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ABSTRACT: Plants need abundant nitrogen and phosphorus for higher yield. Improving plant genetics for higher nitrogen and phosphorus use efficiency would save potentially billions of dollars annually on fertilizers and reduce global environmental pollution. This will require knowledge of molecular regulators for maintaining homeostasis of these nutrients in plants. Previously, we reported that the NITROGEN LIMITATION ADAPTATION (NLA) gene is involved in adaptive responses to low-nitrogen conditions in Arabidopsis, where nla mutant plants display abrupt early senescence. To understand the molecular mechanisms underlying NLA function, two suppressors of the nla mutation were isolated that recover the nla mutant phenotype to wild type. Map-based cloning identified these suppressors as the phosphate (Pi) transport-related genes PHF1 and PHT1.1. In addition, NLA expression is shown to be regulated by the low-Pi induced microRNA miR827. Pi analysis revealed that the early senescence in nla mutant plants was due to Pi toxicity. These plants accumulated over five times the normal Pi content in shoots specifically under low nitrate and high Pi but not under high nitrate conditions. Also the Pi overaccumulator pho2 mutant shows Pi toxicity in a nitrate-dependent manner similar to the nla mutant. Further, the nitrate and Pi levels are shown to have an antagonistic crosstalk as displayed by their differential effects on flowering time. The results demonstrate that NLA and miR827 have pivotal roles in regulating Pi homeostasis in plants in a nitrate-dependent fashion.
PLoS Genetics 03/2011; 7(3):e1002021. · 8.69 Impact Factor
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ABSTRACT: Development of genetic varieties with improved nitrogen use efficiency (NUE) is essential for sustainable agriculture. Generally, NUE can be divided into two parts. First, assimilation efficiency involves nitrogen (N) uptake and assimilation and second utilization efficiency involves N remobilization. Understanding the mechanisms regulating these processes is crucial for the improvement of NUE in crop plants. One important approach is to develop an understanding of the plant response to different N regimes, especially to N limitation, using various methods including transcription profiling, analysing mutants defective in their normal response to N limitation, and studying plants that show better growth under N-limiting conditions. One can then attempt to improve NUE in crop plants using the knowledge gained from these studies. There are several potential genetic and molecular approaches for the improvement of crop NUE discussed in this review. Increased knowledge of how plants respond to different N levels as well as to other environmental conditions is required to achieve this.
Journal of Experimental Botany 10/2010; 62(4):1499-509. · 5.36 Impact Factor
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ABSTRACT: The signaling molecule auxin plays a central role in several aspects of plant growth and developmental processes. Underlying optimization of these processes are complex mechanisms orchestrating the expression of genes involved in controlling auxin level, movement and signalling. The small auxin-up RNA (SAUR) family comprises a large set of genes whose expressions are early auxin-responsive. However, the function of these genes is largely unknown. Loss-of-function mutants in a number of Arabidopsis SAUR genes did not show any marked phenotypic differences to wild-type plants likely due to compensatory functions of conserved members of the SAUR gene family. We have recently shown that a rice SAUR39 gene negatively regulates auxin synthesis and transport in rice. Here we propose a model that constitutive induction of SAUR39 gene expression reduces growth and seed yield in rice plants due to the presence of a lower auxin level, reduced polar auxin transport, less chlorophyll and increased sugar and anthocyanin contents. In wild-type plants, the SAUR39 gene is expressed at a low level and is transiently induced by changes in external auxin or other environmental stimuli, but within hours of this change its expression is reduced to the low constitutive level. This homeostatic mechanism is essential for optimal plant growth and seed yield and its disruption due to the constitutive overexpression of SAUR39 leads to a set of negative pleiotropic phenotypes.
Plant signaling & behavior 12/2009; 4(12):1174-5.
