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Drought stress affects interactions between potato plants, psyllid vectors, and a bacterial pathogen
Abstract
Transmission of insect-borne pathogens is mediated by interactions between insects and plants across variable environments. Water stress, for example, affects the physiology, defense, chemistry, and nutritional balance of plants in ways that alter their tolerance to herbivores and pathogens. However, few studies have explored interactions between water stress and insect-borne pathogens as well as the molecular mechanisms mediating these interactions. Here we address these knowledge gaps by assessing effects of plant water stress on the transmission of a bacterial pathogen, Candidatus Liberibacter solanacearum (CLs), by the vector Bactericera cockerelli Šulc (potato psyllid). We hypothesized that plant water stress would promote pathogen transmission by inducing plant gene transcripts and phytohormones involved in defense. Our results showed water stress was associated with decreased CLs titer with two psyllid haplotypes. Our analysis of plant gene transcripts suggested water stress affected phytohormone pathways in ways that altered plant tolerance to the CLs pathogen. Our study shows that abiotic stressors like drought may mediate the spread of plant pathogens by altering plant signaling pathways in ways that affect pathogen transmission.
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Drought is projected to become more severe and widespread as global warming continues in the 21 st century, but hydroclimatic changes and their drivers are not well examined in the latest projections from the Phase Six of the Coupled Model Inetercomparison Project (CMIP6). Here, precipitation (P), evapotranspiration (E), soil moisture (SM), and runoff (R) from 25 CMIP6 models, together with self-calibrated Palmer Drought Severity Index with Penman-Monteith potential evapotranspiration (scPDSIpm), are analyzed to quantify hydroclimatic and drought changes in the 21 st century and the underlying causes. Results confirm consistent drying in these hydroclimatic metrics across most of the Americas (including the Amazon), Europe and the Mediterranean region, southern Africa, and Australia; although the drying magnitude differs, with the drying being more severe and widespread in surface SM than in total SM. Global drought frequency based on surface SM and scPDSIpm increases by ~25%–100% (50%–200%) under the SSP2-4.5 (SSP5-8.5) scenario in the 21 st century together with large increases in drought duration and areas, which result from a decrease in the mean and flattening of the probability distribution functions of SM and scPDSIpm; while the R-based drought changes are relatively small. Changes in both P and E contribute to the SM change, whereas scPDSIpm decreases result from ubiquitous PET increases and P decreases over subtropical areas. The R changes are determined primarily by P changes, while the PET change explains most of the E increase. Inter-model spreads in surface SM and R changes are large, leading to large uncertainties in the drought projections.
Plants are often attacked by multiple antagonists, and traits of the attacking organisms, and their order of arrival onto hosts, may affect plant defenses. However, few studies have assessed how multiple antagonists, and varying attack order, affect plant defense or nutrition. To address this, we assessed defensive and nutritional responses of Pisum sativum plants after attack by a vector herbivore (Acrythosiphon pisum), a non‐vector herbivore (Sitona lineatus), and a pathogen (Pea enation mosaic virus, PEMV). We show viruliferous A. pisum induced several anti‐pathogen plant defense signals, but these defenses were inhibited by S. lineatus feeding on peas infected with PEMV. In contrast, S. lineatus feeding induced anti‐herbivore defense signals, and these plant defenses were enhanced by PEMV. Sitona lineatus also increased abundance of plant amino acids, but only when they attacked after viruliferous A. pisum. Our results suggest that diverse communities of biotic antagonists alter defense and nutritional traits of plants through complex pathways that depend on the identity of attackers and their order of arrival onto hosts. Moreover, we show interactions among a group of biotic stressors can vary along a spectrum from antagonism to enhancement/synergism based on the identity and order of attackers, and these interactions are mediated by a multitude of phytohormone pathways.
Plant hormones are essential for regulating the interactions between plants and their complex biotic and abiotic environments. Each hormone initiates a specific molecular pathway and these different hormone pathways are integrated in a complex network of synergistic, antagonistic and additive interactions. The inter‐pathway communication is called hormone crosstalk. By influencing the immune network topology, hormone crosstalk is essential for tailoring plant responses to diverse microbes and insects in diverse environmental and internal contexts. Crosstalk provides robustness to the immune system but also drives specificity of induced defense responses against the plethora of biotic interactors. Recent advances in dry‐lab and wet‐lab techniques have greatly enhanced our understanding of the broad‐scale effects of hormone crosstalk on immune network functioning and have revealed underlying principles of crosstalk mechanisms. Molecular studies have demonstrated that hormone crosstalk is modulated at multiple levels of regulation, such as by affecting protein stability, gene transcription and hormone homeostasis. These new insights into hormone crosstalk regulation of plant defense are reviewed here, with a focus on crosstalk acting on the jasmonic acid pathway in Arabidopsis thaliana, pinpointing to the transcription factors MYC2 and ORA59 as major targets for modulation by other hormones.
The impact of drought stress on tripartite plant‐pathogen‐vector interactions constitutes a complex and largely understudied field of plant‐insect interaction. A number of studies explored these topics using aphid vectors of plant pathogens, but few have considered the interactions between drought‐stressed plants and pathogen‐transmitting psyllids. The potato psyllid, Bactericera cockerelli (Šulc) (Hemiptera: Triozidae), is one of the key pests of solanaceous crops in the USA that causes direct injury as well as indirect injury through transmission of a bacterial pathogen, Candidatus Liberibacter solanacearum (Lso), the causal agent of zebra chip. Previous studies explored the impact of Lso infection and drought stress on B. cockerelli development and reproductive rate separately, but no research to date has evaluated whether drought stress and Lso infection alter feeding behavior of the insects. We explored this using the electrical penetration graph (EPG) technique and monitored feeding behavior of Lso‐infected and uninfected potato psyllids on well‐watered and drought‐stressed tomato (Solanum lycopersicum L., Solanaceae). We found that drought stress had a significant effect on feeding behavior associated with salivation into the phloem and phloem ingestion, both linked to Lso transmission. Furthermore, infected potato psyllids in particular produced a higher number of events associated with these feeding behaviors and remained in these phases longer in well‐watered plants than in plants that were under drought stress. We also reported a new and previously undescribed waveform H of unknown biological function that was produced by the psyllids. This is the first study that considered the impact of bacterial infection and concomitant drought stress on feeding behavior of an insect quantified using EPG.
Copper‐based antimicrobial compounds are widely and historically used to control plant diseases, such as late blight caused by Phytophthora infestans, which seriously affects the yield and quality of potato. We previously identified that copper ion (Cu2+) acts as an extremely sensitive elicitor to induce ethylene (ET)‐dependent immunity in Arabidopsis. Here, we found that Cu2+ induces the defence response to P. infestans in potato. Cu2+ suppresses the transcription of the abscisic acid (ABA) biosynthetic genes StABA1 and StNCED1, resulting in decreased ABA content. Treatment with ABA or inhibitor fluridone made potato more susceptible or resistance to late blight, respectively. In addition, potato with knockdown of StABA1 or StNCED1 showed greater resistance to late blight, suggesting that ABA negatively regulates potato resistance to P. infestans. Cu2+ also promotes the rapid biosynthesis of ET. Potato plants treated with 1‐aminocyclopropane‐1‐carboxylate showed enhanced resistance to late blight. Repressed expression of StEIN2 or StEIN3 resulted in enhanced transcription of StABA1 and StNCED1, accumulation of ABA and susceptibility to P. infestans. Consistently, StEIN3 directly binds to the promoter regions of StABA1 and StNCED1. Overall, we concluded that Cu2+ triggers the defence response to potato late blight by activating ET biosynthesis to inhibit the biosynthesis of ABA.
