Supply of nitrogen can reverse senescence processes and affect expression of genes coding for plastidic glutamine synthetase and lysine-ketoglutarate reductase/saccharopine dehydrogenase

Arabidopsis BBX14 negatively regulates nitrogen starvation- and dark-induced leaf senescence

Senescence is a highly regulated process driven by developmental age and environmental factors. Although leaf senescence is accelerated by nitrogen (N) deficiency, the underlying physiological and molecular mechanisms are largely unknown. Here, we reveal that BBX14, a previously uncharacterized BBX-type transcription factor in Arabidopsis, is crucial for N starvation-induced leaf senescence. We find that inhibiting BBX14 by artificial miRNA (amiRNA) accelerates senescence during N starvation and in darkness, while BBX14 overexpression (BBX14-OX) delays it, identifying BBX14 as a negative regulator of N starvation- and dark-induced senescence. During N starvation, nitrate and amino acids like glutamic acid, glutamine, aspartic acid, and asparagine were highly retained in BBX14-OX leaves compared to the wild type. Transcriptome analysis showed a large number of senescence-associated genes (SAGs) to be differentially expressed between BBX14-OX and wild-type plants, including ETHYLENE INSENSITIVE3 (EIN3) which regulates N signaling and leaf senescence. Chromatin immunoprecipitation (ChIP) showed that BBX14 directly regulates EIN3 transcription. Furthermore, we revealed the upstream transcriptional cascade of BBX14. By yeast one-hybrid screen and ChIP, we found that MYB44, a stress-responsive MYB transcription factor, directly binds to the promoter of BBX14 and activates its expression. In addition, Phytochrome Interacting Factor 4 (PIF4) binds to the promoter of BBX14 to repress BBX14 transcription. Thus, BBX14 functions as a negative regulator of N starvation-induced senescence through EIN3 and is directly regulated by PIF4 and MYB44.

Variability and limits of nitrogen and phosphorus resorption during foliar senescence

Foliar nutrient resorption (NuR) plays a key role in ecosystem functioning and plant nutrient economy. Most of this recycling occurs during the senescence of leaves and is actively addressed by cells. Here, we discuss the importance of cell biochemistry, physiology, and subcellular anatomy to condition the outcome of NuR at the cellular level and to explain the existence of limits to NuR. Nutrients are transferred from the leaf in simple metabolites that can be loaded into the phloem. Proteolysis is the main mechanism for mobilization of N, whereas P mobilization requires the involvement of different catabolic pathways, making the dynamics of P in leaves more variable than those of N before, during, and after foliar senescence. The biochemistry and fate of organelles during senescence impose constraints that limit NuR. The efficiency of NuR decreases, especially in evergreen species, as soil fertility increases, which is attributed to the relative costs of nutrient acquisition from soil decreasing with increasing soil nutrient availability, while the energetic costs of NuR from senescing leaves remain constant. NuR is genetically determined, with substantial interspecific variability, and is environmentally regulated in space and time, with nutrient availability being a key driver of intraspecific variability in NuR.

Interplay between ethylene and nitrogen nutrition: How ethylene orchestrates nitrogen responses in plants

The stress hormone ethylene plays a key role in plant adaptation to adverse environmental conditions. Nitrogen (N) is the most quantitatively required mineral nutrient for plants, and its availability is a major determinant for crop production. Changes in N availability or N forms can alter ethylene biosynthesis and/or signaling. Ethylene serves as an important cellular signal to mediate root system architecture adaptation, N uptake and translocation, ammonium toxicity, anthocyanin accumulation, and premature senescence, thereby adapting plant growth and development to external N status. Here, we review the ethylene-mediated morphological and physiological responses and highlight how ethylene transduces the N signals to the adaptive responses. We specifically discuss the N-ethylene relations in rice, an important cereal crop in which ethylene is essential for its hypoxia survival.

