, including its irreversibility.
Results
In silico analyses allowed the complex identification and characterisation of 117 genes involved in epigenetic processes in barley. These genes include those responsible for DNA methylation, post-translational histone modifications, and ATP-dependent chromatin remodelling complexes. We then performed RNAseq analysis after DILS and DLS to evaluate their expression in senescence-dependent leaf metabolism. Principal component analysis revealed that evaluated gene expression in developmental senescence was similar to controls, while induced senescence displayed a distinct profile. Western blot experiments revealed that senescence engages senescence-specific histone modification. During DILS and DLS, the methylation of histone proteins H3K4me3 and H3K9me2 increased. H3K9ac acetylation levels significantly decreased during DILS and remained unchanged during DLS.
Conclusions
The study identified different epigenetic regulations of senescence types in barley leaves. These findings are valuable for exploring epigenetic regulation of senescence-related molecular mechanisms, particularly in response to premature, induced leaf senescence. Based on the results, we suggest the presence of an epigenetically regulated molecular switch between cell survival and cell death in DILS, highlighting an epigenetically driven cell survival metabolic response.
... Leaf cells go through a sequential and organised dismantling process that includes turnover reduction, nucleic acid reduction, and protein degradation (Lutts et al., 1996), leaf pigment breakdown (Matile et al., 1996), peroxidation, membrane disruption, and lipid degradation (Lutts et al., 1996), and peroxidation, membrane disruption, and lipid degradation (Lutts et al., 1996). (Buchanan-Wollaston, 1997; Thompson et al., 1998;Buchanan-Wollaston et al., 2003). Leaf senescence is activated at the grownup stage of leaf development when leaves are wholly expanded. ...
The present study was conducted with the aim of verifying secondar metabolites amplification from the second generation of Phyllanthus odontadenius (P. odontadenius M2) obtained by seeds gamma-rays (Cs-137) irradiation and their effects on Plasmodium falciaprum. The obtained M1 plantlets come from to original seeds from a single plant of P. odontadenius ikrradiated at 150 Gy with Conservatome LISA I Irradiator. These plantes cultivated previously in vitro on MS (Murashige and Skoog) medium were cultivated in the field and which the randomized plants gave seeds which were used to obtain the M2 plants of P. odontadenius. The phytochemical screening of P. odontadenius M2 extracts (aqueous and methanolic extracts) carried out at LACOREN and the aqueous extract in vitro antiplasmodial activity determination carried out at INRB should justify this secondary metabolites amplification of P. odontadenius Plants M2. On the one hand, results showed that sencondary metabolites of both aqueous and methanolic extracts were amplified compared to those obtained previously. On the other hand, the in vitro antiplasmodial activity of aqueous extracts was found to be high (IC50<5 µg/ml) according to WHO recommendation's. This would be a valuable reason for the peaceful use of ionizing radiation in the fight against malaria and which would validate the use of P. odontadenius and its improvement by mutagenesis techniques.
Keywords: Malaria, Phyllanthus odontadenius, Gamma rays, secondary metabolites; in vitro antiplasmodial activity.
... Numerous Senescence-Associated Genes (SAGs) have been identified in Arabidopsis [7,9,19,23]. Using Arabidopsis as a model, approximately 10% of the total genes in the genome are up-regulated during senescence [54] with over 200 transcription factors implicated [8,37,61]. This suggested the involvement of a complex regulatory network in senescence. ...
Background
Whole plant senescence represents the final stage in the life cycle of annual plants, characterized by the decomposition of aging organs and transfer of nutrients to seeds, thereby ensuring the survival of next generation. However, the transcriptomic profile of vegetative organs during this death process remains to be fully elucidated, especially regarding the distinctions between natural programmed death and artificial sudden death induced by herbicide.
Results
Differential genes expression analysis using RNA-seq in leaves and roots of Arabidopsis thaliana revealed that natural senescence commenced in leaves at 45–52 days after planting, followed by roots initiated at 52–60 days. Additionally, both organs exhibited similarities with artificially induced senescence by glyphosate. Transcription factors Rap2.6L and WKRY75 appeared to serve as central mediators of regulatory changes during natural senescence, as indicated by co-expression networks. Furthermore, the upregulation of RRTF1, exclusively observed during natural death, suggested its role as a regulator of jasmonic acid and reactive oxygen species (ROS) responses, potentially triggering nitrogen recycling in leaves, such as the glutamate dehydrogenase (GDH) shunt. Root senescence was characterized by the activation of AMT2;1 and GLN1;3, facilitating ammonium availability for root-to-shoot translocation, likely under the regulation of PDF2.1.
Conclusions
Our study offers valuable insights into the transcriptomic interplay between phytohormones and ROS during whole plant senescence. We observed distinct regulatory networks governing nitrogen utilization in leaf and root senescence processes. Furthermore, the efficient allocation of energy from vegetative organs to seeds emerges as a critical determinant of population sustainability of annual Arabidopsis.
... Senescence is the last developmental stage prior to death of an organ or the whole plant (Lim et al., 2007;Rapp et al., 2015). Macromolecule degradation and nutrient remobilization during senescence allows plants to reuse endogenous nutrients (Himelblau and Amasino, 2001;Buchanan-Wollaston et al., 2003;Gregersen et al., 2008). Developmental senescence is primarily an age-based process, which is controlled by both internal and external seasonal cues and generally mediated by hormones (Jibran et al., 2013). ...
... Leaves are the primary photosynthetic organs during plant growth and maturation (Van Camp, 2005). Senescence of leaves is crucial in plant fitness by recycling nutrients to rapidly growing organs (Lohman et al., 1994), since it economizes nutrients to develop seeds and ensure the continuation of life in annual plants, and redistributes nutrients to stems or roots as storage resources for the development of new leaves or flowers in following season in perennial plants (Buchanan-Wollaston et al., 2003;Schippers et al., 2015;Woo et al., 2019). When leaves enter the stage of senescence, the cells undergo orderly metabolism and gene expression changes, as well as the degradation of chlorophyll, proteins, lipids, nucleic acid, carbohydrates and other macromolecules (Lim et al., 2007;Ren et al., 2010;Woo et al., 2019). ...
Leaf senescence is a complex process regulated by developmental and environmental factors, and plays a pivotal role in the development and life cycle of higher plants. Casein kinase 1 (CK1) is a highly conserved serine/threonine protein kinase in eukaryotes and functions in various cellular processes including cell proliferation, light signaling and hormone effects of plants. However, the biological function of CK1 in plant senescence remains unclear.
Through systemic genetic and biochemical studies, we here characterized the function of Arabidopsis EL1‐like (AEL), a CK1, in promoting leaf senescence by stimulating ethylene biosynthesis through phosphorylating transcription factor WRKY22. Seedlings lacking or overexpressing AELs presented delayed or accelerated leaf senescence, respectively.
AELs interact with and phosphorylate WRKY22 at Thr57, Thr60 and Ser69 residues to enhance whose transactivation activity. Being consistent, increased or suppressed phosphorylation of WRKY22 resulted in the promoted or delayed leaf senescence. WRKY22 directly binds to promoter region and stimulates the transcription of 1‐amino‐cyclopropane‐1‐carboxylate synthase 7 gene to promote ethylene level and hence leaf senescence.
Our studies demonstrated the crucial role of AEL‐mediated phosphorylation in regulating ethylene biosynthesis and promoting leaf senescence by enhancing WRKY22 transactivation activity, which helps to elucidate the fine‐controlled ethylene biosynthesis and regulatory network of leaf senescence.
... Additionally, the complex interplay between various stress factors and the onset of leaf senescence has been a subject of extensive research. This stress accelerate physiological changes typically associated with senescence, such as chlorophyll depletion and nutrient concentration reduction, thus hastening the decline in visual and structural integrity of the leaves [9]. In this regard, Alstroemeria serves as an excellent model for exploring the molecular basis of leaf senescence, offering insights that can be applied to a broader range of floricultural species [10]. ...
Background
The primary challenge in the cut flower industry, specifically in the postharvest phase, is the short vase life of flowers. This issue, along with early leaf yellowing and perianth abscission, significantly diminishes the economic value of flowers due to their accelerated senescence. To tackle this, we conducted a factorial experiment on Alstroemeria cv. Rebecca, utilizing a completely randomized design with three replications. In this experiment the effects of varying concentrations of Salicylic acid (SA) (0, 1.5, and 3 mM) and sucrose (SU) (0% and 3%) were investigated on the postharvest quality of leaves and florets, with systematic evaluations every three days throughout their vase life.
Results
This experiment revealed that the specific treatment combination of 1.5 mM SA + 3% SU (T5) markedly improved various parameters, such as vase life, total chlorophyll content, membrane stability index, relative fresh weight, and water uptake of cut flowers. In our analysis, we observed that this preservative solution not only extended the vase life and enhanced water uptake but also effectively preserved total chlorophyll, mitigated the loss of fresh weight, and reduced membrane deterioration in petals. Additionally, our results showed an increase in the activities of catalase (CAT) and peroxidase (POD) enzymes, as well as total protein content, alongside a decrease in malondialdehyde (MDA) and hydrogen peroxide (H2O2) levels. Moreover, this study noted a decrease in microbial populations in solutions containing different concentrations of salicylic acid.
Conclusions
Our research demonstrated that alstroemeria flowers maintained in a solution with 1.5 mM SA + 3% SU exhibited a significantly prolonged vase life of up to 21 days, in contrast to the 15 days observed in control flowers kept in water. These results are highly beneficial for manufacturers in the cut flower industry, as they provide a viable method to substantially extend the vase life of cut flowers. Such an enhancement in flower longevity can lead to increased market value and customer satisfaction. Furthermore, the reduction in flower senescence and decay rates can contribute to decreased waste and greater efficiency in cut flower distribution and sales, offering a substantial advantage to manufacturers in this competitive market. The extended vase life and reduced senescence observed in alstroemeria flowers treated with 1.5 mM SA and 3% SU are attributed to SA's role in enhancing endogenous defense responses and sucrose's function as an energy source, collectively improving water uptake, and delaying the natural decay process.
... Controlling senescence can greatly improve crop yields and other plant characteristics, such as extending shelf life, especially in light of potential future food shortages and the use of plants as bioenergy sources [32]. Understanding how leaf senescence is regulated will help control senescence in the future, either through genetic modification or the manipulation of key environmental triggers [33]. ...