The plant cell wall acts not only as a physical barrier, but also as a complex and dynamic structure that actively changes under different biotic and abiotic stress conditions. The question is, how are the different cell wall compounds modified during different interactions with exogenous stimuli such as pathogens? Plants exposed to viral pathogens respond to unfavorable conditions on multiple levels. One challenge that plants face under viral stress is the number of processes required for differential cell wall remodeling. The key players in these conditions are the cell wall genes and proteins, which can be regulated in specific ways during the interactions and have direct influences on the rebuilding of the cell wall structure. The cell wall modifications occurring in plants during viral infection remain poorly described. Therefore, this study focuses on cell wall dynamics as an effect of incompatible interactions between the potato virus Y (PVY NTN) and resistant potatoes (hypersensitive plant), as well as compatible (susceptible plant) interactions. Our analysis describes, for the first time, the expression of the potato expansin A3 (StEXPA3) and potato extensin 4 (StEXT4) genes in PVY NTN-susceptible and-resistant potato plant interactions. The results indicated a statistically significant induction of the StEXPA3 gene during a susceptible response. By contrast, we demonstrated the predominantly gradual activation of the StEXT4 gene during the hypersensitive response to PVY NTN inoculation. Moreover, the in situ distributions of expansins (StEXPAs), which are essential cell wall-associated proteins, and the hydroxyproline-rich glycoprotein (HRGP) extensin were investigated in two types of interactions. Furthermore, cell wall loosening was accompanied by an increase in StEXPA deposition in a PVY NTN-susceptible potato, whereas the HRGP content dynamically increased during the hypersensitive response, when the cell wall was reinforced. Ultrastructural localization and quantification revealed that the HRGP extensin was preferably located in the apoplast, but deposition in the symplast was also observed in resistant plants. Interestingly, during the hypersensitive response, StEXPA proteins were mainly located in the symplast area, in contrast to the susceptible potato where StEXPA proteins were mainly observed in the cell wall. These findings revealed that changes in the intracellular distribution and abundance of StEXPAs and HRGPs can be differentially regulated, depending on different types of PVY NTN-potato plant interactions, and confirmed the involvement of apoplast and symplast activation as a defense response mechanism.
Plants regularly encounter stress, and their responses to abiotic and biotic stressors have been a focus of research for decades. Stress caused by drought is one of the most often studied abiotic stresses owing to the increase in the incidence of drought driven by climate change. Severe drought has been shown to elicit a whole-plant response guided by key phytohormones, which not only respond to water stress but also play a critical role in the response of plants to biotic stress imposed by herbivores and pathogens. This is especially relevant for insect-transmitted pathogen systems, where plants, herbivores, and pathogens are linked in a web of direct and indirect interactions. Few studies have thus far explored the complex nature of drought-mediated tripartite interactions, however, and our ability to generalize and predict how plants respond to herbivore-transmitted pathogens while simultaneously countering the consequences of drought remains limited. The goal of this mini-review is to assess the current state of the field regarding the molecular mechanisms underlying plant responses to the combined effects of drought and simultaneous herbivory and pathogen transmission and their ecological consequences. We discuss plant responses to drought, herbivory, and pathogens as distinct and concurrent stresses, and highlight the implications of the tripartite interactions on insect vector and pathogen suppression in agroecosystems. This review provides a framework for future research linking generalized molecular responses in drought-stressed plants to tripartite species interactions and the ecology of insect-transmitted pathogens in the context of modern agriculture and water deficit driven by climate change.
Sucrose is the end product of photosynthesis and the primary sugar transported in the phloem of most plants. Sucrose synthase (SuSy) is a glycosyl transferase enzyme that plays a key role in sugar metabolism, primarily in sink tissues. SuSy catalyzes the reversible cleavage of sucrose into fructose and either uridine diphosphate glucose (UDP-G) or adenosine diphosphate glucose (ADP-G). The products of sucrose cleavage by SuSy are available for many metabolic pathways, such as energy production, primary-metabolite production, and the synthesis of complex carbohydrates. SuSy proteins are usually homotetramers with an average monomeric molecular weight of about 90 kD (about 800 amino acids long). Plant SuSy isozymes are mainly located in the cytosol or adjacent to plasma membrane, but some SuSy proteins are found in the cell wall, vacuoles, and mitochondria. Plant SUS gene families are usually small, containing between four to seven genes, with distinct exon-intron structures. Plant SUS genes are divided into three separate clades, which are present in both monocots and dicots. A comprehensive phylogenetic analysis indicates that a first SUS duplication event may have occurred before the divergence of the gymnosperms and angiosperms and a second duplication event probably occurred in a common angiosperm ancestor, leading to the existence of all three clades in both monocots and dicots. Plants with reduced SuSy activity have been shown to have reduced growth, reduced starch, cellulose or callose synthesis, reduced tolerance to anaerobic-stress conditions and altered shoot apical meristem function and leaf morphology. Plants overexpressing SUS have shown increased growth, increased xylem area and xylem cell-wall width, and increased cellulose and starch contents, making SUS high-potential candidate genes for the improvement of agricultural traits in crop plants. This review summarizes the current knowledge regarding plant SuSy, including newly discovered possible developmental roles for SuSy in meristem functioning that involve sugar and hormonal signaling.
We present new global maps of the Köppen-Geiger climate classification at an unprecedented 1-km resolution for the present-day (1980–2016) and for projected future conditions (2071–2100) under climate change. The present-day map is derived from an ensemble of four high-resolution, topographically-corrected climatic maps. The future map is derived from an ensemble of 32 climate model projections (scenario RCP8.5), by superimposing the projected climate change anomaly on the baseline high-resolution climatic maps. For both time periods we calculate confidence levels from the ensemble spread, providing valuable indications of the reliability of the classifications. The new maps exhibit a higher classification accuracy and substantially more detail than previous maps, particularly in regions with sharp spatial or elevation gradients. We anticipate the new maps will be useful for numerous applications, including species and vegetation distribution modeling. The new maps including the associated confidence maps are freely available via www.gloh2o.org/koppen.
Potato (Solanum tuberosum) plants are exposed to diverse environmental stresses, which may modulate plant–pathogen interactions, and potentially cause further decreases in crop productivity. To provide new insights into interactive molecular responses to heat stress combined with virus infection in potato, we analyzed expression of genes encoding pathogenesis-related (PR) proteins [markers of salicylic acid (SA)-mediated plant defense] and heat shock proteins (HSPs), in two potato cultivars that differ in tolerance to elevated temperatures and in susceptibility to potato virus Y (PVY). In plants of cv. Chicago (thermosensitive and PVY-susceptible), increased temperature reduced PR gene expression and this correlated with enhancement of PVY infection (virus accumulation and symptom production). In contrast, with cv. Gala (thermotolerant and PVY resistant), which displayed a greater increase in PR gene expression in response to PVY infection, temperature affected neither PR transcript levels nor virus accumulation. HSP genes were induced by elevated temperature in both cultivars but to higher levels in the thermotolerant (Gala) cultivar. PVY infection did not alter expression of HSP genes in the Gala cultivar (possibly because of the low level of virus accumulation) but did induce expression of HSP70 and HSP90 in the susceptible cultivar (Chicago). These findings suggest that responses to heat stress and PVY infection in potato have some common underlying mechanisms, which may be integrated in a specific consolidated network that controls plant sensitivity to multiple stresses in a cultivar-specific manner. We also found that the SA pre-treatment subverted the sensitive combined (heat and PVY) stress phenotype in Chicago, implicating SA as a key component of such a regulatory network.