Effect of nitrogen supply on stay-green sorghum in differing post-flowering water regimes

The expression of stay-green (SG) characteristic in sorghum under water stress was related to N supply. SG genotype performed better than a non-stay-green (NSG) genotype at medium and high N levels. The differences in physiological parameters between SG and NSG genotypes were not significant at low N level and severe water stress. Grain sorghum [Sorghum bicolor (L.) Moench] with stay-green (SG) trait has the potential to produce more biomass and use soil water and nitrogen (N) more efficiently under post-flowering water stress. Previous studies were mostly conducted without N deficiency and more information is needed for interactions among soil N availability, SG genotype, and post-flowering water stress. In this study, the differences in leaf growth and senescence, shoot and root biomass, evapotranspiration (ET), water use efficiency (WUE), leaf photosynthetic responses, and nitrogen use efficiency (NUE) between a SG genotype (BTx642) and a non-stay-green (NSG) genotype (Tx7000) were examined. The two genotypes were grown at three N levels (Low, LN; Medium, MN; High, HN) and under three post-flowering water regimes (No water deficit, ND; Moderate water deficit, MD; Severe water deficit, SD). The genotypic difference was generally significant while it frequently interacted with N levels and water regimes. At medium and high N levels, SG genotype consistently had greater green leaf area, slower senescence rate, more shoot biomass and root biomass, and greater WUE and NUE than the NSG genotype under post-flowering drought. However, differences in several variables (e.g., leaf senescence, ET, WUE and NUE) between genotypes were not significant under SD at LN. At HN and MN, photosynthetic function of SG genotype was better maintained under drought. At LN, SG genotype maintained greater green leaf area but had lower photosynthetic activity than the NSG genotype. Nonetheless, adequate N supply is important for SG genotype under drought and greater root biomass may contribute to greater NUE in SG genotype.

Seed Yield and Nitrogen Efficiency in Oilseed Rape After Ammonium Nitrate or Urea Fertilization

In agricultural plant production, nitrate, ammonium, and urea are the major fertilized nitrogen forms, which differ in root uptake and downstream signaling processes in plants. Nitrate is known to stimulate cytokinin synthesis in roots, while for urea no hormonal effect has been described yet. Elevated cytokinin levels can delay plant senescence favoring prolonged nitrogen uptake. As the cultivation of winter oilseed rape provokes high nitrogen-balance surpluses, we tested the hypotheses whether nitrogen use efficiency increases under ammonium nitrate- relative to urea-based nutrition and whether this is subject to genotypic variation. In a 2-year field study, 15 oilseed rape lines were fertilized either with ammonium nitrate or with urease inhibitor-stabilized urea and analyzed for seed yield and nitrogen-related yield parameters. Despite a significant environmental impact on the performance of the individual lines, which did not allow revealing consistent impact of the genotype, ammonium nitrate-based nutrition tended to increase seed yield in average over all lines. To resolve whether the fertilizer N forms act on grain yield via phytohormones, we collected xylem exudates at three developmental stages and determined the translocation rates of cytokinins and N forms. Relative to urea, ammonium nitrate-based nutrition enhanced the translocation of nitrate or total nitrogen together with cytokinins, whereas in the urea treatment translocation rates were lower as long as urea remained stable in the soil solution. At later developmental stages, i.e., when urea became hydrolyzed, nitrogen and cytokinin translocation increased. In consequence, urea tended to increase nitrogen partitioning in the shoot toward generative organs. However, differences in overall nitrogen accumulation in shoots were not present at the end of the vegetation period, and neither nitrogen uptake nor utilization efficiency was consistently different between the two applied nitrogen forms.

Novel Aspects of Nitrate Regulation in Arabidopsis

Nitrogen (N) is one of the most essential macronutrients for plant growth and development. Nitrate (NO₃ ^-), the major form of N that plants uptake from the soil, acts as an important signaling molecule in addition to its nutritional function. Over the past decade, significant progress has been made in identifying new components involved in NO₃ ^- regulation and starting to unravel the NO₃ ^- regulatory network. Great reviews have been made recently by scientists on the key regulators in NO₃ ^- signaling, NO₃ ^- effects on plant development, and its crosstalk with phosphorus (P), potassium (K), hormones, and calcium signaling. However, several novel aspects of NO₃ ^- regulation have not been previously reviewed in detail. Here, we mainly focused on the recent advances of post-transcriptional regulation and non-coding RNA (ncRNAs) in NO₃ ^- signaling, and NO₃ ^- regulation on leaf senescence and the circadian clock. It will help us to extend the general picture of NO₃ ^- regulation and provide a basis for further exploration of NO₃ ^- regulatory network.