Hydrogen peroxide (H2O2) is relatively stable among ROS (reactive oxygen species) and could act as a signal in plant cells. In the present work, detached tomato leaves were treated with exogenous H2O2 at 10 mmol/L for 8 h to study the mechanism of how H2O2 regulates leaf senescence. The data indicated that H2O2 treatment significantly accelerated the degradation of chlorophyll and led to the upregulation of the expression of leaf senescence-related genes (NYC1, PAO, PPH, SGR1, SAG12 and SAG15) during leaf senescence. H2O2 treatment also induced the accumulation of H2O2 and malondialdehyde (MDA), decreased POD and SOD enzyme activities and inhibited H2S production by reducing the expression of LCD1/2 and DCD1/2. A correlation analysis indicated that H2O2 was significantly and negatively correlated with chlorophyll, the expression of leaf senescence−related genes, and LCD1/2 and DCD1/2. The principal component analysis (PCA) results show that H2S showed the highest load value followed by O2•−, H2O2, DCD1, SAG15, etc. Therefore, these findings provide a basis for studying the role of H2O2 in regulating detached tomato leaf senescence and demonstrated that H2O2 plays a positive role in the senescence of detached leaves by repressing antioxidant enzymes and H2S production.
... So, there is a need to discover a new and more effective technique to identify genomic areas of crops under waterlogged stress for comprehensive molecular linkage maps. Such would help with traits of marker-assisted selection to improve crops that tolerate submerging or waterlogging conditions, thereby preventing some effects such as depletion of chlorophyll, degradation of protein and RNA, reduction of levels of nutrients such as nitrogen, phosphorus, metal ions and minerals in plant shoot (Buchanan-Wollaston et al. 2003). Root rot disease hindered plants' growth and increased product yield loss during and after the flooding (Schmitthenner 1995). ...
The threats of climate change on water and crop in agriculture show food security with possible zero hunger. The effects necessitate assessing specific water management and crop varieties adaptation that the rain-fed farmers use for sustainable agricultural production. The study used 1600 copies of the questionnaire to gather farmers’ specific water and crop varieties adaptation across four (Benue, Edo, Niger and Ondo) states in Nigeria. The retrieved copies were coded in an Excel spreadsheet, descriptively analysed in SPSS v23 and subjected to the stepwise method of multiple regression analysis. The results entered three water management practices (riverside cultivation, irrigation practising and mulching) into the Benue model, each for Ondo, Edo and Niger, respectively. All the crop variety practices (selecting different technology, high-yielding varieties, water-resistant varieties, drought-resistant varieties except disease-resistant varieties) were entered into the Benue model, drought-resistant varieties for Edo, water- and drought-resistant varieties for Niger while disease-resistant varieties for Ondo. The data of each state was significant (p < 0.05). Models of Benin and Niger states explained the highest variance of 12.5% for water management and 21.7% for crop-resistant varieties. The Durbin-Watson value was less than 2 for all the models to indicate positive autocorrelation. Mulching (a water management variable) worked best for the farmers across the four states. Both water- and drought-resistant variety adaptations did not work in Ondo and Edo. As the choices of specific adaptation differed across the study states, the study suggested different climate change adaptations for farmers on water management and crop varieties.
... Finally, salicylic acid (SA) or jasmonic acid (JA) signaling pathways are activated, resulting in the production of pathogenesis-related (PR) proteins [1], which are produced by plants exposed to some abiotic stresses, such as low temperature and mechanical damage [2,3]. In addition, PRs are secreted when the plant matures or undergoes senescence [4]. However, it has also been reported that PRs accumulate during the growth and development of plants such as tobacco, mung beans and tomatoes [5]. ...
Tea is a healthy beverage made from the leaves of the tea plant [Camellia sinensis (L.) O. Kuntze]. The tea plant is a perennial evergreen plant that is widely distributed in tropical and subtropical regions. PR proteins (pathogenesis-related proteins, PRs) are defense-related proteins induced under pathogenesis-related conditions. Currently, there are a few studies on PRs in plants. Tea leaf color mutants have been of wide interest to scientists as special materials. A tea plant displaying variegated spontaneous albinism found in a local tea plantation in Huangshan City, Anhui Province, China, was used as the material sample in this study. An analysis of preexisting transcriptomic and proteomic data revealed the significant upregulation of three classes of PRs. In order to investigate the correlation between PRs and variegated tea plants, a series of studies were conducted on PR genes. The results revealed the identification of 17 CsPR1, 3 CsPR4 and 31 CsPR5 genes in tea plants using bioinformatics methods, and their structures and promoter sequences. The expression of three PR1, two PR4 and one PR5 genes was determined to be induced in stress treatment experiments involving mechanical damage, insect bites, low temperature treatment, and fungal infections. Additionally, the ribonuclease activity of CsPR4 was successfully verified. This is the first study to report the ribonuclease activity of CsPR4 in tea plants. The results can serve as a reference for future studies on PRs in tea plants, offering new insights into information on albinism in tea plants.
... The protein and RNA degradation parallels a loss in photosynthetic activity and the majority of the processes have occurred by the time yellowing of leaf can be seen (Buchanan-Wollaston et al., 2003). The degradation processes are transported one of leaves to the part of the plants. ...
In the recent years, increasing labour cost and shortage of labour being
the major constraints to follow cotton manual harvesting in staggered
manner. Further, it is very expensive and they would like to increasingly opt
for mechanical harvesting. In this context, it was suggested that research
should focus to reduce cost of cultivation substantially by promoting the use
of synchronized maturity varieties in cotton and use of defoliants to
encourage mechanical harvesting. In agriculture, defoliants are used to
eliminate the leaves of a crop plant so they will not interfere with the
harvesting by machinery. Early harvesting with good boll opening can also
be achieved by use of defoliants. The use of defoliants also reduces the trash
content in picked cotton which will also help in improving the quality of
cotton. There is an urgent need to identify suitable defoliant with suitable
dose and time of application so as to facilitate mechanical harvesting in
rainfed cotton, Some of the successful defoliants for uniform boll bursting
and higher yield of cotton such as Ethrel, Ethepon, Mepiquat chloride (MC),
Sodium salt and DU (Dropp Ultra) are the hormonal defoliants and TDZ
Thidiazuron Butifos, merphos, tribufos and tribute are the herbicidal
defoliants. The best combination of Thidiazuron + diuron (DCMU),
pyraflufen ethyl, thidiazuron + diuron cellular isozyme, Ethephon +
AMADS, Ethephon + cyclanilide Ethephon + tribufos Mepiquat chloride
(MC) + cyclanilide.
... Systematic examination of source-sink-regulated senescence (SSRS) has been hindered by the difficulty in untangling this process from other signals leading to senescence. Senescence can be triggered by several factors, including extreme temperature, drought, oxidative stress, nutrient starvation, darkness, and disease pressure (Buchanan-Wollaston et al. 2003;Lim et al. 2007;Sakuraba et al. 2020;Zhu et al. 2022). To disentangle SSRS from other related processes, uniform and reproducible induction of senescence by removal of grain sink, easily achieved by preventing pollination, offers an excellent system that can be scaled up for genetic studies. ...
Source and sink interactions play a critical but mechanistically poorly understood role in the regulation of senescence. To disentangle the genetic and molecular mechanisms underlying source-sink regulated senescence (SSRS), we performed a phenotypic, transcriptomic, and systems genetics analysis of senescence induced by the lack of a strong sink in maize (Zea mays). Comparative analysis of genotypes with contrasting SSRS phenotypes revealed that feedback inhibition of photosynthesis, a surge in reactive oxygen species, and the resulting endoplasmic reticulum (ER) stress were the earliest outcomes of weakened sink demand. Multi-environmental evaluation of a biparental population and a diversity panel identified 12 quantitative trait loci and 24 candidate genes, respectively, underlying SSRS. Combining the natural diversity and coexpression networks analyses identified seven high-confidence candidate genes involved in proteolysis, photosynthesis, stress response, and protein folding. The role of a cathepsin B like protease 4 (ccp4), a candidate gene supported by systems genetic analysis, was validated by analysis of natural alleles in maize and heterologous analyses in Arabidopsis (Arabidopsis thaliana). Analysis of natural alleles suggested that a 700-bp polymorphic promoter region harboring multiple ABA-responsive elements is responsible for differential transcriptional regulation of ccp4 by ABA and the resulting variation in SSRS phenotype. We propose a model for SSRS wherein feedback inhibition of photosynthesis, ABA signaling, and oxidative stress converge to induce ER stress manifested as programmed cell death and senescence. These findings provide a deeper understanding of signals emerging from loss of sink strength and offer opportunities to modify these signals to alter senescence program and enhance crop productivity.
... Although beautiful autumn colours are widely appreciated by the public, senescence regulation at the molecular level is not well understood. Research efforts 1-3 , such as genetic approaches, transcriptomics, metabolomics and external applications of phytohormones have identified genes where mutations lead to premature or delayed senescence, senescence-associated genes (SAGs) that are up-or downregulated in senescing leaves, and metabolic signals such as reactive oxygen species (ROS) that are involved in the process [4][5][6][7][8] . Yet, there is no consensus on the senescence trigger, which metabolites, genes or post-translational mechanisms are the most important ones-across species-for the process, or which environmental factors could consistently explain why senescence starts at a certain time in trees in nature. ...
Deciduous trees exhibit a spectacular phenomenon of autumn senescence driven by the seasonality of their growth environment, yet there is no consensus which external or internal cues trigger it. Senescence starts at different times in European aspen (Populus tremula L.) genotypes grown in same location. By integrating omics studies, we demonstrate that aspen genotypes utilize similar transcriptional cascades and metabolic cues to initiate senescence, but at different times during autumn. The timing of autumn senescence initiation appeared to be controlled by two consecutive “switches”; 1) first the environmental variation induced the rewiring of the transcriptional network, stress signalling pathways and metabolic perturbations and 2) the start of senescence process was defined by the ability of the genotype to activate and sustain stress tolerance mechanisms mediated by salicylic acid. We propose that salicylic acid represses the onset of leaf senescence in stressful natural conditions, rather than promoting it as often observed in annual plants.