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ABSTRACT: The phytohormone auxin plays a critical role for plant growth by regulating the expression of a set of genes. One large auxin-responsive gene family of this type is the small auxin-up RNA (SAUR) genes, although their function is largely unknown. The expression of the rice (Oryza sativa) SAUR39 gene showed rapid induction by transient change in different environmental factors, including auxin, nitrogen, salinity, cytokinin, and anoxia. Transgenic rice plants overexpressing the SAUR39 gene resulted in lower shoot and root growth, altered shoot morphology, smaller vascular tissue, and lower yield compared with wild-type plants. The SAUR39 gene was expressed at higher levels in older leaves, unlike auxin biosynthesis, which occurs largely in the meristematic region. The transgenic plants had a lower auxin level and a reduced polar auxin transport as well as the down-regulation of some putative auxin biosynthesis and transporter genes. Biochemical analysis also revealed that transgenic plants had lower chlorophyll content, higher levels of anthocyanin, abscisic acid, sugar, and starch, and faster leaf senescence compared with wild-type plants at the vegetative stage. Most of these phenomena have been shown to be negatively correlated with auxin level and transport. Transcript profiling revealed that metabolic perturbations in overexpresser plants were largely due to transcriptional changes of genes involved in photosynthesis, senescence, chlorophyll production, anthocyanin accumulation, sugar synthesis, and transport. The lower growth and yield of overexpresser plants was largely recovered by exogenous auxin application. Taken together, the results suggest that SAUR39 acts as a negative regulator for auxin synthesis and transport.
Plant physiology 09/2009; 151(2):691-701. · 6.53 Impact Factor
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Yong-Mei Bi, Surya Kant,
Joseph Clarke,
Joseph Clark,
Satinder Gidda,
Feng Ming,
Jianyao Xu,
Amanda Rochon,
Barry J Shelp,
Lixin Hao,
Rong Zhao,
Robert T Mullen,
Tong Zhu,
Steven J Rothstein
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ABSTRACT: Development of genetic varieties with improved nitrogen-use efficiency (NUE) is essential for sustainable agriculture. In this study, we developed a growth system for rice wherein N was the growth-limiting factor, and identified N-responsive genes by a whole genome transcriptional profiling approach. Some genes were selected to test their functionality in NUE by a transgenic approach. One such example with positive effects on NUE is an early nodulin gene OsENOD93-1. This OsENOD93-1 gene responded significantly to both N induction and N reduction. Transgenic rice plants over-expressing the OsENOD93-1 gene had increased shoot dry biomass and seed yield. This OsENOD93-1 gene was expressed at high levels in roots of wild-type (WT) plants, and its protein product was localized in mitochondria. Transgenic plants accumulated higher concentrations of total amino acids and total N in roots. A higher concentration of amino acids in xylem sap was detected in transgenic plants, especially under N stress. In situ hybridization revealed that OsENOD93-1 is expressed in vascular bundles, as well as in epidermis and endodermis. This work demonstrates that transcriptional profiling, coupled with a transgenic validation approach, is an effective strategy for gene discovery. The knowledge gained from this study could be applied to other important crops.
Plant Cell and Environment 09/2009; 32(12):1749-60. · 5.22 Impact Factor
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ABSTRACT: In Arabidopsis thaliana, the regulation of hexose levels by the large monosaccharide transporter (MST) gene family influences many aspects of plant growth. The cloning and transgenic expression of one family member (STP13) enabled the manipulation of carbon (C) and nitrogen (N) metabolism in Arabidopsis. Transgenic seedlings constitutively over-expressing STP13 (STP13OX) had increased rates of glucose uptake, higher endogenous sucrose levels and accumulated more total C and biomass per plant when grown on soil-less media supplemented with 55 mM glucose and sufficient N (9 mM nitrate). Furthermore, STP13OX seedlings acquired 90% more total N than the Col-0 seedlings, and had higher levels of expression of the nitrate transporter NRT2.2. In addition, STP13OX seedlings were larger and had higher biomass than Col-0 seedlings when grown under a limiting N condition (3 mM nitrate). Transgene analysis of STP13 reveals that its gene product is localized to the plasma membrane (PM) in tobacco BY-2 suspension cells, that it encodes a functional MST in planta, and that the STP13 promoter directs GUS expression to the vasculature and to leaf mesophyll cells. This work highlights the link between C and N metabolism, demonstrating that a plant's N use may be improved by increasing the availability of C.