Characterization of S40-like proteins and their roles in response to environmental cues and leaf senescence in rice

BACKGROUND

Senescence affects the quality and yield of plants by regulating different traits of plants. A few members of S40 gene family, the barley HvS40 and the Arabidopsis AtS40-3, have been shown to play a role in leaf senescence in Barley and Arabidopsis. Although we previously reported that S40 family exist in most of plants, up to now, no more function of S40 members in plant has been demonstrated. The aim of this study was to provide the senescence related information of S40 gene family in rice as rice is a major crop that feeds about half of the human population in the world.

RESULTS

A total of 16 OsS40 genes were identified from the genome database of Oryza sativa L. japonica by bioinformatics analysis. Phylogenetic analysis reveals that the 16 OsS40 proteins are classified into five groups, and 4 of the 16 members belong to group I to which also the HvS40 and AtS40-3 is assigned. S40 genes of rice show high structural similarities, as 13 out of the 16 genes have no intron and the other 3 genes have only 1 or 2 introns. The expression patterns of OsS40 genes were analyzed during natural as well as stress-induced leaf senescence in correspondence with senescence marker genes. We found that 6 of them displayed differential but clearly up-regulated transcript profiles under diverse situations of senescence, including darkness, nitrogen deficiency, hormone treatments as well as pathogen infection. Furthermore, three OsS40 proteins were identified as nuclear located proteins by transient protoplast transformation assay.

CONCLUSIONS

Taking all findings together, we concluded that OsS40-1, OsS40-2, OsS40-12 and OsS40-14 genes have potential regulatory function of crosstalk among abiotic, biotic and developmental senescence in rice. Our results provide valuable baseline for functional validation studies of the rice S40 genes in rice leaf senescence.

Impact of Alternatively Polyadenylated Isoforms of ETHYLENE RESPONSE FACTOR4 with Activator and Repressor Function on Senescence in Arabidopsis thaliana L

Leaf senescence is highly regulated by transcriptional reprogramming, implying an important role for transcriptional regulators. ETHYLENE RESPONSE FACTOR4 (ERF4) was shown to be involved in senescence regulation and to exist in two different isoforms due to alternative polyadenylation of its pre-mRNA. One of these isoforms, ERF4-R, contains an ERF-associated amphiphilic repression (EAR) motif and acts as repressor, whereas the other form, ERF4-A, is lacking this motif and acts as activator. Here, we analyzed the impact of these isoforms on senescence. Both isoforms were able to complement the delayed senescence phenotype of the erf4 mutant with a tendency of ERF4-A for a slightly better complementation. However, overexpression led to accelerated senescence of 35S:ERF4-R plants but not of 35S:ERF4-A plants. We identified CATALASE3 (CAT3) as direct target gene of ERF4 in a yeast-one-hybrid screen. Both isoforms directly bind to the CAT3 promoter but have antagonistic effects on gene expression. The ratio of ERF4-A to ERF4-R mRNA changed during development, leading to a complex age-dependent regulation of CAT3 activity. The RNA-binding protein FPA shifted the R/A-ratio and fpa mutants are pointing towards a role of alternative polyadenylation regulators in senescence.

Changes in nitrogen availability lead to a reprogramming of pyruvate metabolism

BACKGROUND

Low availability of nitrogen (N) severely affects plant growth at different levels, which can be reverted by the resupply of N. To unravel the critical steps in primary metabolism underlying the growth adjustment in response to changes in N availability, transcriptomic and comprehensive metabolite analyses were performed in barley using primary leaves at early and later stages of N deprivation, and after N resupply to N-deficient plants.