... Yet one gene cannot be approved as reliable for senescence, the procedures of senescence are evidently in control of genetic (Nam 1997). Several developments had been got via the classification and characterization of many SAGs and mutants related to senescence that offer higher awareness of leaf senescence at the level of molecules (Buchanan-Wollaston et al. 2003). Senescing tissues carried the Arabidopsis SAG29 putatively coding a protein with 2 saliva/MtN3 domains. ...
Identification of orthologues is very crucial in the fields of comparative genomics for the characterization and exploitation of functionally preserved gene products in crops. SAG29/SWEET15 a senescence-associated gene is involved in nutrient re-mobilization upon aging. SAG29 transcript abundance is higher not only in leaves upon abiotic stresses and seeds, showing its crucial involvement to regulate sink remobilization. Current study is carried out to elucidate the gene regulatory network of SAG29 and its orthologues using in silico approach. To this end, a comparative genome-wide bioinformatic analysis was performed for the identification of evolutionarily conserved non-coding sequences (CNSs) in the 1000-bp promoter region (counted from the translation initiation codon). Consensus sequence revealed a highly conserved motif at-673, Zea may at-720, and Glycine max at-517 upstream from ATG. The consensus sequence of this novel putative cis-regulatory element is designated from the sequence logo as "NAA(T/A)[ATA]NN(C/G)NNGNAA(T/A)AA" with "ATA" core binding site for upstream transcription factors controlling the expression of SAG29. These newly discovered putative cis-regulatory elements might have the conserved role in controlling SAG29 and its orthologues biological role upon stress and development of plants. Moreover, SAG29 expression is also known to be induced in roots of Glycine max (harboring SAG29 orthologue), current research also revealed the negative growth effect induced by sodium selenite, in terms of growth retardation, reduced chlorophyll production, and declined endogenous IAA, indicating the growth inhibitory effect the selenium supplementation in Glycine max. Therefore, SAG29 is a good candidate for the generation of abiotic stress-tolerant Glycin max.
... The balance of hormones like cytokinins and ethylene is known to affect the time of senescence, both accelerating and delaying the process. When these hormones are overexpressed or suppressed, this results in alterations to the timing of senescence (Buchanan- Wollaston et al., 2003). An relationship between higher hormone contents and higher chlorophyll and nutrient contents, as well as the maintenance of photosynthesis in older tissues, reduced the degenerative consequences of senescence in transgenic tobacco plants that were induced for high cytokinin production (Jordi et al., 2000). ...
... Senescence is the last stage of leaf development that determines the leaf lifespan. Leaf senescence is a genetically controlled process that is coordinated at the cell, organ, and whole-plant levels [15,16]. It consists of a series of biochemical processes in which cell organelles, membranes, and macromolecules are degraded. ...
High-altitude tree populations are exposed to severe natural environmental conditions. Among abiotic factors, variable temperatures, early frosts, and high radiation are the factors affecting tree growth at high altitudes. Fagus sylvatica L. exhibits a variety of physiological and genetic traits that allow it to adapt to different forest habitats. This study examines the differences in the biochemical properties of senescing beech leaves between populations originating from different altitudes using a common-garden experiment. Leaves were collected from five-year-old plants from the beginning of August to the end of October for two years. Based on the changes in senescence marker levels the genetic differences and significant correlations between populations’ altitude origin and their biochemical characteristics were identified. According to the free radical theory of leaf senescence, reactive oxygen species (ROS) and senescence markers were highly correlated. In this study, populations from higher altitudes were characterized by earlier and greater increases in ROS content and oxidative stress, which resulted in higher antioxidative system activity. Increases in ROS in high-altitude populations play a controlling role to initiate earlier senescence processes that allow the trees to adapt to harsh climatic conditions. Earlier senescence allows beech trees to maintain a balance between nitrogen metabolism and photosynthetic activity. It allows for remobilization of nitrogen compounds more efficiently and protects the trees from nitrogen loss and prepares them for winter dormancy.
... L'attaque d'un champignon modifie la teneur en azote des plantes, considéré comme un puit supplémentaire pour la plante(Tavernier et al., 2007) et active d'importants changements métaboliques dans la plante. Des études ont montré que le métabolisme de la plante au cours d'un stress abiotique et de l'infection par des pathogènes est semblable à celui observé pendant le processus de sénescence(Buchanan-Wollaston et al., 2003;Pageau et al., 2006). La remobilisation des nutriments par le champignon conduit ainsi à des changements drastiques dans la relation source / puits analogue au processus de remobilisation observé lors de la sénescence des feuilles(Masclaux et al., 2000). ...
Le dessèchement précoce du tournesol résulte d'une infection de bas de tige par Phoma macdonaldii qui provoque une nécrose encerclante et un rétrécissement du diamètre du collet. Cette attaque entraine une sénescence foliaire brutale occasionnant des pertes dommageables pour la culture. Des contaminations artificielles par des spores ou du mycélium de P. macdonaldii ont permis de reproduire artificiellement des pieds secs et confortent l'implication de ce champignon dans ce syndrome. Dans un contexte où la protection chimique et la résistance variétale sont limitées, la maîtrise de P. macdonaldii par le contrôle cultural pourrait constituer une alternative efficace. Des essais au champ et en serre menés de 2006 à 2009 à l'INRA et au Cetiom ont permis de mettre en évidence l'importance de la conduite de culture du tournesol sur la fréquence et la gravité du dessèchement précoce et de mieux comprendre l'étiologie de ce syndrome. Une alimentation azotée importante, un peuplement dense et à un régime hydrique limitant en post floraison forment trois facteurs particulièrement favorables au développement du syndrome de desséchement précoce attribué à P. macdonaldii. L'alimentation azotée, au travers de la fertilisation, est le principal facteur favorisant la maladie ; le régime hydrique limitant et la densité de peuplement amplifient la maladie. Le faible diamètre de bas de tige (sous de fortes densités) associé à la prolifération du champignon dans le xylème (azote), occasionnerait une réduction de la conductance hydraulique par obstruction des vaisseaux. Amplifiée lorsque la disponibilité en eau du sol est limitante, cet arrêt de la transpiration se traduit par une sénescence anticipée de 40 jours par rapport à la maturité physiologique de peuplement sains.
Leaf senescence, the final phase of leaf development, plays a crucial role in plant fitness and crop improvement, as it enables nutrient remobilization from leaves to reproductive organs like developing seeds. This process involves extensive reprogramming of gene expression, governed by intricate regulatory networks operating across multiple layers of control. The employment of systems approaches using omics-based technologies and the characterization of key regulators has been instrumental in uncovering newly emerging regulatory mechanisms, providing valuable insights into how this orderly degeneration process is fine-tuned. In this review, we present a comprehensive overview of the current research on epigenetic mechanisms as a key layer within regulatory networks, influencing transcription factor activity and modulating the expression of senescence-associated genes. We also discuss recent advances in identifying the role of non-coding RNAs, RNA methylation, and peptide hormones during leaf senescence, which contributes to a deeper understanding of the complex regulatory pathways involved.
Leaf senescence, highly regulated by plant hormones and environmental factors, represents the final stage of leaf development. Therefore, the strategies to delay leaf senescence might extent the limitation of growth and yield for crop. Although previous studies had demonstrated the potential of melatonin to delay leaf senescence, its mechanism remained many mysteries. Here, we reported the role of melatonin in delaying WRKY75-induced leaf senescence. Exogenous melatonin increased chlorophyll content and reduced the accumulation of ROS in plants. In addition, it up-regulated the expression of the SNAT gene and increased its melatonin level. Exogenous melatonin also decreased SA level and down-regulated SID2 gene expression. Furthermore, the expression of SAG13 and WRKY75 , both positive senescence-related genes, was found to decrease after melatonin treatment. These findings suggest that melatonin counteracts the effects of a network structure comprising SA, ROS, and WRKY75 on senescence, thereby regulating various events related to plant senescence and delaying leaf senescence.
Plant senescence is a highly coordinated process that is intricately regulated by numerous endogenous and environmental signals. The involvement of phytic acid in various cell signaling and plant processes has been recognized, but the specific roles of phytic acid metabolism in Arabidopsis leaf senescence remain unclear. Here, we demonstrate that in Arabidopsis thaliana the multiple inositol phosphate phosphatase (AtMINPP) gene, encoding an enzyme with phytase activity, plays a crucial role in regulating leaf senescence by coordinating the ethylene signal transduction pathway. Through overexpressing AtMINPP (AtMINPP–OE), we observed early leaf senescence and reduced chlorophyll contents. Conversely, a loss-of-function heterozygous mutant (atminpp/+) exhibited the opposite phenotype. Correspondingly, the expression of senescence-associated genes (SAGs) was significantly upregulated in AtMINPP–OE but markedly decreased in atminpp/+. Yeast one-hybrid and chromatin immunoprecipitation assays indicated that the EIN3 transcription factor directly binds to the promoter of AtMINPP. Genetic analysis further revealed that AtMINPP–OE could accelerate the senescence of ein3–1eil1–3 mutants. These findings elucidate the mechanism by which AtMINPP regulates ethylene-induced leaf senescence in Arabidopsis, providing insights into the genetic manipulation of leaf senescence and plant growth.
ENVIRONMENTAL ASSESSMENT OF SOME HEAVY METALS, PHTHALATE ESTERS, AND POLYCHLORINATED BIPHENYLS FROM MUNICIPAL DUMPSITE IN ILORIN, NIGERIA
Phosphorus (P) is a crucial macronutrient for plant growth, development, and reproduction. The effects of low P (LP) stress on leaf senescence and the role of PHR1 in LP‐induced leaf senescence are still unknown. Here, we report that PHR1 plays a crucial role in LP‐induced leaf senescence, showing delayed leaf senescence in phr1 mutant and accelerated leaf senescence in 35S:PHR1 transgenic Arabidopsis under LP stress. The transcriptional profiles indicate that 763 differentially expressed SAGs (DE‐SAGs) were upregulated and 134 DE‐SAGs were downregulated by LP stress. Of the 405 DE‐SAGs regulated by PHR1, 27 DE‐SAGs were involved in P metabolism and transport. PHR1 could bind to the promoters of six DE‐SAGs ( RNS1 , PAP17 , SAG113 , NPC5 , PLDζ2 , and Pht1;5 ), and modulate them in LP‐induced senescing leaves. The analysis of RNA content, phospholipase activity, acid phosphatase activity, total P and phosphate content also revealed that PHR1 promotes P liberation from senescing leaves and transport to young tissues under LP stress. Our results indicated that PHR1 is one of the crucial modulators for P recycling and redistribution under LP stress, and the drastic decline of P level is at least one of the causes of early senescence in P‐deficient leaves.