Abiotic factors inducing osmotic stress can affect plant immunity and resistance against pathogen attack. Although a number of studies have characterized grapevine responses to various forms of biotic and abiotic stresses, the relationships between osmotic stress response and susceptibility of mature berries to Botrytis cinerea still remain unknown. In this study, we investigated the effects of osmotic stress and abscisic acid (ABA) on defense responses of mature grapevine berries before and after B. cinerea infection. We focused on the possible involvement of polyamines in the interaction between osmotic stress response and susceptibility to B. cinerea. We showed that osmotic stress induced by PEG or sucrose, and exogenous ABA induce transient but low defense responses, including weak expression of PR genes and phytoalexin synthesis in mature berries. This was accompanied by an upregulation of NCED2 involved in ABA biosynthesis and a large production of free polyamines. However, osmotic stress followed by B. cinerea infection primed berries for enhanced accumulation of polyamines, but slowed down the defense responses and increased susceptibility to the pathogen. A weak increase of diamine- and polyamine-oxidase activities was also recorded in stressed berries, but declined after pathogen infection. The pretreatment of stressed berries with appropriate inhibitors of diamine- and polyamine-oxidases further increased polyamine level and greatly lowered defense responses, leading to higher susceptibility to B. cinerea. These results suggest that increased polyamine titer through low activation of their oxidative degradation in grape berries may contribute at least in part to the weakening of defense responses and subsequent disease susceptibility.
The gene NCED1 encodes 9-cis-epoxycarotenoid dioxygenase, which catalyzes oxidative cleavage of 9-cis-epoxycarotenoids neoxanthin and violaxanthin to xanthoxin, a key step in the biosynthesis of abscisic acid in higher plants. In the present study, the complete NCED1 of 1 917 bp was cloned and characterized from rice (Oryza sativa L. cv. N22) as no earlier reports were available for its characterization from indica cultivar. The NCED1 had no intron and encoded a protein of 639 amino acids with a predicted molecular mass of 68.62 kD and pI of 6.07. The aliphatic index and grand average of hydropathicity were found to be 77.04 and -0.148, respectively. Multiple alignment analysis revealed that the sequence shared a high identity with the Oryza sativa japonica group (100 %) followed by Triticum aestivum (90 %), Hordeum vulgare (90 %), and Zea mays (89 %). The enzyme had a RPE65 domain of 476 amino acid residues. The RPE65 domain requires Fe(II) as a cofactor coordinated with 4 histidine residues and 3 glutamic acid residues. The phylogenic tree shows that NCED1 of japonica rice and NCED1 of indica rice were in the same group. They might have been evolved from a common ancestor. Analysis with a PSORT III tool shows that NCED is a chloroplastic protein. The real-time quantitative PCR and RNA-sequencing studies show that the expression of NCED1 was progressively reduced with increasing water stress, and a negative correlation between expression of OsNCED1 and severity of stress was established. Further, NCED1 expression negatively correlated with abscisic acid (ABA) accumulation under water stress whereas in some other species its expression increased along with ABA accumulation. This might be due to feedback inhibition of the ABA biosynthesis in rice.
The response of a phytopathogen vector to pathogen-induced plant volatiles was investigated, as well as the response of the phytopathogen vector's parasitoid to herbivore-induced plant volatiles released from plants with and without drought stress. 2. These experiments were performed with Asian citrus psyllid (Diaphorina citri), vector of the plant pathogen Candidatus Liberibacter asiaticus (CLas) and its parasitoid Tamarixia radiata as models. Candidatus Liberibacter asiaticus is the presumed causal pathogen of huanglongbing (HLB), also called citrus greening disease. 3. Diaphorina citri vectors were attracted to headspace volatiles of CLas-infected citrus plants at 95% of their water-holding capacity (WHC); such attraction to infected plants was much lower under drought stress. Attraction of the vector to infected and non-stressed plants was correlated with greater release of methyl salicylate (MeSA) as compared with uninfected and non-stressed control citrus plants. Drought stress decreased MeSA release from CLas-infected plants as compared with non-stressed and infected plants. 4. Similarly, T. radiata was attracted to headspace volatiles released from D. citri-infested citrus plants at 95% of their WHC. However, wasps did not show preference between headspace volatiles of psyllid-infested and uninfested plants when they were at 35% WHC, suggesting that herbivore-induced defences did not activate to recruit this natural enemy under drought stress. 5. Our results demonstrate that herbivore- and pathogen-induced responses are environmentally dependent and do not occur systematically following damage. Drought stress affected both pathogen- and herbivore-induced plant volatile release, resulting in concomitant decreases in behavioural response of both the pathogen's vector and the vector's primary parasitoid.
Global transcriptome studies demonstrated the existence of unique plant responses under combined stress which are otherwise not seen during individual stresses. In order to combat combined stress plants use signaling pathways and ‘cross talk’ mediated by hormones involved in stress and growth related processes. However, interactions among hormones’ pathways in combined stressed plants are not yet known. Here we studied dynamics of different hormones under individual and combined drought and pathogen infection in Arabidopsis thaliana by liquid chromatography-mass spectrometry (LC-MS) based profiling. Our results revealed abscisic acid (ABA) and salicylic acid (SA) as key regulators under individual drought and pathogen stress respectively. Under combined drought and host pathogen stress (DH) we observed non-induced levels of ABA with an upsurge in SA and jasmonic acid (JA) concentrations, underscoring their role in basal tolerance against host pathogen. Under a non-host pathogen interaction with drought (DNH) stressed plants, ABA, SA and JA profiles were similar to those under DH or non-host pathogen alone. We propose that plants use SA/JA dependent signaling during DH stress which antagonize ABA biosynthesis and signaling pathways during early stage of stress. The study provides insights into hormone modulation at different time points during combined stress.
Background
Starch is the principle constituent of potato tubers and is of considerable importance for food and non-food applications. Its metabolism has been subject of extensive research over the past decades. Despite its importance, a description of the complete inventory of genes involved in starch metabolism and their genome organization in potato plants is still missing. Moreover, mechanisms regulating the expression of starch genes in leaves and tubers remain elusive with regard to differences between transitory and storage starch metabolism, respectively. This study aimed at identifying and mapping the complete set of potato starch genes, and to study their expression pattern in leaves and tubers using different sets of transcriptome data. Moreover, we wanted to uncover transcription factors co-regulated with starch accumulation in tubers in order to get insight into the regulation of starch metabolism.
Results
We identified 77 genomic loci encoding enzymes involved in starch metabolism. Novel isoforms of many enzymes were found. Their analysis will help to elucidate mechanisms of starch biosynthesis and degradation. Expression analysis of starch genes led to the identification of tissue-specific isoenzymes suggesting differences in the transcriptional regulation of starch metabolism between potato leaf and tuber tissues. Selection of genes predominantly expressed in developing potato tubers and exhibiting an expression pattern indicative for a role in starch biosynthesis enabled the identification of possible transcriptional regulators of tuber starch biosynthesis by co-expression analysis.
Conclusions
This study provides the annotation of the complete set of starch metabolic genes in potato plants and their genomic localizations. Novel, so far undescribed, enzyme isoforms were revealed. Comparative transcriptome analysis enabled the identification of tuber- and leaf-specific isoforms of starch genes. This finding suggests distinct regulatory mechanisms in transitory and storage starch metabolism. Putative regulatory proteins of starch biosynthesis in potato tubers have been identified by co-expression and their expression was verified by quantitative RT-PCR.
Electronic supplementary material
The online version of this article (doi:10.1186/s12864-016-3381-z) contains supplementary material, which is available to authorized users.