RESULT

N deficiency in leaves caused differential regulation of 1947 genes, mostly belonging to the functional classes photosynthesis, cell wall degradation, lipid degradation, amino acid degradation, transcription factors, phytohormone metabolism and receptor-like kinases. Interestingly, 62% of the genes responding to low N were regulated in the opposite direction after two days of N resupply. Reprogramming of gene transcription was linked to metabolic rearrangements and affected the metabolism of amino acids and sugars. The levels of major amino acids, including Glu, Asp, Ser, Gln, Gly, Thr, Ala, and Val, decreased during primary leaf age and, more pronounced, during low N-induced senescence, which was efficiently reverted after resupply of N. A significant decrease was observed for pyruvate and metabolites involved in the TCA cycle under low N, and this was reverted to initial levels after 5 days of N resupply. Correspondingly, transcript levels of genes coding for pyruvate kinase, pyruvate dehydrogenase, and pyruvate orthophosphate dikinase followed the same trend as related metabolites.

CONCLUSION

Our results show that upon N limitation a specific pathway for remobilization at the link between glycolysis and TCA cycle in barley is established that is at least partly regulated by a strict reprogramming of the gene coding for pyruvate orthophosphate dikinase. Further analysis of this pathway, its regulatory levels and biochemical changing of pyruvate metabolism enzymes in response to N availability is needed to determine the link between N status and primary metabolism.

Morphophysiological mechanism of rice yield increase in response to optimized nitrogen management

The yield-increasing mechanism of an optimized nitrogen fertilizer application (OFA) in rice was reported in this work through a three-year test. Results showed that the number of branches and spikelets increased, panicle length, the diameter and vascular bundle number of panicle-neck internode improved with OFA. Under the condition of OFA, high effective leaf areas, especially for the flag and the second upper leaf areas, increased, the net photosynthetic rate of the upper three leaves promoted, so the photosynthetic productivity went up by a large margin; moreover, the content of soluble protein and chlorophyll of leaf also increased, and the content of soluble sugar and malondialdehyde (MDA) decreased, as a result in slowing down the senescence speed in leaves, and increasing the photosynthetic time. Gene expression level, including MOC1, LAX1, SP1, GS1;1, were up-regulated obviously in different panicle initiation stage under OFA condition, which conduced to the increase in the secondary branches and spikelets. So we concluded that the changes in organ formation and panicle structure, together with the responses in physiological and molecular made the photosynthetic area, rate and time all increased with OFA, which provided the matter basis for the big panicle development, consequently, got a higher yield.

ES7, encoding a ferredoxin-dependent glutamate synthase, functions in nitrogen metabolism and impacts leaf senescence in rice

Glutamate synthase (GOGAT) is a key enzyme for nitrogen metabolism and ammonium assimilation in plants. In this study, an early senescence 7 (es7) mutant was identified and characterized. The leaves of the es7 mutant begin to senesce at the tillering stage about 60day after sowing, and become increasingly senescent as the plants develop at the heading stage. When es7 plants are grown under photorespiration-suppressed conditions (high CO₂), the senescence phenotype and chlorophyll content are rescued. qRT-PCR analysis showed that senescence- associated genes were up-regulated significantly in es7. A map-based cloning strategy was used to identify ES7, which encodes a ferredoxin-dependent glutamate synthase (Fd-GOGAT). ES7 was expressed constitutively, and the ES7 protein was localized in chloroplast. qRT-PCR analysis indicated that several genes related to nitrogen metabolism were differentially expressed in es7. Further, we also demonstrated that chlorophyll synthesis-associated genes were significantly down-regulated in es7. In addition, when seedlings are grown under increasing nitrogen concentrations (NH₄NO₃) for 15days, the contents of chlorophyll a, chlorophyll b and total chlorophyll were significantly lower in es7. Our results demonstrated that ES7 is involved in nitrogen metabolism, effects chlorophyll synthesis, and may also associated with photorespiration, impacting leaf senescence in rice.

Basipetal auxin versus acropetal cytokinin transport, and their interaction with NO3 fertilisation in cotyledon senescence and sink:source relationships in cucumber (Cucumis sativus L.)