Senescence is the final stage of leaf development. For leafy vegetables such as pak choi, leaf senescence is adverse to yield due to the harvest period shortening. However, the regulatory mechanisms of leaf senescence are largely unknown in leafy vegetables. Here, we isolated and characterized a NAC gene, BcNAC056, in pak choi [Brassica campestris (syn. Brassica rapa) ssp. chinensis cv. 49caixin]. BcNAC056-GFP was located in the nucleus at the subcellular level, and BcNAC056 was responsive to leaf senescence and different hormones at the transcriptional level. Heterologous overexpression of BcNAC056 in Arabidopsis promoted leaf senescence, accompanied by the increased expression of senescence-associated genes (SAGs), whereas virus-induced gene silencing-based silencing in pak choi delayed leaf senescence. The following transcriptome analysis showed that heterologous overexpression of BcNAC056 enhanced some AtSAG transcripts in Arabidopsis. Electrophoretic mobility shift assay (EMSA) and dual-luciferase (LUC) reporter assay revealed that BcNAC056 activated SAG12 by directly binding to the promoter. In addition, with the LUC reporter and transient overexpression assays, we proposed that BcNAC056-BcWRKY1 interaction promoted the activation of BcSAG12. Taken together, our findings revealed a new regulatory mechanism of leaf senescence in pak choi.
Redox homeostasis in plant cells is critical for maintaining normal growth and development, as reactive oxygen species (ROS) can function as signaling molecules or toxic compounds. However, how plants fine-tune redox homeostasis during natural or stress-induced senescence remains unclear. Cut roses (Rosa hybrida), an economically important ornamental product worldwide, often undergo stress-induced precocious senescence at the postharvest bud stage. Here we identified RhPLATZ9, an age- and dehydration-induced PLATZ (plant AT-rich sequence and zinc-binding) protein and determined that it functions as a transcriptional repressor in rose flowers during senescence. We also showed that RhWRKY33a regulates RhPLATZ9 expression during flower senescence. RhPLATZ9-silenced flowers and RhWRKY33a-silenced flowers showed accelerated senescence, with higher ROS contents than the control. By contrast, overexpression of RhWRKY33a or RhPLATZ9 delayed flower senescence, and overexpression in rose calli showed lower ROS accumulation than the control. RNA-seq analysis revealed that apoplastic NADPH oxidase genes (RhRbohs) were enriched among the upregulated differentially expressed genes in RhPLATZ9-silenced flowers compared to wild-type flowers. Yeast one-hybrid assays, electrophoretic mobility shift assays, dual luciferase assays, and chromatin immunoprecipitation quantitative PCR confirmed that the RhRbohD gene is a direct target of RhPLATZ9. These findings suggest that the RhWRKY33a-RhPLATZ9-RhRbohD regulatory module acts as a brake to help maintain ROS homeostasis in petals and thus antagonize age- and stress-induced precocious senescence in rose flowers.
Rice (Oryza sativa) is the major source of food as over half of the world's population relies on rice as their major source of nutrition. Drought stress is the main factor negatively affecting rice growth and productivity worldwide. As DREB transcriptional factors are important for plant growth and abiotic stress responses, they are promising molecular plant breeding candidates. Overexpression of DREB1A gene in plants improves drought tolerance but also causes various growth abnormalities such as retarded growth, delay in flowering and reduced seed setting. In the present work, DREB1A gene under the control of inducible Oshox24 promoter was expressed in rice to study plant growth and abiotic stress tolerance. The inheritance of DREB1A in individual transgenic rice plants was confirmed using PCR analysis. The overexpression of POshox24; DREB1A indicated its positive role in delaying hormone-induced leaf senescence and chlorophyll breakdown induced by Methyl-Jasmonates (MeJA) Abscisic Acid (ABA), and Ethylene (ETH). The upper four leaves of POshox24:DREB1A-OX#2 and POshox24:DREB1A-OX#5 exhibited higher chlorophyll contents compared to control plants. Under drought stress treatment POshox24:DREB1A-OX#2 and POshox24:DREB1A-OX#5 exhibited 67% (27/40) and 63% (25/40) survival rate compared to control plants 35% (13/40). Our results suggest that stress-inducible expression of DREB1A under Oshox24 promoter produces no pleiotropic effects on plant growth and development and confers drought stress tolerance.
The study of plant death is completed by defining plant cell senescence. Senescence is an inseparable part of plant growth. Like many other plant processes, this is a genetic control program that is regulated by a set of environmental and genetic factors. These factors and their mechanisms respond to the relationship between sources and sinks, in which glucose signals and hormonal regulation and cellular genetics play a major role. During senescence, nutrients such as nitrogen, phosphorus, and glucose are released from the breakdown of macromolecules in leaf cells, to be assigned to growing organs or storage tissues. Symptoms of leaf senescence include chlorophyll loss, dehydration, and eventually plant death. Recognition of senescence is very necessary because with sufficient knowledge, senescence can be manipulated in the desired direction. For example, current molecular genetic approaches are used to postpone senescence, like blocking ethylene production or increasing cytokinin production. They are based on plant hormone mechanisms. Plant storage products increase their life span. In order to understand leaf senescence, many genetic SAGs, especially transcription factors and encoding components of transmission pathways, various gene functional methods are described.
Senescence has long been a concern of postharvest scientists. The essential interests are two-fold. On the one hand, the premature senescence of many commodities, often accelerated due to adverse handling and storage conditions, renders them useless to the consumer. On the other hand, for many other commodities, notably fruits and cut flowers, the early manifestations of senescence form the basis of desirable quality attributes. The objectives of postharvest handling practices in the latter case are to: a) arrest continued developm ent during storage and b) ensure that normal development occurs following storage. For these reasons, an appreciation of the underlying causes of senescence becomes crucial to our understanding of how best to control and regulate the process. The symposium speakers will focus on the mechanisms and control of senescence in three horticultural commodity groups: leafy vegetables, fruits, and cut flowers.
Arabidopsis RD21 is a cysteine protease of the papain family. Unlike other members of the papain family, RD21 has a C-terminal extension sequence composed of two domains, a 2-kD proline-rich domain and a 10-kD domain homologous to animal epithelin/granulin family proteins. The RD21 protein was accumulated as 38-and 33-kD proteins in Arabidopsis leaves. An immunoblot showed that the 38-kD protein had the granulin domain, whereas the 33-kD protein did not. A pulse-chase experiment with Bright-Yellow 2 transformant cells expressing RD21 showed that RD21 was synthesized as a 57-kD precursor and was then slowly processed to make the 33-kD mature protein via the 38-kD intermediate. After a 12-h chase, the 38-kD intermediate was still detected in the cells. These results indicate that the N-terminal propeptide was first removed from the 57-kD precursor, and the C-terminal granulin domain was then slowly removed to yield the 33-kD mature protein. Subcellular fractionation of the Bright-Yellow 2 transformant showed that the intermediate and mature forms of RD21 were localized in the vacuoles. Under the acidic conditions of the vacuolar interior, the intermediate was found to be easily aggregated. The intermediate and the mature protein were accumulated in association with leaf senescence. Taken together, these results indicate that the intermediate of RD21 was accumulated in the vacuoles as an aggregate, and then slowly matured to make a soluble protease by removing the granulin domain during leaf senescence.
The aim of this study was to investigate the interactions between cytokinin, sugar repression, and light in the senescence-related decline in photosynthetic enzymes of leaves. In transgenic tobacco (Nicotiana tabacum) plants that induce the production of cytokinin in senescing tissue, the age-dependent decline in NADH-dependent hydroxypyruvate reductase (HPR), ribulose-1,5-bisphosphate carboxylase/oxygenase, and other enzymes involved in photosynthetic metabolism was delayed but not prevented. Glucose (Glc) and fructose contents increased with leaf age in wild-type tobacco and, to a greater extent, in transgenic tobacco. To study whether sugar accumulation in senescing leaves can counteract the effect of cytokinin on senescence, discs of wild-type leaves were incubated with Glc and cytokinin solutions. The photorespiratory enzyme HPR declined rapidly in the presence of 20 mm Glc, especially at very low photon flux density. Although HPR protein was increased in the presence of cytokinin, cytokinin did not prevent the Glc-dependent decline. Illumination at moderate photon flux density resulted in the rapid synthesis of HPR and partially prevented the negative effect of Glc. Similar results were obtained for the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase. It is concluded that sugars, cytokinin, and light interact during senescence by influencing the decline in proteins involved in photosynthetic metabolism.
With the completion of the Arabidopsis genome sequencing project, the next major challenge is the large-scale determination of gene function. As a model organism for agricultural biotechnology, Arabidopsis presents the opportunity to provide key insights into the way that gene function can affect commercial crop production. In an attempt to aid in the rapid discovery of gene function, we have established a high throughput phenotypic analysis process based on a series of defined growth stages that serve both as developmental landmarks and as triggers for the collection of morphological data. The data collection process has been divided into two complementary platforms to ensure the capture of detailed data describing Arabidopsis growth and development over the entire life of the plant. The first platform characterizes early seedling growth on vertical plates for a period of 2 weeks. The second platform consists of an extensive set of measurements from plants grown on soil for a period of ∼2 months. When combined with parallel processes for metabolic and gene expression profiling, these platforms constitute a core technology in the high throughput determination of gene function. We present here analyses of the development of wild-type Columbia (Col-0) plants and selected mutants to illustrate a framework methodology that can be used to identify and interpret phenotypic differences in plants resulting from genetic variation and/or environmental stress.