Little is known about how water stress including drought and flooding modifies the ability of plants to resist simultaneous attack by insect feeding and transmission of insect-vectored pathogen. We analyzed insect population growth, feeding behaviors, virus transmission, and plant amino acid profiles and defense gene expression to characterize mechanisms underlying the interaction between water stress, soybean aphid and aphid-transmitted, Soybean mosaic virus, on soybean plants. Population growth of non-viruliferous aphids was reduced under drought stress and saturation, likely because the aphids spent less time feeding from the sieve element on these plants compared to well-watered plants. Water stress did not impact population growth of viruliferous aphids. However, virus incidence and transmission rate was lowest under drought stress and highest under saturated conditions since viruliferous aphids took the greatest amount time to puncture cells and transmit the virus under saturated conditions and lowest time under drought stress. Petiole exudates from drought-stressed plants had the highest level of total free amino acids including asparagine and valine that are critical for aphid performance. Aphids did not benefit from improved phloem sap quality as indicated by their lower densities on drought-stressed plants. Saturation, on the other hand, resulted in low amino acid content compared to all of the other treatments. Drought and saturation had significant and opposing effects on expression of marker genes involved in abscisic acid (ABA) signaling. Drought alone significantly increased expression of ABA marker genes, which likely led to suppression of salicylic acid (SA)- and jasmonic acid (JA)-related genes. In contrast, ABA marker genes were down-regulated under saturation, while expression of SA- and JA-related genes was up-regulated. We propose that the apparent antagonism between ABA and SA/JA signaling pathways contributed to an increase in aphid densities under drought and their decrease under saturation. Taken together, our findings suggests that plant responses to water stress is complex involving changes in phloem amino acid composition and signaling pathways, which can impact aphid populations and virus transmission.
The activation of the abscisic acid (ABA) signaling pathway reduces water loss from plants challenged by drought stress. The effect of drought-induced ABA signaling on the defense and nutrition allocation of plants is largely unknown. We postulated that these changes can affect herbivorous insects. We studied the effects of drought on different feeding stages of pea aphids in the wild-type A17 of Medicago truncatula and ABA signaling pathway mutant sta-1. We examined the impact of drought on plant water status, induced plant defense signaling via the abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA) pathways, and on the host nutritional quality in terms of leaf free amino acid content. During the penetration phase of aphid feeding, drought decreased epidermis/mesophyll resistance but increased mesophyll/phloem resistance of A17 but not sta-1 plants. Quantification of transcripts associated with ABA, JA and SA signaling indicated that the drought-induced up-regulation of ABA signaling decreased the SA-dependent defense but increased the JA-dependent defense in A17 plants. During the phloem-feeding phase, drought had little effect on the amino acid concentrations and the associated aphid phloem-feeding parameters in both plant genotypes. In the xylem absorption stage, drought decreased xylem absorption time of aphids in both genotypes because of decreased water potential. Nevertheless, the activation of the ABA signaling pathway increased water-use efficiency of A17 plants by decreasing the stomatal aperture and transpiration rate. In contrast, the water potential of sta-1 plants (unable to close stomata) was too low to support xylem absorption activity of aphids; the aphids on sta-1 plants had the highest hemolymph osmolarity and lowest abundance under drought conditions. Taken together this study illustrates the significance of cross-talk between biotic-abiotic signaling pathways in plant-aphid interaction, and reveals the mechanisms leading to alter aphid fecundity in water stresses plants.
The potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), is a vector of the bacterium “Candidatus Liberibacter solanacearum,” the putative causal agent of potato zebra chip disease that has seriously affected the potato industry in the Central and Southwestern United States for the past decade. The 2011 potato growing season saw the first report of zebra chip disease in Washington, Oregon, and Idaho; however, B. cockerelli has been recorded in this region every season at least for the past 7 yr. Studies were conducted to determine the relationship between psyllids collected from the Pacific Northwest potatoes in 2011 and those from the Southwestern and Central United States. High resolution melting analysis of the B. cockerelli mitochondrial Cytochrome C Oxidase subunit I-like gene was conducted on over 450 psyllids collected from numerous locations across the Central and Western United States. Results suggest that at least three potato psyllid haplotypes exist in the United States, correlating to the Central, Western, and Northwestern United States geographical regions. The high resolution melting analysis results were subsequently supported by DNA sequencing data.
This paper aims to discuss various aspects of the use of reference genes in qPCR technique used in the thousands of present studies. Most frequently, these are housekeeping genes and they must meet several criteria so that they can lay claim to the name. Lots of papers report that in different conditions, for different organisms and even tissues the basic assumption-the constant level of the expression is not maintained for many genes that seem to be perfect candidates. Moreover, their transcription can not be affected by experimental factors. Sounds simple and clear but a great number of designed protocols and lack of consistency among them brings confusion on how to perform experiment properly. Since during selection of the most stable normalizing gene we can not use any reference gene, different ways and algorithms for their selection were developed. Such methods, including examples of best normalizing genes in some specific cases and possible mistakes are presented based on available sources. Numerous examples of reference genes applications, which are usually in too few numbers in relevant articles not allowing to make a solid fundament for a reader, will be shown along with instructive compilations to make an evidence for presented statements and an arrangement of future qPCR experiments. To include all the pitfalls and problems associated with the normalization methods there is no way not to begin from sample preparation and its storage going through candidate gene selection, primer design and statistical analysis. This is important because numerous short reviews available cover the topic only in lesser extent at the same time giving the reader false conviction of complete topic recognition.
The plant hormone abscisic acid (ABA) is involved in a wide variety of plant processes, including the initiation of stress-adaptive responses to various environmental cues. Recently, ABA also emerged as a central factor in the regulation and integration of plant immune responses, although little is known about the underlying mechanisms. Aiming to advance our understanding of ABA-modulated disease resistance, we have analyzed the impact, dynamics and interrelationship of ABA and the classic defense hormone salicylic acid (SA) during progression of rice infection by the leaf blight pathogen Xanthomonas oryzae pv. oryzae (Xoo). Consistent with ABA negatively regulating resistance to Xoo, we found that exogenously administered ABA renders rice hypersusceptible to infection, whereas chemical and genetic disruption of ABA biosynthesis and signaling, respectively, led to enhanced Xoo resistance. In addition, we found successful Xoo infection to be associated with extensive reprogramming of ABA biosynthesis and response genes, suggesting that ABA functions as a virulence factor for Xoo. Interestingly, several lines of evidence indicate that this immune-suppressive effect of ABA is due at least in part to suppression of SA-mediated defenses that normally serve to limit pathogen growth. Resistance induced by the ABA biosynthesis inhibitor fluridone, however, appears to operate in a SA-independent manner and is likely due to induction of non-specific physiological stress. Collectively, our findings favor a scenario whereby virulent Xoo hijacks the rice ABA machinery to cause disease and highlight the importance of ABA and its crosstalk with SA in shaping the outcome of rice-Xoo interactions.
Little is known about how drought stress influences plant secondary metabolite accumulation and how this affects plant defense against different aphids. We therefore cultivated Arabidopsis thaliana (L.) plants under well-watered, drought, and water-logged conditions. Two aphid species were selected for this study: the generalist Myzus persicae (Sulzer) and the crucifer specialist Brevicoryne brassicae (L.). Metabolite concentrations in the phloem sap, which influence aphid growth, changed particularly under drought stress. Levels of sucrose and several amino acids, such as glutamic acid, proline, isoleucine, and lysine increased, while concentrations of 4-methoxyindol-3-ylmethyl glucosinolate decreased. M. persicae population growth was highest on plants under drought stress conditions. However, B. brassicae did not profit from improved phloem sap quality under drought stress and performed equally in all water treatments. Water stress and aphids generally had an opposite effect on the accumulation of secondary metabolites in the plant rosettes. Drought stress and water-logging led to increased aliphatic glucosinolate and flavonoid levels. Conversely, aphid feeding, especially of M. persicae, reduced levels of flavonoids and glucosinolates in the plants. Correspondingly, transcript levels of aliphatic biosynthetic genes decreased after feeding of both aphid species. Contrary to M. persicae, drought stress did not promote population growth of B. brassicae on these plants. The specialist aphid induced expression of CYP79B2, CYP79B3, and PAD3 with corresponding accumulation of indolyl glucosinolates and camalexin. This was distinct from M. persicae, which did not elicit similarly strong camalexin accumulation, which led to the hypothesis of a specific defense adaptations against the specialist aphid.