The paramount role of cytokinins (CKs) in initiation, as well as prevention, of senescence is well established. In recent years, experimental methods have become available to raise and lower the CK concentration and experimentally manipulate senescence. Decapitating the apical shoot and adding the synthetic auxin naphthylacetic acid to the cut stem reduced endogenous CKs to low levels. Conversely, if no auxin was applied, xylem and leaf CK levels increased dramatically, indicating that basipolar auxin transport is a key determinant in the synthesis of CKs and is potentially more important than NO(3). Manipulating the concentration of applied NO(3) caused considerable variation in leaf CK levels and concomitant changes in senescence. These and other results suggest that the frequently discussed decrease in nitrogen use efficiency (NUE) may be more highly regulated by CKs than by NO(3). Analysis of the re-metabolisation and re-allocation of chlorophyll, proteins, amino acids and starch in three different cucumber cultivars indirectly showed that these metabolites were significantly affected by the concentration of CKs in the leaves. Further research in this area may allow leaf senescence and plant yield to be more efficiently regulated by manipulating CKs and/or basipolar auxin transport instead of nitrate.

Transcriptional networks in leaf senescence

Plant senescence is a natural phenomenon known for the appearance of beautiful autumn colors and the ripening of cereals in the field. Senescence is a controlled process that plants utilize to remobilize nutrients from source leaves to developing tissues. While during the past decades, molecular components underlying the onset of senescence have been intensively studied, knowledge remains scarce on the age-dependent mechanisms that control the onset of senescence. Recent advances have uncovered transcriptional networks regulating the competence to senesce. Here, gene regulatory networks acting as internal timing mechanisms for the onset of senescence are highlighted, illustrating that early and late leaf developmental phases are highly connected.

Rice Phytochrome B (OsPhyB) Negatively Regulates Dark- and Starvation-Induced Leaf Senescence

Light regulates leaf senescence and light deprivation causes large-scale transcriptional reprogramming to dismantle cellular components and remobilize nutrients to sink organs, such as seeds and storage tissue. We recently reported that in Arabidopsis (Arabidopsis thaliana), Phytochrome-Interacting Factor4 (PIF4) and PIF5 promote dark-induced senescence and natural senescence by directly activating the expression of typical senescence-associated genes (SAGs), including ORESARA1 (ORE1) and ETHYLENE INSENSITIVE3 (EIN3). In contrast, phytochrome B (PhyB) inhibits leaf senescence by repressing PIF4 and PIF5 at the post-translational level. Although we found how red light signaling represses leaf senescence in Arabidopsis, it remains unknown whether PhyB and/or PhyA are involved in leaf senescence in rice (Oryza sativa). Here we show that rice phyB knockout mutants (osphyB-1, -2, and -3) exhibited an early senescence phenotype during dark-induced senescence, but an osphyA knockout mutant (osphyA-3) senesced normally. The RT-qPCR analysis revealed that several senescence-associated genes, including OsORE1 and OsEIN3, were significantly up-regulated in osphyB-2 mutants, indicating that OsPhyB also inhibits leaf senescence, like Arabidopsis PhyB. We also found that leaf segments of osphyB-2 senesced faster even under light conditions. Supplementation with nitrogen compounds, such as KNO₃ and NH₄NO₃, rescued the early senescence phenotype of osphyB-2, indicating that starvation is one of the major signaling factors in the OsPhyB-dependent leaf senescence pathway.

Carbon/Nitrogen Imbalance Associated with Drought-Induced Leaf Senescence in Sorghum bicolor