Systematic analysis of the Arabidopsis genome provides a basis for detailed studies of genome structure and evolution. Members of multigene families were mapped, and random sequence alignment was used to identify regions of extended similarity in the Arabidopsis genome. Detailed analysis showed that the number, order, and orientation of genes were conserved over large regions of the genome, revealing extensive duplication covering the majority of the known genomic sequence. Fine mapping analysis showed much rearrangement, resulting in a patchwork of duplicated regions that indicated deletion, insertion, tandem duplication, inversion, and reciprocal translocation. The implications of these observations for evolution of the Arabidopsis genome as well as their usefulness for analysis and annotation of the genomic sequence and in comparative genomics are discussed.
The last phase of leaf development, generally referred to as leaf senescence, is an integral part of plant development that involves massive programmed cell death. Due to a sharp decline of photosynthetic capacity in a leaf, senescence limits crop yield and forest plant biomass production. However, the biochemical components and regulatory mechanisms underlying leaf senescence are poorly characterized. Although several approaches such as differential cDNA screening, differential display, and cDNA subtraction have been employed to isolate senescence-associated genes (SAGs), only a limited number of SAGs have been identified, and information regarding the regulation of these genes is fragmentary. Here we report on the utilization of enhancer trap approach toward the identification and analysis of SAGs. We have developed a sensitive large-scale screening method and have screened 1,300 Arabidopsis enhancer trap lines and have identified 147 lines in which the reporter gene GUS (-glucuronidase) is expressed in senescing leaves but not in non-senescing ones. We have systematically analyzed the regulation of-glucuronidase expression in 125 lines (genetically, each contains single T-DNA insertion) by six senescence-promoting factors, namely abscisic acid, ethylene, jasmonic acid, brassinosteroid, darkness, and dehydration. This analysis not only reveals the complexity of the regulatory circuitry but also allows us to postulate the existence of a network of senescence-promoting pathways. We have also cloned three SAGs from randomly selected enhancer trap lines, demonstrating that reporter expression pattern reflects the expression pattern of the endogenous gene.
Chlorophyllase (Chlase) is the first enzyme involved in chlorophyll (Chl) degradation and catalyzes the hydrolysis of ester
bond to yield chlorophyllide and phytol. In the present study, we isolated the Chlase cDNA. We synthesized degenerate oligo
DNA probes based on the internal amino acid sequences of purified Chlase from Chenopodium album, screened the C. album cDNA library, and cloned a cDNA (CaCLH, C. album chlorophyll-chlorophyllido hydrolase). The deduced amino acid sequence (347 aa residues) had a lipase motif overlapping with
an ATP/GTP-binding motif (P-loop). CaCLH possibly was localized in the extraplastidic part of the cell, because a putative signal sequence for endoplasmic reticulum
is at the N terminus. The amino acid sequence shared 37% identity with a function-unknown gene whose mRNA is inducible by
coronatine and methyl jasmonate (MeJA) in Arabidopsis thaliana (AtCLH1). We expressed the gene products of AtCLH1 and of CaCLH in Escherichia coli, and they similarly exhibited Chlase activity. Moreover, we isolated another full-length cDNA based on an Arabidopsis genomic fragment and expressed it in E. coli, demonstrating the presence of the second Arabidopsis CLH gene (AtCLH2). No typical feature of signal sequence was identified in AtCLH1, whereas AtCLH2 had a typical signal sequence for chloroplast. AtCLH1 mRNA was induced rapidly by a treatment of MeJA, which is known to promote senescence and Chl degradation in plants, and
a high mRNA level was maintained up to 9 h. AtCLH2, however, did not respond to MeJA.
We have compared the time course of leaf senescence in pea (Pisum sativum L. cv Messire) plants subjected to a mild water deficit to that of monocarpic senescence in leaves of three different ages in well-watered plants and to that of plants in which leaf senescence was delayed by flower excision. The mild water deficit (with photosynthesis rate maintained at appreciable levels) sped up senescence by 15 d (200°Cd), whereas flower excision delayed it by 17 d (270°Cd) compared with leaves of the same age in well-watered plants. The range of life spans in leaves of different ages in control plants was 25 d (340°Cd). In all cases, the first detected event was an increase in the mRNA encoding a cysteine-proteinase homologous to Arabidopsis SAG2. This happened while the photosynthesis rate and the chlorophyll and protein contents were still high. The 2-fold variability in life span of the studied leaves was closely linked to the duration from leaf unfolding to the beginning of accumulation of this mRNA. In contrast, the duration of the subsequent phases was essentially conserved in all studied cases, except in plants with excised flowers, where the degradation processes were slower. These results suggest that senescence in water-deficient plants was triggered by an early signal occurring while leaf photosynthesis was still active, followed by a program similar to that of monocarpic senescence. They also suggest that reproductive development plays a crucial role in the triggering of senescence.
Exposure to UV‐B radiation resulted in a loss of chlorophyll and an increase in lipid damage in a similar manner to that induced during natural senescence. In addition, exposure to UV‐B led to the induction of a number of genes associated with senescence ( SAG12, 13, 14 , and 17 ). These results show, for the first time, that exposure to UV‐B can lead to cellular decline through active and regulated processes involving many genes also associated with natural senescence.
accelerated cell death 2 (acd2) mutants of Arabidopsis have spontaneous spreading cell death lesions and constitutive activation of defenses in the absence of pathogen infection. Lesion formation in acd2 plants can be triggered by the bacterial toxin coronatine through a light-dependent process. Coronatine-triggered and spontaneous lesion spreading in acd2 plants also requires protein translation, indicating that cell death occurs by an active process. We have cloned the ACD2 gene; its predicted product shows significant and extensive similarity to red chlorophyll catabolite reductase, which catalyzes one step in the breakdown of the porphyrin component of chlorophyll [Wüthrich, K. L., Bovet, L., Hunziger, P. E., Donnison, I. S. & Hörtensteiner, S. (2000) Plant J. 21, 189–198]. Consistent with this, ACD2 protein contains a predicted chloroplast transit peptide, is processed in vivo, and purifies with the chloroplast fraction in subcellular fractionation experiments. At some stages of development, ACD2 protein also purifies with the mitochondrial fraction. We hypothesize that cell death in acd2 plants is caused by the accumulation of chlorophyll breakdown products. Such catabolites might be specific triggers for cell death or they might induce cellular damage through their ability to absorb light and emit electrons that generate free radicals. In response to infection by Pseudomonas syringae, transgenic plants expressing excess ACD2 protein show reduced disease symptoms but not reduced growth of bacteria. Thus, breakdown products of chlorophyll may act to amplify the symptoms of disease, including cell death and yellowing. We suggest that economically important plants overexpressing ACD2 might also show increased tolerance to pathogens and might be useful for increasing crop yields.
For some time it has been conventional to regard leaf senescence as a species of catastrophe in which the activities of the mature, carbon-exporting leaf lose both their integrity and coordination in an essentially unregulated manner. This view assumes parallels with gerontological changes occurring in humans and other animals. However, it is becoming increasingly apparent that leaf senescence is least understood when considered in these terms and that degenerative sequences in plants and animals, whilst having underlying similarities, also have fundamental differences.
Sugars are key regulatory molecules that affect diverse processes in higher plants. Hexokinase is the first enzyme in hexose metabolism and may be a sugar sensor that mediates sugar regulation. We present evidence that hexokinase is involved in sensing endogenous levels of sugars in photosynthetic tissues and that it participates in the regulation of senescence, photosynthesis, and growth in seedlings as well as in mature plants. Transgenic tomato plants overexpressing the Arabidopsis hexokinase-encoding gene AtHXK1 were produced. Independent transgenic plants carrying single copies of AtHXK1 were characterized by growth inhibition, the degree of which was found to correlate directly to the expression and activity of AtHXK1. Reciprocal grafting experiments suggested that the inhibitory effect occurred when AtHXK1 was expressed in photosynthetic tissues. Accordingly, plants with increased AtHXK1 activity had reduced chlorophyll content in their leaves, reduced photosynthesis rates, and reduced photochemical quantum efficiency of photosystem II reaction centers compared with plants without increased AtHXK1 activity. In addition, the transgenic plants underwent rapid senescence, suggesting that hexokinase is also involved in senescence regulation. Fruit weight, starch content in young fruits, and total soluble solids in mature fruits were also reduced in the transgenic plants. The results indicate that endogenous hexokinase activity is not rate limiting for growth; rather, they support the role of hexokinase as a regulatory enzyme in photosynthetic tissues, in which it regulates photosynthesis, growth, and senescence.
In the green alga Chlamydomonas reinhardtii , the ClpP protease is encoded by an essential chloroplast gene. Mutating its AUG translation initiation codon to AUU reduced ClpP accumulation to 25 to 45% of that of the wild type. Both the mature protein and the putative precursor containing its insertion sequence were present in reduced amounts. Attenuation of ClpP did not affect growth rates under normal conditions but restricted the ability of the cells to adapt to elevated CO 2 levels. It also affected the rate of degradation of the cytochrome b 6 f complex of the thylakoid membrane in two experimental situations: (1) during nitrogen starvation, and (2) in mutants deficient in the Rieske iron‐sulfur protein. The ClpP level also controls the steady state accumulation of a mutated version of the Rieske protein. In contrast, attenuation of ClpP did not rescue the fully unassembled subunits in other cytochrome b 6 f mutants. We conclude that proteolytic disposal of fully or partially assembled cytochrome b 6 f is controlled by the Clp protease.
The expression patterns of senescence-related genes were determined during ozone (O3) exposure in Arabidopsis. Rosettes were treated with 0.15 μL L⁻¹ O3 for 6 h d⁻¹ for 14 d. O3-treated leaves began to yellow after 10 d of exposure, whereas yellowing was not apparent in control leaves until d 14. Transcript levels for eight of 12 senescence related genes characterized showed induction by O3. SAG13(senescence-associated gene), SAG21, ERD1(early responsive to dehydration), and BCB (blue copper-binding protein) were induced within 2 to 4 d of O3 treatment; SAG18, SAG20, and ACS6 (ACC synthase) were induced within 4 to 6 d; and CCH (copper chaperone) was induced within 6 to 8 d. In contrast, levels of photosynthetic gene transcripts,rbcS (small subunit of Rubisco) and cab(chlorophyll a/b-binding protein), declined after 6 d. Other markers of natural senescence, SAG12,SAG19, MT1 (metallothionein), andAtgsr2 (glutamine synthetase), did not show enhanced transcript accumulation. When SAG12promoter-GUS (β-glucuronidase) andSAG13 promoter-GUS transgenic plants were treated with O3, GUS activity was induced in SAG13-GUS plants after 2 d but was not detected in SAG12-GUS plants.SAG13 promoter-driven GUS activity was located throughout O3-treated leaves, whereas control leaves generally showed activity along the margins. The acceleration of leaf senescence induced by O3 is a regulated event involving many genes associated with natural senescence.