This review examines the direct effects of climate change on insect herbivores. Temperature is identified as the dominant abiotic factor directly affecting herbivorous insects. There is little evidence of any direct effects of CO2 or UVB. Direct impacts of precipitation have been largely neglected in current research on climate change. Temperature directly affects development, survival, range and abundance. Species with a large geographical range will tend to be less affected. The main effect of temperature in temperate regions is to influence winter survival; at more northerly latitudes, higher temperatures extend the summer season, increasing the available thermal budget for growth and reproduction. Photoperiod is the dominant cue for the seasonal synchrony of temperate insects, but their thermal requirements may differ at different times of year. Interactions between photoperiod and temperature determine phenology; the two factors do not necessarily operate in tandem. Insect herbivores show a number of distinct life-history strategies to exploit plants with different growth forms and strategies, which will be differentially affected by climate warming. There are still many challenges facing biologists in predicting and monitoring the impacts of climate change. Future research needs to consider insect herbivore phenotypic and genotypic flexibility, their responses to global change parameters operating in concert, and awareness that some patterns may only become apparent in the longer term.
A new huanglongbing (HLB) “Candidatus Liberibacter” species is genetically characterized, and the bacterium is designated “Candidatus Liberibacter psyllaurous.” This bacterium infects the psyllid Bactericera cockerelli and its solanaceous host plants potato and tomato, potentially resulting in “psyllid yellowing.” Host plant-dependent HLB
transmission and variation in psyllid infection frequencies are found.
Experiments were performed on three abscisic acid (ABA)-deficient tomato (Lycopersicon esculentum Mill.) mutants, notabilis, flacca, and sitiens, to investigate the role of ABA and jasmonic acid (JA) in the generation of electrical signals and Pin2 (proteinase inhibitor II) gene expression. We selected these mutants because they contain different levels of endogenous ABA. ABA levels in the mutant sitiens were reduced to 8% of the wild type, in notabilis they were reduced to 47%, and in flacca they were reduced to 21%. In wild-type and notabilis tomato plants the induction of Pin2 gene expression could be elicited by heat treatment, current application, or mechanical wounding. In flacca and sitiens only heat stimulation induced Pin2 gene expression. JA levels in flacca and sitiens plants also accumulated strongly upon heat stimulation but not upon mechanical wounding or current application. Characteristic electrical signals evolved in the wild type and in the notabilis and flacca mutants consisting of a fast action potential and a slow variation potential. However, in sitiens only heat evoked electrical signals; mechanical wounding and current application did not change the membrane potential. In addition, exogenous application of ABA to wild-type tomato plants induced transient changes in membrane potentials, indicating the involvement of ABA in the generation of electrical signals. Our data strongly suggest the presence of a minimum threshold value of ABA within the plant that is essential for the early events in electrical signaling and mediation of Pin2 gene expression upon wounding. In contrast, heat-induced Pin2 gene expression and membrane potential changes were not dependent on the ABA level but, rather, on the accumulation of JA.
In the natural world, plants have to withstand and counteract stress havoc like biotic and abiotic stressors throughout their life cycle. To survive and perform vital biological processes, plants have evolved a variety of complex ways to detect environmental cues and respond appropriately. Phytohormones such as abscisic acid, salicylic acid, jasmonic acid, and ethylene, regulate plant defenses through synergistic and antagonistic actions. Cross-talk among these hormones enables the plant to utilize less energy for defense response and allocate it for growth and development. Crosstalk helps in prioritizing and optimizing the response according to the severity of specific stress. With the elucidation of important gene structure and functions through omics approaches and their experimental validation, functional mutant studies have revealed several unique and shared mechanisms involved in cross-talk between different signaling networks. In this review, we discuss how plant hormones coordinate with various signaling components such as transcription factors, signaling proteins, mitogen-activated protein kinase, and reactive oxygen species to orchestrate a fine balance of the primary and secondary stress responses during biotic and abiotic stress. Further, the emerging roles of novel molecules such as indoleamine serotonin, melatonin, methylglyoxal, spermine, etc. as moderators of cross-talk have been discussed. The final section of this article deals with the regulation of phytohormone networks by small RNAs and epigenetic regulations.
Diverse molecular processes regulate the interactions between plants and insect herbivores. Here, we review genes and proteins that are involved in plant–herbivore interactions and discuss how their discovery has structured the current standard model of plant–herbivore interactions. Plants perceive damage-associated and, possibly, herbivore-associated molecular patterns via receptors that activate early signaling components such as Ca2+, reactive oxygen species, and MAP kinases. Specific defense reprogramming proceeds via signaling networks that include phytohormones, secondary metabolites, and transcription factors. Local and systemic regulation of toxins, defense proteins, physical barriers, and tolerance traits protect plants against herbivores. Herbivores counteract plant defenses through biochemical defense deactivation, effector-mediated suppression of defense signaling, and chemically controlled behavioral changes. The molecular basis of plant–herbivore interactions is now well established for select model systems. Expanding molecular approaches to unexplored dimensions of plant–insect interactions should be a future priority. Expected final online publication date for the Annual Review of Plant Biology Volume 70 is April 29, 2019. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Pathogen spread by arthropod vectors is the outcome of pathogen-vector-plant interactions, as well as how these interactions are impacted by abiotic and biotic factors. While plant water stress impacts each component of the Pierce's disease pathosystem (Xylella fastidiosa Wells et al., insect vectors, and grapevines), the outcome of interactions in relation to pathogen spread is unknown. The objectives of this study were 1) to determine the role of plant water stress on vector acquisition and inoculation of X. fastidiosa under choice and no-choice conditions for source or recipient vines, and 2) to provide insights into the effects of vineyard irrigation regimes on spread of X. fastidiosa by using a host-vector epidemic model. Under no-choice conditions, pathogen acquisition increased as water stress increased in source plants, while inoculation was not affected by water status of recipient vines. Thus, under no-choice conditions, plant water stress increased transmission of X. fastidiosa. However, when vectors had a choice of an uninfected well-watered versus an infected water-stressed grapevine, transmission efficiency declined as water stress levels increased. While our experimental results produced wide uncertainty estimates, the epidemiological modeling suggested a non-linear relationship between water stress and pathogen spread: moderate water stress enhances pathogen spread but severe or no stress produce equivalent spread. In summary, both host plant condition and vector host preference interacted to determine transmission efficiency of X. fastidiosa.
Zebra Chip disease vectored by the potato psyllid Bactericera cockerelli (Šulc) was first reported in Idaho and the Columbia Basin of Oregon and Washington in 2011. Since then growers have incurred significant costs for managing the disease. Thus, we conducted an expert opinion survey to estimate expenditure on insecticides dedicated to controlling potato psyllids in the largest potato producing regions of Idaho, Oregon, and Washington. Results highlight a total of about 9 million US dollars spent on active ingredients targeted at psyllid control. When application costs are added to the cost of insecticides, expenditures total about 11 million US dollars.