Drought stress triggers mature leaf senescence, which supports plant survival and remobilization of nutrients; yet leaf senescence also critically decreases post-drought crop yield. Drought generally results in carbon/nitrogen imbalance, which is reflected in the increased carbon:nitrogen (C:N) ratio in mature leaves, and which has been shown to be involved in inducing leaf senescence under normal growth conditions. Yet the involvement of the carbon/nitrogen balance in regulation of drought-induced leaf senescence is unclear. To investigate the role of carbon/nitrogen balance in drought-induced senescence, sorghum seedlings were subjected to a gradual soil drought treatment. Leaf senescence symptoms and the C:N ratio, which was indicated by the ratio of non-structural carbohydrate to total N content, were monitored during drought progression. In this study, leaf senescence developed about 12 days after the start of drought treatment, as indicated by various senescence symptoms including decreasing photosynthesis, photosystem II photochemistry efficiency (Fv/Fm) and chlorophyll content, and by the differential expression of senescence marker genes. The C:N ratio was significantly enhanced 10 to 12 days into drought treatment. Leaf senescence occurred in the older (lower) leaves, which had higher C:N ratios, but not in the younger (upper) leaves, which had lower C:N ratios. In addition, a detached leaf assay was conducted to investigate the effect of carbon/nitrogen availability on drought-induced senescence. Exogenous application of excess sugar combined with limited nitrogen promoted drought-induced leaf senescence. Thus our results suggest that the carbon/nitrogen balance may be involved in the regulation of drought-induced leaf senescence.

Senescence Meets Dedifferentiation

Senescence represents the final stage of leaf development but is often induced prematurely following exposure to biotic and abiotic stresses. Leaf senescence is manifested by color change from green to yellow (due to chlorophyll degradation) or to red (due to de novo synthesis of anthocyanins coupled with chlorophyll degradation) and frequently culminates in programmed death of leaves. However, the breakdown of chlorophyll and macromolecules such as proteins and RNAs that occurs during leaf senescence does not necessarily represent a one-way road to death but rather a reversible process whereby senescing leaves can, under certain conditions, re-green and regain their photosynthetic capacity. This phenomenon essentially distinguishes senescence from programmed cell death, leading researchers to hypothesize that changes occurring during senescence might represent a process of trans-differentiation, that is the conversion of one cell type to another. In this review, we highlight attributes common to senescence and dedifferentiation including chromatin structure and activation of transposable elements and provide further support to the notion that senescence is not merely a deterioration process leading to death but rather a unique developmental state resembling dedifferentiation.

Reversal of senescence by N resupply to N-starved Arabidopsis thaliana: transcriptomic and metabolomic consequences

Leaf senescence is a developmentally controlled process, which is additionally modulated by a number of adverse environmental conditions. Nitrogen shortage is a well-known trigger of precocious senescence in many plant species including crops, generally limiting biomass and seed yield. However, leaf senescence induced by nitrogen starvation may be reversed when nitrogen is resupplied at the onset of senescence. Here, the transcriptomic, hormonal, and global metabolic rearrangements occurring during nitrogen resupply-induced reversal of senescence in Arabidopsis thaliana were analysed. The changes induced by senescence were essentially in keeping with those previously described; however, these could, by and large, be reversed. The data thus indicate that plants undergoing senescence retain the capacity to sense and respond to the availability of nitrogen nutrition. The combined data are discussed in the context of the reversibility of the senescence programme and the evolutionary benefit afforded thereby. Future prospects for understanding and manipulating this process in both Arabidopsis and crop plants are postulated.

Identification of predominant genes involved in regulation and execution of senescence-associated nitrogen remobilization in flag leaves of field grown barley

The transcriptomes of senescing flag leaves collected from barley field plots with standard or high nitrogen supply were compared to identify genes specifically associated with nitrogen remobilization during leaf senescence under agronomically relevant conditions. In flag leaves collected in field plots with high nitrogen supply, the decline in chlorophyll content was delayed. By comparing changes in gene expression for the two nitrogen levels, it was possible to discriminate genes related to nitrogen remobilization during senescence and genes involved in other processes associated with the late development of leaves under field conditions. Predominant genes that were more strongly upregulated during senescence of flag leaves from plants with standard nitrogen supply included genes encoding the transcription factor HvNAC026, serine type protease SCPL51, and the autophagy factors APG7 and ATG18F. Elevated expression of these genes in senescing leaves from plants with standard nitrogen supply indicates important roles of the corresponding proteins in nitrogen remobilization. In comparison, the genes upregulated in both flag leaf samples might have roles in general senescence processes associated with late leaf development. Among these genes were the transcription factor genes HvNAC001, HvNAC005, HvNAC013, HvWRKY12 and MYB, genes encoding the papain-like cysteine peptidases HvPAP14 and HvPAP20, as well as a subtilase gene.