Cell-cell and extracellular cell matrix (ECM) interactions provide cells with information essential for controlling morphogenesis, cell-fate specification, and cell death. In animals, one of the major groups of enzymes that degrade the ECM is the matrix metalloproteinases (MMPs). Here, we report the characterization of the cucumber (Cucumis sativus L. cv Marketmore)Cs1-MMP gene encoding such an enzyme likely to play a role in plant ECM degradation. Cs1-MMP has all the hallmark motif characteristics of animal MMPs and is a pre-pro-enzyme having a signal peptide, propeptide, and zinc-binding catalytic domains. Cs1-MMP also displays functional similarities with animal MMPs. For example, it has a collagenase-like activity that can cleave synthetic peptides and type-I collagen, a major component of animal ECM. Cs1-MMP activity is completely inhibited by a hydroxamate-based inhibitor that binds at the active site of MMPs in a stereospecific manner. TheCs1-MMP gene is expressed de novo at the end stage of developmental senescence, prior to the appearance of DNA laddering in cucumber cotyledons leaf discs and male flowers. As the steady-state level of Cs1-MMP mRNA peaks late in senescence and the pro-enzyme must undergo maturation and activation, the protease is probably not involved in nutrient remobilization during senescence but may have another function. The physiological substrates for Cs1-MMP remain to be determined, but the enzyme represents a good candidate for plant ECM degradation and may be involved in programmed cell death (PCD). Our results suggest that PCD occurs only at the culmination of the senescence program or that the processes are distinct with PCD being triggered at the end of senescence.
Numerous studies have shown that transcription factors are important in regulating plant responses to environmental stress. However, specific functions for most of the genes encoding transcription factors are unclear. In this study, we used mRNA profiles generated from microarray experiments to deduce the functions of genes encoding known and putative Arabidopsis transcription factors. The mRNA levels of 402 distinct transcription factor genes were examined at different developmental stages and under various stress conditions. Transcription factors potentially controlling downstream gene expression in stress signal transduction pathways were identified by observed activation and repression of the genes after certain stress treatments. The mRNA levels of a number of previously characterized transcription factor genes were changed significantly in connection with other regulatory pathways, suggesting their multifunctional nature. The expression of 74 transcription factor genes responsive to bacterial pathogen infection was reduced or abolished in mutants that have defects in salicylic acid, jasmonic acid, or ethylene signaling. This observation indicates that the regulation of these genes is mediated at least partly by these plant hormones and suggests that the transcription factor genes are involved in the regulation of additional downstream responses mediated by these hormones. Among the 43 transcription factor genes that are induced during senescence, 28 of them also are induced by stress treatment, suggesting extensive overlap responses to these stresses. Statistical analysis of the promoter regions of the genes responsive to cold stress indicated unambiguous enrichment of known conserved transcription factor binding sites for the responses. A highly conserved novel promoter motif was identified in genes responding to a broad set of pathogen infection treatments. This observation strongly suggests that the corresponding transcription factors play general and crucial roles in the coordinated regulation of these specific regulons. Although further validation is needed, these correlative results provide a vast amount of information that can guide hypothesis-driven research to elucidate the molecular mechanisms involved in transcriptional regulation and signaling networks in plants.
Branched-chain α-ketoacid dehydrogenase (BCKDH) has been known in mammals to be a key enzyme of the catabolic pathway of branched-chain amino acids. We have isolated two cDNA clones encoding the E1β and E2 subunits of BCKDH, respectively, fromArabidopsis thaliana. Proteins encoded in these cDNA sequences had putative mitochondrial targeting sequences and conserved domains reported for their mammalian counterparts. Northern blot and immunoblot analyses showed that transcripts from the respective genes and E2 protein markedly accumulated in leaves kept in the dark. We found that the activity of BCKDH in the leaf extracts also increased when plants were placed in the dark. Addition of sucrose to detached leaves inhibited the accumulation of transcripts, whereas application of a photosynthesis inhibitor strongly induced the expression of these genes even under light illumination. These observations indicate that the cellular sugar level is likely responsible for the dark-induced expression of these genes. The transcript levels of these genes were also high in senescing leaves, in which photosynthetic activity is low and free amino acids from degraded protein are likely to serve as an alternative energy source.
Two genes recently identified in Arabidopsis thaliana may be involved in sequestering free copper ions in the cytoplasm and delivering copper to post-Golgi vesicles. The genes COPPER CHAPERONE and RESPONSIVE TO ANTAGONIST1 are homologous to copper-trafficking genes from yeast and humans. This plant copper-delivery pathway is required to create functional ethylene receptors. The pathway may also facilitate the transport of copper from senescing leaf tissue. In addition, several other genes have been identified recently that may have a role in copper salvage during senescence.
Postharvest senescence in broccoli (Brassica oleracea L. var Italica) florets results in phenotypic changes similar to those seen in developmental leaf senescence. To compare these two processes in more detail, we investigated molecular and biochemical changes in broccoli florets stored at two different temperatures after harvest. We found that storage at cooler temperatures delayed the symptoms of senescence at both the biochemical and gene expression levels. Changes in key biochemical components (lipids, protein, and chlorophyll) and in gene expression patterns occurred in the harvested tissue well before any visible signs of senescence were detected. Using previously identified senescence-enhanced genes and also newly isolated, differentially expressed genes, we found that the majority of these showed a similar enhancement of expression in postharvest broccoli as in developmental leaf senescence. At the biochemical level, a rapid loss of membrane fatty acids was detected after harvest, when stored at room temperature. However, there was no corresponding increase in levels of lipid peroxidation products. This, together with an increased expression of protective antioxidant genes, indicated that, in the initial stages of postharvest senescence, an orderly dismantling of the cellular constituents occurs, using the available lipid as an energy source. Postharvest changes in broccoli florets, therefore, show many similarities to the processes of developmental leaf senescence.
Senescence is a sequence of biochemical and physiological events that constitute the final stage of development. The identification of genes that alter senescence has practical value and is helpful in revealing pathways that influence senescence. However, the genetic mechanisms of senescence are largely unknown. The leaf of the oresara9 (ore9) mutant of Arabidopsis exhibits increased longevity during age-dependent natural senescence by delaying the onset of various senescence symptoms. It also displays delayed senescence symptoms during hormone-modulated senescence. Map-based cloning of ORE9 identified a 693–amino acid polypeptide containing an F-box motif and 18 leucine-rich repeats. The F-box motif of ORE9 interacts with ASK1 (Arabidopsis Skp1-like 1), a component of the plant SCF complex. These results suggest that ORE9 functions to limit leaf longevity by removing, through ubiquitin-dependent proteolysis, target proteins that are required to delay the leaf senescence program in Arabidopsis.
Senescence-associated genes are up-regulated during plant senescence and many have been implicated in encoding enzymes involved in the metabolism of senescing tissues. Using the differential display technique, we identified a SAG in bean (Phaseolus vulgaris) leaf that was exclusively expressed during senescence and was designated senescence-associated receptor-like kinase (SARK). The deduced SARK polypeptide consists of a signal peptide, a leucine-rich repeat in the extracellular region, a single membrane-spanning domain, and the characteristic serine/threonine protein kinase domain. The mRNA level for SARK increased prior to the loss of chlorophyll and the decrease of chlorophylla/b-binding protein mRNA. Detached mature bean leaves, which senesce at an accelerated rate compared with leaves on intact plants, showed a similar temporal pattern of SARK message accumulation. Light and cytokinin, which delayed the initiation of leaf senescence, also delayed SARK gene expression; in contrast, darkness and ethylene, which accelerated senescence, advanced the initial appearance of the SARK transcript. SARK protein accumulation exhibited a temporal pattern similar to that of its mRNA. A possible role for SARK in the regulation of leaf senescence was considered.
Senescence is a highly organized and well‐regulated process. As much as 75% of total cellular nitrogen may be located in mesophyll
chloroplasts of C3‐plants. Proteolysis of chloroplast proteins begins in an early phase of senescence and the liberated amino acids can be exported
to growing parts of the plant (e.g. maturing fruits). Rubisco and other stromal enzymes can be degraded in isolated chloroplasts,
implying the involvement of plastidial peptide hydrolases. Whether or not ATP is required and if stromal proteins are modified
(e.g. by reactive oxygen species) prior to their degradation are questions still under debate. Several proteins, in particular
cysteine proteases, have been demonstrated to be specifically expressed during senescence. Their contribution to the general
degradation of chloroplast proteins is unclear. The accumulation in intact cells of peptide fragments and inhibitor studies
suggest that multiple degradation pathways may exist for stromal proteins and that vacuolar endopeptidases might also be involved
under certain conditions. The breakdown of chlorophyll‐binding proteins associated with the thylakoid membrane is less well
investigated. The degradation of these proteins requires the simultaneous catabolism of chlorophylls. The breakdown of chlorophylls
has been elucidated during the last decade. Interestingly, nitrogen present in chlorophyll is not exported from senescencing
leaves, but remains within the cells in the form of linear tetrapyrrolic catabolites that accumulate in the vacuole. The degradation
pathways for chlorophylls and chloroplast proteins are partially interconnected.
Tomato plants in which the synthesis of ethylene-forming enzyme (EFE) had been inhibited by an anti-sense gene were used to study the role of ethylene in fruit ripening at the biochemical and molecular level. A reduction in total lycopene accumulation was observed in EFE-antisense fruit ripened both on and off the plant. The rate of over-ripening and fruit spoilage was also reduced in the EFE-antisense fruit. The degree of inhibition of ripening was shown to be dependent upon the stage of development at which the fruit were detached from the plant. The effects on ripening in EFE-antisense fruit were much more pronounced when fruit were detached from the vine before the onset of colour change, and were associated with changes in the level of accumulation of mRNAs homologous to a number of previously characterized ripening-related cDNAs. The retarded ripening of detached EFE-antisense fruit could be partially restored by ethylene treatment. In addition to changes observed in ripening fruit, a temporal delay in the onset of foliar senescence was observed in EFE-antisense plants, indicating that the physiological effects of the EFE-antisense gene, and the associated reduction in ethylene evolution, are not confined specifically to fruit ripening.