Recent outbreaks in plant diseases associated with Liberibacter pathogens have impacted large areas of western and southern North America. The increase in frequency and severity of drought could render plants more susceptible to the colonization by insect vectors. Experiments were conducted in a laboratory setting to evaluate the influence of water scarcity on drought stress of tomato plants, Solanum lycopersicum L. (Solanaceae). Weekly water treatment of 200, 100, and 50 ml resulted in unstressed (control, Ψw = −0.55 MPa), lowly drought-stressed (LDS, Ψw = −0.70 MPa), and moderately drought-stressed (MDS, Ψw = −0.87 MPa) plants, respectively. By controlling for both water availability and plant drought stress, the effect of drought stress on S. lycopersicum susceptibility to potato psyllid, Bactericera cockerelli (Šulc) (Hemiptera: Triozidae), was evaluated. In a no-choice experiment, MDS plants had significantly more B. cockerelli nymphs than control plants. However, plant susceptibility to B. cockerelli colonization was not due to the oviposition preference for MDS, but rather to the higher B. cockerelli nymphal survival on MDS than on control plants. Nymphal survival of B. cockerelli on MDS plants was consistently and significantly higher than on control plants. Throughout all nymphal stages, B. cockerelli had higher survival on MDS plants than on control plants. Drought stress not only enhanced B. cockerelli survival on S. lycopersicum but it also resulted in 60% more adults produced on water-stressed plants than on control plants. Therefore, as adults can move from plant to plant, drought stress could increase B. cockerelli's dispersion potential. Although plant drought stress improved B. cockerelli survival, it did not affect B. cockerelli oviposition. No difference in number of offspring was found between B. cockerelli adults that developed on MDS vs. control plants. These results might be relevant to B. cockerelli outbreaks and Liberibacter epidemics.
Potato psyllid, Bactericera cockerelli (Šulc), causes economic damage to potato crops throughout the major potato growing regions of western North America. When cultivated crops are not available, potato psyllid often occurs on non-crop hosts. In the southern U.S. and northern Mexico, native species of Lycium (Solanaceae) are important non-crop hosts for the psyllid. We determined whether Old World species of Lycium now widespread in the Pacific Northwest are reservoirs of potato psyllid in this growing region. We examined Lycium spp. across a wide geographic region in Washington, Oregon, and Idaho at irregular intervals during three growing seasons. Potato psyllids were present at all locations. To determine whether Lycium is also a host during intervals of the year in which the potato crop is not available, we monitored a subset of these sites over the entire year. Six sites were monitored at 1- to 3-week intervals from June 2014 to June 2016. Psyllids were present on Lycium throughout the year at all sites, including during winter, indicating that Lycium is also a host when the potato crop is seasonally not available. Psyllid populations included a mixture of Northwestern and Western haplotypes. We observed well-defined spring and fall peaks in adult numbers, with peaks separated by long intervals in which psyllid numbers were very low. Seasonal patterns in psyllid numbers on these non-native Lycium hosts were very similar to what has been observed on native Lycium in the desert southwest region of the U.S. Our findings demonstrate that potato psyllid associates with Lycium across a broad geographic region within the Pacific Northwest. These results will assist in predicting sources of potato psyllid colonizing potatoes in this important growing region.
1. Understanding environmentally dependent variation in interspecific interactions is needed for
evaluating how agroecosystems respond to abiotic stressors, including climate change. Both
biotic and abiotic conditions shape crop responses to stress events, but interactions between
environmental conditions and insect-borne plant pathogens remain poorly understood.
2. We tested the hypothesis that drought stress, as applied by experimental water deprivation,
drives conditional outcomes in host–pathogen and host–vector interactions using a cereal–aphid–
virus association and greenhouse experiments.
3. Under conditions of ample water supply, infection of wheat plants with barley yellow dwarf
virus (BYDV) resulted in reduced above-ground growth, seed set, seed yields, and seed
germination compared with plants exposed only to non-infected (non-viruliferous) aphids or
control plants not subjected to aphid infestation. However, when water was chronically limiting,
infection with BYDV did not significantly affect plant performance.
4. When wheat was subjected to acute drought stress, plants infected with BYDV surpassed
both control plants and plants exposed to non-infected aphids in all measured performance traits.
5. Feeding experiments with aphid vectors (Rhopalosiphum padi) and subsequent life table
analysis revealed that aphid fecundity improved by 47% when feeding on BYDV-infected plants
when water inputs were chronically low. However, when plants received ample water, aphid
fecundity was enhanced by only 23% from feeding on BYDV-infected plants.
6. Synthesis and applications. Collectively, our experiments suggest that wheat–barley yellow
dwarf virus (BYDV) interactions shift along gradients of water stress severity and duration.
When BYDV infection preceded water deprivation plant performance was not reduced from
virus infection, and infected plants recovered from severe stress events more readily than non-infected plants. However, vector–pathogen mutualism resulting in enhanced reproduction of
aphids on virus-infected plants is likely to amplify direct plant injury from herbivory in the field.
Our findings indicate that during periods of drought, management of BYDV infection may not
be needed and infection could benefit wheat under conditions of acute water stress.
Salicylic acid (SA), is an important phytohormones that plays a role in response to biotic stresses and pathogenesis. Apart from this role, recent studies have demonstrated that SA also participates in the signaling of abiotic stress responses, such as drought, high and low temperature, salinity, ozone, UV radiation, and heavy metals. In addition, abiotic stresses also induce endogenous SA accumulation. The appropriate application of SA could provide protection against several types of environmental stresses. SA may cause oxidative stress, partially through accumulation of hydrogen peroxide. A low concentration of hydrogen peroxide also improves the antioxidative capacity of plants and stimulates the synthesis of protective compounds, leading to enhanced tolerance to abiotic stresses. The effect of SA application depends on numerous factors such as the species and developmental stage of the plant, the mode of application, and the concentration of applied and endogenous SA levels. This chapter reviews the effects of SA on different abiotic stresses, and possible mechanisms for abiotic stress responses controlled by SA.
The potato psyllid, Bactericera cockerelli (ulc), is a vector of the bacterium 'Candidatus Liberibacter solanacearum' (Lso) that has been linked to the economically devastating zebra chip disease of potato. To date, four haplotypes of the potato psyllid have been identified and include Central, Western, Northwestern, and Southwestern haplotypes. Zebra chip was reported in potato crops in the Pacific Northwestern United States for the first time in 2011, and the Lso-infected psyllids collected from zebra chip-affected fields were identified as the Western haplotype. Additional studies have reported a mix of the Western and Northwestern psyllid haplotypes in the Pacific Northwest. The present study further examined psyllid population dynamics over the duration of the 2012 potato season in the Pacific Northwest by haplotype analysis of 864 potato psyllids collected from potato fields in Washington, Oregon, and Idaho. In the Yakima Valley of Washington and the lower Columbia Basin of Washington and Oregon, the Northwestern haplotype was predominant (78 %), and was detected earlier in the season than the Western haplotype. Interestingly, in south-central Idaho, all four psyllid haplotypes were identified, but the predominant haplotype was the Western haplotype (77 %). Here, Northwestern psyllids were detected early in the season from June to mid-August, whereas Central psyllids were detected in late July and thereafter. These results suggest that haplotype composition of psyllid populations in potato fields throughout the 2012 growing season in south-central Idaho differed greatly from those in Washington and Oregon. Additionally, all psyllids were analyzed for the presence of Lso, and no Lso-positive psyllids were found in Washington and Oregon, whereas Lso-positive psyllids were found in south-central Idaho. These Lso-positive psyllids consisted of the Western, Northwestern, and Central haplotypes.