Plant Cell and Environment 12/2008; 32(3):271-85. · 5.22 Impact Factor
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ABSTRACT: A comprehensive knowledge of mechanisms regulating nitrogen (N) use efficiency is required to reduce excessive input of N fertilizers while maintaining acceptable crop yields under limited N supply. Studying plant species that are naturally adapted to low N conditions could facilitate the identification of novel regulatory genes conferring better N use efficiency. Here, we show that Thellungiella halophila, a halophytic relative of Arabidopsis (Arabidopsis thaliana), grows better than Arabidopsis under moderate (1 mm nitrate) and severe (0.4 mm nitrate) N-limiting conditions. Thellungiella exhibited a lower carbon to N ratio than Arabidopsis under N limitation, which was due to Thellungiella plants possessing higher N content, total amino acids, total soluble protein, and lower starch content compared with Arabidopsis. Furthermore, Thellungiella had higher amounts of several metabolites, such as soluble sugars and organic acids, under N-sufficient conditions (4 mm nitrate). Nitrate reductase activity and NR2 gene expression in Thellungiella displayed less of a reduction in response to N limitation than in Arabidopsis. Thellungiella shoot GS1 expression was more induced by low N than in Arabidopsis, while in roots, Thellungiella GS2 expression was maintained under N limitation but was decreased in Arabidopsis. Up-regulation of NRT2.1 and NRT3.1 expression was higher and repression of NRT1.1 was lower in Thellungiella roots under N-limiting conditions compared with Arabidopsis. Differential transporter gene expression was correlated with higher nitrate influx in Thellungiella at low (15)NO(3)(-) supply. Taken together, our results suggest that Thellungiella is tolerant to N-limited conditions and could act as a model system to unravel the mechanisms for low N tolerance.
Plant physiology 08/2008; 147(3):1168-80. · 6.53 Impact Factor
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ABSTRACT: Abiotic stresses are a primary cause of crop loss worldwide. The convergence of stress signalling pathways to a common set of transcription factors suggests the existence of upstream regulatory genes that control plant responses to multiple abiotic stresses. To identify such genes, data from published Arabidopsis thaliana abiotic stress microarray analyses were combined with our presented global analysis of early heat stress-responsive gene expression, in a relational database. A set of Multiple Stress (MST) genes was identified by scoring each gene for the number of abiotic stresses affecting expression of that gene. ErmineJ over-representation analysis of the MST gene set identified significantly enriched gene ontology biological processes for multiple abiotic stresses and regulatory genes, particularly transcription factors. A subset of MST genes including only regulatory genes that were designated 'Multiple Stress Regulatory' (MSTR) genes, was identified. To validate this strategy for identifying MSTR genes, mutants of the highest-scoring MSTR gene encoding the circadian clock protein CCA1, were tested for altered sensitivity to stress. A double mutant of CCA1 and its structural and functional homolog, LATE ELONGLATED HYPOCOTYL, exhibited greater sensitivity to salt, osmotic and heat stress than wild-type plants. This work provides a reference data set for further study of MSTR genes.
Plant Cell and Environment 07/2008; 31(6):697-714. · 5.22 Impact Factor
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ABSTRACT: Two genes encoding Arabidopsis (Arabidopsis thaliana) DEAD-box RNA helicases were identified in a functional genomics screen as being down-regulated by multiple abiotic stresses. Mutations in either gene caused increased tolerance to salt, osmotic, and heat stresses, suggesting that the helicases suppress responses to abiotic stress. The genes were therefore designated STRESS RESPONSE SUPPRESSOR1 (STRS1; At1g31970) and STRS2 (At5g08620). In the strs mutants, salt, osmotic, and cold stresses induced enhanced expression of genes encoding the transcriptional activators DREB1A/CBF3 and DREB2A and a downstream DREB target gene, RD29A. Under heat stress, the strs mutants exhibited enhanced expression of the heat shock transcription factor genes, HSF4 and HSF7, and the downstream gene HEAT SHOCK PROTEIN101. Germination of mutant seed was hyposensitive to the phytohormone abscisic acid (ABA), but mutants showed up-regulated expression of genes encoding ABA-dependent stress-responsive transcriptional activators and their downstream targets. In wild-type plants, STRS1 and STRS2 expression was rapidly down-regulated by salt, osmotic, and heat stress, but not cold stress. STRS expression was also reduced by ABA, but salt stress led to reduced STRS expression in both wild-type and ABA-deficient mutant plants. Taken together, our results suggest that STRS1 and STRS2 attenuate the expression of stress-responsive transcriptional activators and function in ABA-dependent and ABA-independent abiotic stress signaling networks.