Morphological characterisation and agronomic evaluation was conducted on 12 transgenic broccoli lines containing a tomato antisense 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase gene. Plants of three cultivars: Shogun (Sh), Green Beauty (Gy) and Dominator (D), were regenerated from hairy root cultures after co-cultivation with Agrobacterium rhizogenes strain A4T harbouring the binary vector pLN35. The T-DNA of pLN35 contains genes encoding a tomato antisense ACC oxidase gene (35S-ACC-5′7′) and a neomycin phosphotransferase II gene (NOS-NPTII-NOS) for kanamycin resistance. The transgenic plants were transferred to a greenhouse and fertile plants obtained. Integration of the foreign DNA into the broccoli genome was confirmed by the polymerase chain reaction and Southern analyses. Transgenic plants showed evidence of hairy root (HR)-induced morphological changes to varying degrees. Of the 12 characterised transgenic lines, three lines (Gy/7, D/1 and D/2) performed within the limits of acceptability for all head quality parameters analysed (size, density, colour, shape and leafiness). The ethylene production from stalks of four field-grown transgenic lines of Green Beauty broccoli showed significant reductions in activity relative to the control 98 h after harvest. The Dominator transgenic lines D/1 and D/2 showed significant improvements in head colour relative to the control from 48 h after harvest. These results are consistent with the ethylene production patterns determined previously for these lines. The head colour results are consistent with previous results suggesting that two enzyme systems may be involved in broccoli senescence, giving two bursts of ethylene production, with only the second burst inhibited by the antisense ACC oxidase gene used.
A spontaneous mutation arising in Festuca pratensis has the effect of stabilizing the pigmentproteolipid complexes of thylakoid membranes so that leaf tissue does not turn yellow during senescence. Inheritance of the non-yellowing character was analysed in crosses between the wild-type cultivar Rossa and a mutant line Bf 993. Electrophoretic variants of cytoplasmic phosphoglucoisomerase coded by alleles of the nuclear gene Pgi-2 were used to identify hybrids during intercrossing. About 96% of the F1 progeny were heterozygous and all were phenotypically yellowing. In the F2 generation yellow ∶ green segregated in a ratio of 2.14∶1, not significantly different from 3∶1. In the backcross between F1 and Bf 993 the ratio was 1∶1 yellow ∶ green. There was no indication of linkage to Pgi-2. Senescence of detached Bf 993 and Rossa leaves was compared with that of the F1 hybrid. The hybrid behaved in an essentially identical fashion to the wildtype parent, and in marked contrast to the mutant, in all aspects of the senescence syndrome investigated, including loss of chlorophyll, carotenoids and the light-harvesting chlorophyll-protein of thylakoid membranes, and elevation of the particulate protein ∶ chlorophyll ratio in the terminal stages. It is concluded that there exists in Festuca pratensis a nuclear gene, designated Sid (senescence-induced degradation) which regulates turnover of hydrophobic components of photosynthetic membranes in ageing leaf tissue and which occurs in at least two allelic forms, y (yellow) dominant over g (green).
Changes in gene expression during foliar senescence and fruit ripening in tomato (Lycopersicon esculentum Mill.) were examined using in-vitro translation of isolated RNA and hybridization against cDNA clones.During the period of chlorophyll loss in leaves, changes occurred in mRNA in-vitro translation products, with some being reduced in prevalence, whilst others increased. Some of the translation products which changed in abundance had similar molecular weights to those known to increase during tomato fruit ripening. By testing RNA from senescing leaves against a tomato fruit ripening-related cDNA library, seven cDNA clones were identified for mRNAs whose prevalence increased during both ripening and leaf senescence. Using dot hybridization, the pattern of expression of the mRNAs corresponding to the seven clones was examined. Maximal expression of the majority of the mRNAs coincided with the time of greatest ethylene production, in both leaves and fruit. Treatment of mature green leaves or unripe fruit with the ethylene antagonist silver thiosulphate prevented the onset of senescence or ripening, and the expression of five of the seven ripening- and senescence-related genes.The results indicate that senescence and ripening in tomato involve the expression of related genes, and that ethylene may be an important factor in controlling their expression.
A senescence-associated gene of Arabidopsis thaliana, sen 1, was previously shown to be distinctively regulated by several senescence-inducing factors including dark treatment. In this study, the promoter region of the sen 1 gene was fused to the β-glucuronidase (GUS) reporter gene and introduced into tobacco plants to examine the regulation mechanism of this gene by dark treatment. The sen 1 gene expression was highly induced by dark and abscisic acid treatment in transgenic tobacco as in Arabidopsis. The promoter activity was rapidly induced by dark treatment but the values of senescence parameters (chlorophyll content, photochemical efficiency, soluble protein content) changed only slightly during dark incubation for 4 days, showing that dark activation of the senl promoter was not closely associated with major symptoms of leaf senescence. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) treatment of the excised transgenic tobacco leaves in light caused activation of the sen 1 promoter and this was concomitant with a decrease in sugar levels, indicating that sugar content may be related to the regulation of the senl promoter. Darkinduced activation of the sen 1 promoter was highly suppressed by addition of exogenous sucrose at physiological concentrations. Glucose and fructose were also effective in suppressing the activity of the sen 1 promoter. However, mannitol or 3-O-methyl-D-glucose, a non-metabolizable analog of glucose did not suppress the promoter activity. We further confirmed that the dark activation of the sen 1 promoter was associated with a decrease in sugar content. Our results clearly demonstrate that the promoter of the sen 1 gene is negatively regulated by sugar and the dark-induced activation of the sen 1 promoter is due to release of the suppression of the sen 1 promoter activity by sugar. These data imply that the senescence-associated gene, sen 1, is activated upon sugar starvation, recognizing sugar starvation as one of senescence signals.
: Availability of nitrogen almost always limits plant growth. Therefore, efficient use of nitrogen is essential for the plants. In upright plants, especially when they form dense plant stands, old, lower leaves are shaded by young, upper leaves. Nitrogenous compounds in such shaded leaves are degraded and re-allocated to the developing young, upper leaves. These processes raise efficiency of nitrogen use in photosynthetic production of the plant. For this to occur in the most effective way, leaves would need to sense their photosynthetic status in a plant and increase, maintain or decrease their photosynthetic capacity accordingly. Hypotheses that explain how a leaf can sense its photosynthetic status in the plant are reviewed. They include systems involving phytochrome, sugar-sensing, or cytokinin. Our experimental results with Helianthus annuus and Phaseolus vulgaris plants, which were subject to various shading treatments, are examined in the light of these hypotheses. Our experimental results favoured the sugar-sensing hypothesis: A leaf can sense demand of other plant parts for photosynthates produced by it and nitrogen abundance or deficiency by monitoring its sugar concentration. Problems that are to be challenged in the near future are also pointed out.
Chlorophyll breakdown in senescent leaves proceeds in essentially three steps: dephytylation by the action of chlorophyllase; conversion of chlorophyllide to phaeophorbide by Mg-dechelatase; and oxygenolytic cleavage of the chlorine-macrocycle by a newly discovered dioxygenase. The metabolic lesion responsible for high retention of chlorophyll during foliar senescence in a mutant genotype of meadow fescue (Festuca pratensis Huds.) was located in the third step of the breakdown pathway. Senescent leaves of both the normally yellowing reference genotype, c.v Rossa, and the non-yellowing mutant Bf993 were shown to be competent with regard to chlorophyllase and Mg-dechelatase. On the other hand, thylakoids isolated from senescent leaves of cv. Rossa were able to carry out oxygenolysis of phaeophorbide into a colourless fluorescent catabolite in vitro, whereas Bf993 thylakoids were deficient in this activity. It is concluded that the Sid locus, a mutant allele of which is responsible for the stay-green character, encodes or regulates the gene for, phaeophorbide a dioxygenase.
cDNA clones representing genes that are expressed during leaf senescence in Brassica napus were identified by differential screen- ing of a cDNA library made from RNA isolated from leaves at different stages of senescence. The expression of these genes at different stages of leaf development was examined by northern blot analysis, and several different patterns of expression were observed. One of the clones, LSC54, represented a gene that is expressed at high levels during leaf senescence. Analysis of this gene indicated strong expression in flowers as well as in senescing leaves. DNA sequence analysis of the LSC54 cDNA indicated a similarity between the deduced amino acid sequence and several metallothionein-like proteins previously identified in plants. Leaf senescence is a key developmental step in the life cycle of an annual plant, since it is the time during which material built up by the plant during its growth phase is mobilized into the developing seed to prepare for the next generation. Leaf senescence is a period of massive mobiliza- tion of nitrogen, carbon, and minerals from the mature leaf to other parts of the plant. The process appears to be a highly regulated, ordered series of events involving breakdown of leaf proteins, loss of photosynthetic capability, disintegration of chloroplasts, loss of Chl, and export of metabolites (Nooden, 1988). In annual plants, which include most agri- cultural crops, mobilizable nutrients from the entire plant are eventually stored in the developed seeds. A considerable percentage of nitrogen for seed filling is derived from vege- tative plant parts; therefore, remobilization from senescing leaves is critica1 for the nutrient budget in seed crops (Feller and Keist, 1986).
In soybean (Glycine max [L.] Merr.) the homozygous combination of the recessive alleles dI and d2 (i.e., dldld2d2) at two different nuclear loci or the cytoplasmic gene cytG inhibit chlorophyll degradation during senescence; i.e. their leaves are green when they are shed. The main objectives of the present work were: (J) to determine whether these “stay-green” genes also interfere with the loss of the bulk of leaf soluble proteins and ribulose bisphospnate carboxylase/oxygensase (Rubisco; EC 4.1.1.39) during senescence and (2) to relate this to alterations in leaf proteolytic activity. Leaves of the normal. Yellowing cvs Clark and Harosoy lost about 90% of their soluble proteins before abscission. The abscising leaves of these cultivars contained no detectable Rubisco. By contrast, protein degradation was significantly less in leaves of near-isogenic lines of Clark and Harosoy carrying dIdId2d2, with or without G (a dominant nuclear gene in a third locus causing green seed coats). These leaves still retained 50% of the soluble protein and large amounts of both subunits of Rubisco at the time of abscission. Alone, neither dl nor d2 had any effect. The cytoplasmic gene cytG slowed the loss of Rubisco. although eventually when leaves were shed they contained as little Rubisco as Clark. Despite inhibition (i.e. dIdId2d2 and GGdIdId2d2) or retardation (i.e. cytG) of protein loss, these mutant genotypes did not differ from Clark in the breakdown of endogenous Rubisco by leaf extracts (“autodigestion”). The wild-type alleles in the dI and d2 loci may control a central regulatory process of the senescence syndrome.