The potato/tomato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae) has been a major pest of solanaceous crops for decades. This pest can cause damage to crop plants by direct feeding and, as has been recently discovered, by transmitting the bacterial pathogen Candidatus Liberibacter psyllaurous (a.k.a. Ca. L. solanacearum). Many studies have been conducted to determine the relationship of this pest to plant injury and to develop management strategies to alleviate the damage caused by this pest in a wide variety of solanaceous plants. Studies in the past decade have documented substantial genetic variability in this invasive species, enhanced our rapidly-evolving understanding of the interactions between the insect and the pathogen it carries, and improved our appreciation of the invasive potential of the pest. This review seeks to provide a comprehensive update to B. cockerelli life history, relationship to plant diseases, and the current state of management strategies against B. cockerelli.
1 The plant stress, plant vigour and pulsed stress hypotheses describe the relationships between drought stress, plant quality and herbivore performance. We used an aphid‐Brassica system to test these hypotheses under different drought treatments. 2 The quantity of water added per plant/week was 75%, 50% and 25% of the control (unstressed) water regime for low, medium and high drought stress, respectively, and 50% applied fortnightly for pulsed drought stress. The performance of a ‘senescence’ (generalist) and a ‘flush’ feeder (specialist) aphid species and host plant quality were assessed. 3 Drought treatments had a similar effect on the fecundity and intrinsic rate of increase of both aphid species. Aphid performance on unstressed and highly drought‐stressed plants was significantly lower compared with medium drought stress. On average, 20% greater fecundity and 40% greater intrinsic rates of increase were recorded for both aphid species at medium drought stress compared with unstressed plants. 4 Plant biomass and relative water contents were significantly greater for unstressed plants compared with high and pulsed drought treatments. Foliar nitrogen concentration was significantly greater in the high drought stress and pulsed treatments, and the dominant glucosinolate (glucobrassicin) concentration was significantly greater in drought stress treatments. 5 The present study supports the plant stress hypothesis, although the plant vigour and pulsed stress hypotheses are not supported by our data. The implications of these findings for plant–herbivore interactions under changing environmental conditions are discussed.
Zebra chip (ZC), a new and economically important disease of potato (Solanum tuberosum L.), has been documented to occur in commercial potato fields in the United States, Mexico, Central America, and New Zealand. This disease has caused millions of dollars in losses to the potato industry. Whole crops might be rejected because of ZC, often leading to abandonment of entire fields. Plant growth and yield are severely affected by the disease. Additionally, chips or fries processed from ZC-infected tubers exhibit dark stripes that become markedly more visible with frying, and hence are commercially unacceptable. The disease causes serious losses to the fresh market, tablestock and export potato industry as well. ZC-infected tubers usually do not sprout and if they do, produce hair sprouts or weak plants. Finally, there are indications that ZC symptoms might develop in tubers during storage. ZC has been associated with a previously undescribed species of liberibacter, tentatively named “Candidatus Liberibacter solanacearum”, also known as “Ca. L. psyllaurous”. The bacterium is transmitted to potato by the potato psyllid, Bactericera cockerelli (Šulc). All commercial potato cultivars appear to be susceptible to ZC, and management tactics targeted against the potato psyllid are currently the only means to effectively manage the disease. Furthermore, there are concerns about quarantine and trade issues in psyllid-affected regions because some countries may require that shipments of potatoes from certain growing regions be tested for the disease before the shipments are allowed entry. ZC history, geographic distribution, biology, epidemiology, and management are discussed herein.
One goal of phytohormonal ecology is to study the interactions between biotic and abiotic stress at hierarchical levels of biological organization. From an ecological perspective, exposure to one stress may alter the plant's probability of being exposed to another stress. From a mechanistic perspective, hormonal and biochemical signaling in- teractions between responses to each stress may influence the severity or ability to adaptively respond to the subsequent stress. In this article, we consider the relationship between plant water and salt stress and attack by pathogens and herbivores. Empirical data suggest that water stress and the probability of attack by pathogens and herbivores are correlated between habitats. Biochemical interactions between plant responses to water and salt stress and insect and pathogen attack are also interrelated. Initial biochemical models indicated that abscisic acid (ABA), an important hormone in responses to water and salt stress, had a synergistic positive role with jasmonate-induced defenses against herbivores and an an- tagonistic role with salicylate-based resistance to some pathogens. Based on this back- ground, we developed predictions about how water and salt stress would alter plant resis- tance to insects and pathogens and tested the predictions using tomato plants as a model system. We used polyphenol oxidase activity as a marker of the jasmonate response and pathogenesis-related protein P4 as a marker of the salicylate response. First, we examined levels of chemical defense in wild-type and ABA-deficient plants and the ability of these plants to resist insect and pathogen attack. In the second experiment, we exposed plants to short-term salinity stress and tested their subsequent resistance to a chewing insect Spodoptera exiguaand the bacterial speck pathogen Pseudomonas syringaepv. tomato. We have two key findings. First, ABA-deficient plants had higher levels of salicylate-mediated responses and were more resistant to bacterial speck disease, consistent with the proposed role of salicylate in defense against pathogens. This suggests linkage between water avail- ability to the plant and salicylate action in pathogenesis through ABA signaling. ABA- deficient plants had reduced resistance to the insectSpodoptera exigua, suggesting a positive correlation between responses to water stress and herbivory. The lack of difference in chemical expression of the jasmonate (JA) response (polyphenol oxidase activity) between wild-type and ABA-deficient plants did not support the proposed mechanism of synergism with the jasmonate response. Second, salt stress reduced the chemical induction (e.g., pathogenesis-related protein P4) of the salicylate response, but this did not affect resistance to the pathogen. Salt stress did not alter resistance to the herbivore Trichoplusia ni, but did alter the negative signal interaction between the jasmonate and salicylate responses. Under control conditions, the jasmonate and salicylate responses are antagonistic to one another, with induction of one response reducing the inducibility of the other. Under salt stress conditions, the negative effect of salicylate on the jasmonate response was reduced. Thus, complex interactions occur between ABA, JA, and SA, hormones that are important regulators of abiotic and biotic stress responses. Phytohormonal ecology is attempting to link ecological and hormonal interactions to develop a predictive framework for how and why plants coordinate responses to the environment.
Abstract 1. Water stress may increase or reduce the suitability of plants for herbivores. The recently proposed ‘pulsed stress hypothesis’ suggests consideration of stress phenology (pulsed vs. continuous stress) to explain these conflicting effects of plant water stress on herbivore performance.
2. This hypothesis was tested for the effect of differing stress intensity on performance and preference of insect herbivores belonging to different feeding guilds, namely leaf-chewing insects (Spodoptera littoralis caterpillars) and phloem-feeding insects (Aphis pomi aphids), on apple plants (Malus domestica). The plants were non-stressed or exposed to a low or high intensity of pulsed water stress.
3. Plant responses to the different stress levels were generally monotonic. Growth, stomatal conductance (gs), leaf water, and old-leaf nitrogen concentration decreased, whereas young-leaf nitrogen concentration and leaf mass per area (LMA) increased with increasing stress intensity. The stable isotope composition of foliar carbon (δ13C) responded non-monotonically to the drought treatments. The δ13C values were highest in low-stress plants, intermediate in high-stress plants, and lowest in non-stressed plants.
4. The preference and performance responses of the caterpillars were also non-monotonic. Non-stressed plants were intermediately, low-stress plants least, and high-stress plants most attractive or suitable. Aphid population growth was highest on non-stressed plants and lowest on low-stress plants.
5. The results highlight the importance of water stress intensity for the outcome of interactions between herbivores and drought-affected plants. They show that pulsed water stress may enhance or reduce insect herbivore performance and plant resistance, depending on stress intensity.