Plant physiology 12/2007; 145(3):814-30. · 6.53 Impact Factor
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ABSTRACT: Productivity of cereal crops is restricted in saline soils but may be improved by nitrogen nutrition. In this study, the effect of ionic nitrogen form on growth, mineral content, protein content and ammonium assimilation enzyme activities of barley (Hordeum vulgare cv. Alexis L.) irrigated with saline water, was determined. Leaf and tiller number as well as plant fresh and dry weights declined under salinity (120 mM NaCl). In non-saline conditions, growth parameters were increased by application of NH(4)(+)/NO(3)(-) (25:75) compared to NO(3)(-) alone. Under saline conditions, application of NH(4)(+)/NO(3)(-) led to a reduction of the detrimental effects of salt on growth. Differences in growth between the two nitrogen regimes were not due to differences in photosynthesis. The NH(4)(+)/NO(3)(-) regime led to an increase in total N in control and saline treatments, but did not cause a large decrease in plant Na(+) content under salinity. Activities of GS (EC 6.3.1.2), GOGAT (EC 1.4.1.14), PEPC (EC 4.1.1.31) and AAT (EC 2.6.1.1) increased with salinity in roots, whereas there was decreased activity of the alternative ammonium assimilation enzyme GDH (EC 1.4.1.2). The most striking effect of nitrogen regime was observed on GDH whose salinity-induced decrease in activity was reduced from 34% with NO(3)(-) alone to only 14% with the mixed regime. The results suggest that the detrimental effects of salinity can be reduced by partial substitution of NO(3)(-) with NH(4)(+) and that this is due to the lower energy cost of N assimilation with NH(4)(+) as opposed to NO(3)(-) nutrition.
Journal of Plant Physiology 04/2007; 164(3):303-11. · 2.79 Impact Factor
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ABSTRACT: Salt-sensitive glycophytes and salt-tolerant halophytes employ common mechanisms to cope with salinity, and it is hypothesized that differences in salt tolerance arise because of changes in the regulation of a basic set of salt tolerance genes. We explored the expression of genes involved in two key salt tolerance mechanisms in Arabidopsis thaliana and the halophytic A. thaliana relative model system (ARMS), Thellungiella halophila. Salt overly sensitive 1 (SOS1) is a plasma membrane Na+/H+ antiporter that retrieves and loads Na+ ions from and into the xylem. Shoot SOS1 transcript was more strongly induced by salt in T. halophila while root SOS1 was constitutively higher in unstressed T. halophila. This is consistent with a lower salt-induced rise in T. halophila xylem sap Na+ concentration than in A. thaliana. Thellungiella halophila contained higher unstressed levels of the compatible osmolyte proline than A. thaliana, while under salt stress, T. halophila accumulated more proline mainly in shoots. Expression of the A. thaliana ortholog of proline dehydrogenase (PDH), involved in proline catabolism, was undetectable in T. halophila shoots. The PDH enzyme activity was lower and T. halophila seedlings were hypersensitive to exogenous proline, indicating repression of proline catabolism in T. halophila. Our results suggest that differential gene expression between glycophytes and halophytes contributes to the salt tolerance of halophytes.
Plant Cell and Environment 08/2006; 29(7):1220-34. · 5.22 Impact Factor