The plant hormone ethylene influences many aspects of plant growth and development, including some specialized forms of programmed senescence such as fruit ripening and flower petal senescence. To study the relationship between ethylene and leaf senescence, etr1-1, an ethylene-insensitive mutant in Arabidopsis, was used. Comparative analysis of rosette leaf senescence between etr1-1 and wild-type plants revealed that etr1-1 leaves live approximately 30% longer than the wild-type leaves. Delayed leaf senescence in etr1-1 coincided with delayed induction of senescence-associated genes (SAGs) and higher expression levels of photosynthesis-associated genes (PAGs). In wild-type plants, exogenous ethylene was able to further accelerate induction of SAGs and decrease expression of PAGs. The extended period of leaf longevity in etr1-1 was associated with low levels of photosynthetic activity. Therefore, the leaves in etr1-1 functionally senesced even though the apparent life span of the leaf was prolonged.
Tobamoviruses represent a well-characterized system used to examine viral infection, whereas Arabidopsis is a choice plant for most genetic experiments. It would be useful to combine both approaches into one experimental system for virus–plant interaction. Most tobamoviruses, however, are not pathogenic in Arabidopsis. Here, we describe infection of Arabidopsis by a recently discovered crucifer-infecting turnip vein clearing tobamovirus (TVCV). Using this system, we determined patterns and kinetics of viral local and systemic movement within Arabidopsis plants. Localization studies showed that the virus infects both vegetative and reproductive plant tissues. However, there may be a transport barrier between the seed coat and the embryo which virions cannot cross, preventing seed transmission of TVCV. The ability to move both locally and systemically in Arabidopsis, causing mild and fast-developing symptoms but allowing survival and fertility of the infected plants, distinguish TVCV infection of Arabidopsis as a model system to study virus–plant interaction.
Leaf senescence was characterised in two Zea mays lines, earlier senescence (ES) and later senescence (LS). Loss of chlorophyll was delayed in LS compared with ES, but the decline in photosynthesis occurred simultaneously in the two lines. Western analysis detected transition points during senescence of both lines when major quantitative and qualitative changes occurred in a number of leaf proteins. Differences in the pattern of translatable mRNAs were apparent earlier than alterations in pigment or protein levels. A cDNA library was constructed using mRNA from ES leaves early in senescence and differential screening was employed to isolate senescence-related clones. Two senescence-enhanced cDNAs showed sequence homology with cDNAs for seed proteins - a cysteine protease and a protein-processing enzyme. These findings suggest that there are similarities between gene expression during seed maturation, germination and leaf senescence. Other senescence-enhanced cDNAs were related to genes implicated in gluconeogenesis and chlorophyll breakdown.
Chloroplastic glutamine synthetase (GS: EC 6·3·1·2), the octamer of the 44 kDa subunit, is rapidly degraded under photo-oxidative stress conditions in leaves, chloroplasts, and chloroplast lysates. Recent studies have suggested that chloroplastic GS might be cleaved by the hydroxyl radical under such conditions (Thoenen & Feller 1998; Australian Journal of Plant Physiology 25, 279–286; Palatnik, Carrillo & Valle 1999, Plant Physiology 121, 471–478). Herein, we present evidence which supports the above hypothesis. When the purified GS from wheat (Triticum aestivum L.) chloroplasts was exposed to the hydroxyl radical-generating system comprising H2O2–FeSO4–ascorbic acid or FeCl3–ascorbic acid, the GS subunit was degraded into four distinct fragments having apparent molecular masses of 39, 35, 32 and 28 kDa. The apparent molecular masses and isoelectric points of these fragments were identical to those of the respective fragments found in the illuminated lysates of chloroplasts. In addition, the appearance of the GS fragments was completely suppressed in the presence of the scavenger for the hydroxyl radical, n-propyl gallate, in the illuminated lysates of chloroplasts. These results strongly support the hypothesis that the primary cleavage of GS is directly driven by the hydroxyl radical, formed by Fenton reaction under photo-oxidative stress conditions in chloroplasts.
The metallothionein gene,LSC54, shows increased expression during leaf senescence inBrassica napusandArabidopsis thaliana. A number of abiotic and biotic stresses have been shown to induce senescence-like symptoms in plants and, to investigate this further, the promoter of theLSC54gene was cloned and fused to the GUS gene and transformed intoArabidopsis. The promoter was highly induced during leaf senescence and also in response to wounding; histochemical analysis indicated that this induction was localised to a few cells close to the wound site. The transgenicArabidopsistissue was infected with compatible and incompatible isolates of both the fungal biotroph,Peronospora parasiticaand the bacterial necrotroph,Pseudomonas syringae.Incompatible isolates induced rapid cell death (the hypersensitive response) at the site of infection and, with both pathogens, early, localised expression of the GUS gene was observed. In contrast, relatively slow induction of the GUS gene was seen in the compatible interaction and this was correlated with the appearance of senescence-like symptoms in the biotrophic interaction and cell death by necrosis that occurred in response to the necrotrophic pathogen. These results suggest that there are common steps in the signalling pathways that lead to cell death in the hypersensitive response, pathogen induced necrosis and senescence.
Unlike leaves of the normal genotype cv. Rossa, green tissue of Bf993, a nuclear gene mutant of Festuca pratensis, does not turn yellow either during the process of sequential senescence in the intact plant or during senescence induced by detachment. Proteins of detached leaf tissue were labelled with [35S]-methionine and analysed by electrophoresis and fluorography. Many novel proteins were made during senescence and there were several clear genotypic differences. The nature and abundances of gene transcripts in senescing detached and attached leaves were analysed by translation in a cell free system and by northern blotting. There were few discernible changes in the majority of translatable mRNAs during development and senescence of leaves on the intact plant, but a small number of new mRNA species consistently appeared de novo as a result of ageing, a few became undetectable and several others increased or decreased significantly. Northern analysis confirmed that, amongst the messages decreasing in abundance with leaf age and in isolated tissue, there are transcripts of nuclear- and plastome-encoded plastid proteins. Moreover, the previously-reported abnormal stability of intrinsic thylakoid membrane proteins in the mutant is not a consequence of abnormal synthesis or turnover of the corresponding transcripts. A consistent difference in gene expression was observed between the normal genotype and lines homozygous or heterozygous for the mutant gene: one or two characteristic polypeptides (M, 60-65 x 103) were resolved by both one- and two-dimensional electrophoresis amongst the translation products of mutant as compared with normal mRNAs. These polypeptides were also observed amongst the translation products of RNA isolated from polyribosomes, suggesting that the corresponding messages associate with, and are probably therefore translatable by, the protein synthesis machinery in vivo.
Summary • The cellular events associated with programmed cell death during leaf senescence in rice (Oryza sativa) plants are reported here. • The cytological sequence of senescence-related changes in rice leaves was studied by transmission electron microscopy, in situ terminal deoxynucleotidyl transferase-mediated dUTP nick end-labelling (TUNEL) assay and DNA ladder assay. • Cell death in senescing mesophyll cells was marked by depletion of cytoplasm in a tightly controlled manner. However, no apparent morphological feature associated with apoptosis was observed. Nuclear DNA fragmentation was detectable as early as during leaf unfolding and the subsequent developmental and senescent stages. The occurrence of DNA fragmentation correlated well with the size-shift of chromosomal DNA on agarose gel after electrophoresis. However, DNA fragmentation was not accompanied by generation of oligonucleosomal DNA fragments. • These features of cell death occurring during leaf senescence in monocot rice are quite different from features characteristic of apoptosis in animals. The implications of these results for cellular events associated with rice leaf senescence are discussed.
To extend our understanding of the physiology of asparagus after harvest, changes in respiration rate, protein and amino acid complement, and ultrastructure of tip sections (0–30 mm) of asparagus spears (Asparagus officinalis L. cv. Limbras 10) were investigated. Spears had been stored for up to 4 days in the dark at 20°C. Respiration rate (carbon dioxide efflux) declined rapidly after harvest before stabilizing at 12 h at ca 50% of the rate at harvest. Protein, amino acid, and ammonium content of tip sections of 180 mm spears (intact tip sections) during storage, and comparable sections; excised from spears at harvest and subsequently stored (excised tip sections), were compared. Total protein content of intact and excised tip sections increased ca 10% 6–12 h after harvest, and then declined to ca 85% of harvest levels at 48 h. Gel electrophoresis in the presence of sodium dodecyl sulphate revealed the net loss of specific proteins at 48 h. Free amino acid content of excised tip sections declined to ca 75% of harvest levels 12 h after harvest, and then increased to 150% of harvest levels by 48 h. Glutamine levels declined rapidly after harvest, and asparagine content increased ca 200% at 24 h. Similar trends in free amino acid content were found in sections of intact tips. Ammonia (ammonium ions) accumulated to ca 0.3% dry weight at 48 h in both intact and excised tip sections. Ultrastructural studies revealed that tonoplast breakdown commenced 48–96 h after harvest. Results are discussed in relation to the sequence of physiological events following harvest and the timing of mechanisms responsible for their initiation.
Leaf senescence is a hiphly-controlled sequence of events comprising the final stage of development. Cells remain viable during the process and new gene expression is required. There is some similarity between senescence in plants and programmed cell death in animals. In this review, different classes of senescence-related genes are defined and progress towards isolating such genes is reported. A range of internal and external factors which appear to cause leaf senescence is considered and various models for the mechanism of senescence- initiation are described. The current understanding of senescence at the wrganelle and molecular levels is presented. Finally, same ideas are mooted as to why senescence occurs and why it should be studied further.