Plant responses to different stresses are highly complex and involve changes at the transcriptome, cellular, and physiological
levels. Recent evidence shows that plants respond to multiple stresses differently from how they do to individual stresses,
activating a specific programme of gene expression relating to the exact environmental conditions encountered. Rather than
being additive, the presence of an abiotic stress can have the effect of reducing or enhancing susceptibility to a biotic
pest or pathogen, and vice versa. This interaction between biotic and abiotic stresses is orchestrated by hormone signalling
pathways that may induce or antagonize one another, in particular that of abscisic acid. Specificity in multiple stress responses
is further controlled by a range of molecular mechanisms that act together in a complex regulatory network. Transcription
factors, kinase cascades, and reactive oxygen species are key components of this cross-talk, as are heat shock factors and
small RNAs. This review aims to characterize the interaction between biotic and abiotic stress responses at a molecular level,
focusing on regulatory mechanisms important to both pathways. Identifying master regulators that connect both biotic and abiotic
stress response pathways is fundamental in providing opportunities for developing broad-spectrum stress-tolerant crop plants.
Traditionally, herbivorous insects are thought to exhibit enhanced performance and outbreak dynamics on water-stressed host plants due to induced changes in plant physiology. Recent experimental studies, however, provide mixed support for this historical view. To test the plant-stress hypothesis (PSH), we employed two methods (the traditional vote-counting approach and meta-analysis) to assess published studies that investigated insect responses to experimentally induced water-deficit in plants. For insects, we examined how water deficit affects survivorship, fecundity, density, relative growth rate, and oviposition preference. Responses were analyzed by major feeding guild (sap-feeding insects and chewing insects) and for the subguilds of sap-feeders (phloem, mesophyll, and xylem feeders) and chewing insects (free-living chewers, borers, leaf miners, and gall-formers). Both vote counting and meta-analysis found strong negative effects of water stress on the performance of sap-feeding insects at large and on members of the phloem- and mesophyll-feeding subguilds in particular. Both analytical techniques demonstrated a nonsignificant response for chewing insects at large due to the offsetting effects of water stress on the different subguilds. For example, our analyses found consistent positive responses for borers, negative responses for gall-formers, and in consistent responses for free-living species and leaf miners. Overall, our analyses strongly challenge the historical view that herbivorous insects exhibit. elevated performance and outbreak dynamics on water-stressed plants. Rather, there is widespread evidence that many phytophagous insects, especially sap-feeders, are adversely affected by continuous water stress. Despite enhanced foliar nitrogen during times of plant stress, concurrent reductions in turgor and water content interfere with an herbivore's ability to access or utilize nitrogen. To explain the discrepancy between the observed outbreaks of phytophagous insects on water-stressed plants in nature and the negative effects detected in many experimental studies where plants are continuously stressed, we propose a "pulsed stress hypothesis" whereby bouts of stress and the recovery of turgor allow sap-feeders to benefit from stress-induced increases in plant nitrogen. Our finding that phloem-feeding insects respond positively on intermittently stressed plants but exhibit poor performance on continuously stressed ones is consistent with this hypothesis and suggests that the phenology of water stress as it mediates nitrogen availability may hold the key to understanding how water stress affects the population dynamics of insect herbivores.
Drought stress alters the chemical composition of plants, which can influence their tolerance to insect herbivory. To evaluate plant chemical responses to drought stress, broccoli, Brassica oleracea L. var. italica Plenck (Brassicaceae), was grown under well-watered, drought, and water-logged conditions. The glucosinolate (GS) levels and the performance of two aphid species, the specialist Brevicoryne brassicae (L.) and the generalist Myzus persicae (Sulzer) (both Hemiptera: Aphididae), in relation to water stress conditions were studied. High Performance Liquid Chromatography analysis showed that water stress changed the levels of GS in broccoli plants. Plants grown for 2 weeks under drought stress were significantly smaller and showed decreased levels of total GS when compared with GS contents of well-watered plants, whereas water-logged conditions led to a slight increase in the GS contents. A substantial decrease in indolyl GS was detected in water-deficient plants, whereas aliphatic GS decreased slightly. Analysis of sugar levels in phloem sap of broccoli plants revealed that plants under water-logged conditions contained the highest amounts of sugars followed by drought-stressed and well-watered plants. The two aphid species responded differently to water stress-induced changes in their host plants. Significantly larger populations of M. persicae were recorded on plants with a limited water supply than on plants grown under well-watered or water-logged conditions. Brevicoryne brassicae was less affected by water stress, and similar population sizes were found on plants that were subject to different treatments. Analysis of covariance showed a significant effect of the plants' water condition but no significant effect of GS content on the performance of M. persicae. However, the specialist B. brassicae remained unaffected by changes induced under water stress conditions.
Global food demand is increasing rapidly, as are the environmental impacts of agricultural expansion. Here, we project global demand for crop production in 2050 and evaluate the environmental impacts of alternative ways that this demand might be met. We find that per capita demand for crops, when measured as caloric or protein content of all crops combined, has been a similarly increasing function of per capita real income since 1960. This relationship forecasts a 100-110% increase in global crop demand from 2005 to 2050. Quantitative assessments show that the environmental impacts of meeting this demand depend on how global agriculture expands. If current trends of greater agricultural intensification in richer nations and greater land clearing (extensification) in poorer nations were to continue, ~1 billion ha of land would be cleared globally by 2050, with CO(2)-C equivalent greenhouse gas emissions reaching ~3 Gt y(-1) and N use ~250 Mt y(-1) by then. In contrast, if 2050 crop demand was met by moderate intensification focused on existing croplands of underyielding nations, adaptation and transfer of high-yielding technologies to these croplands, and global technological improvements, our analyses forecast land clearing of only ~0.2 billion ha, greenhouse gas emissions of ~1 Gt y(-1), and global N use of ~225 Mt y(-1). Efficient management practices could substantially lower nitrogen use. Attainment of high yields on existing croplands of underyielding nations is of great importance if global crop demand is to be met with minimal environmental impacts.
The two most commonly used methods to analyze data from real-time, quantitative PCR experiments are absolute quantification and relative quantification. Absolute quantification determines the input copy number, usually by relating the PCR signal to a standard curve. Relative quantification relates the PCR signal of the target transcript in a treatment group to that of another sample such as an untreated control. The 2(-Delta Delta C(T)) method is a convenient way to analyze the relative changes in gene expression from real-time quantitative PCR experiments. The purpose of this report is to present the derivation, assumptions, and applications of the 2(-Delta Delta C(T)) method. In addition, we present the derivation and applications of two variations of the 2(-Delta Delta C(T)) method that may be useful in the analysis of real-time, quantitative PCR data.
Plants are obliged to defend themselves against a wide range of biotic and abiotic stresses. Complex regulatory signaling networks mount an appropriate defense response depending on the type of stress that is perceived. In response to abiotic stresses such as drought, cold, and salinity, the function of abscisic acid (ABA) is well documented: elevation of plant ABA levels and activation of ABA-responsive signaling result in regulation of stomatal aperture and expression of stress-responsive genes. In response to pathogens, the role of ABA is more obscure and is a research topic that has long been overlooked. This article aims to evaluate and review the reported modes of ABA action on pathogen defense and highlight recent advances in deciphering the complex role of ABA in plant-pathogen interactions. The proposed mechanisms responsible for positive or negative effects of ABA on pathogen defense are discussed, as well as the regulation of ABA signaling and in planta ABA concentrations by beneficial and pathogenic microorganisms. In addition, the fast-growing number of reports that characterize antagonistic and synergistic interactions between abiotic and biotic stress responses point to ABA as an essential component in integrating and fine-tuning abiotic and biotic stress-response signaling networks.