Identification of differentially senescing mutants of wheat and impacts on yield, biomass and nitrogen partitioning

Increasing photosynthetic capacity by extending canopy longevity during grain filling using slow senescing stay-green genotypes is a possible means to improve yield in wheat. Ethyl methanesulfonate (EMS) mutated wheat lines (Triticum aestivum L. cv. Paragon) were screened for fast and slow canopy senescence to investigate the impact on yield and nitrogen partitioning. Stay-green and fast-senescing lines with similar anthesis dates were characterised in detail. Delayed senescence was only apparent at higher nitrogen supply with low nitrogen supply enhancing the rate of senescence in all lines. In the stay-green line 3 (SG3), on a whole plant basis, tiller and seed number increased whilst thousand grain weight (TGW) decreased; although a greater N uptake was observed in the main tiller, yield was not affected. In fast-senescing line 2 (FS2), yield decreased, principally as a result of decreased TGW. Analysis of N-partitioning in the main stem indicated that although the slow-senescing line had lower biomass and consequently less nitrogen in all plant parts, the proportion of biomass and nitrogen in the flag leaf was greater at anthesis compared to the other lines; this contributed to the grain N and yield of the slow-senescing line at maturity in both the main tiller and in the whole plant. A field trial confirmed senescence patterns of the two lines, and the negative impact on yield for FS2 and a positive impact for SG3 at low N only. The lack of increased yield in the slow-senescing line was likely due to decreased biomass and additionally a possible sink limitation.

Regulation of photosynthesis and transcription factor expression by leaf shading and re-illumination in Arabidopsis thaliana leaves

Leaf senescence of annual plants is a genetically programmed developmental phase. The onset of leaf senescence is however not exclusively determined by tissue age but is modulated by various environmental factors. Shading of individual attached leaves evokes dark-induced senescence. The initiation and progression of dark-induced senescence depend on the plant and the age of the affected leaf, however. In several plant species dark-induced senescence is fully reversible upon re-illumination and the leaves can regreen, but the regreening ability depends on the duration of dark incubation. We studied the ability of Arabidopsis thaliana leaves to regreen after dark-incubation with the aim to identify transcription factors (TFs) that are involved in the regulation of early dark-induced senescence and regreening. Two days shading of individual attached leaves triggers the transition into a pre-senescence state from which the leaves can largely recover. Longer periods of darkness result in irreversible senescence. Large scale qRT-PCR analysis of 1872 TF genes revealed that 649 of them are regulated in leaves during normal development, upon shading or re-illumination. Leaf shading triggered upregulation of 150 TF genes, some of which are involved in controlling senescence. Of those, 39 TF genes were upregulated after two days in the dark and regained pre-shading expression level after two days of re-illumination. Furthermore, a larger number of 422 TF genes were down regulated upon shading. In TF gene clusters with different expression patterns certain TF families are over-represented.

General regulatory patterns of plant mineral nutrient depletion as revealed by serat quadruple mutants disturbed in cysteine synthesis

Sulfate is an essential macronutrient for plants. Plants have developed strategies to cope with sulfate deficiency, and other nutrient ion limitations. However, the regulation of these adaptive responses and the coordinating signals that underlie them are still poorly characterized. O-acetylserine (OAS) is a marker metabolite of sulfate starvation and has been speculated to have a signaling function. OAS is synthesized by the enzyme serine acetyltransferase (SERAT), which is encoded by five distinct genes in Arabidopsis. We investigated quadruple knockout mutants of SERAT that retained only one functional isoform. These mutants displayed symptoms of sulfate starvation. Furthermore, some of them displayed phenotypes typical of prolonged sulfate starvation, in particular, developmental programs associated with senescence or stress responses. Thus, we compared metabolite and transcriptome data from these mutants with N-, P-, K-, and S-depleted plants. This revealed many similarities with general nutrient-depletion-induced senescence (NuDIS), indicating the recruitment of existing regulatory programs for nutrient-starvation responses. Several candidate genes that could be involved in these processes were identified, including transcription factors and other regulatory proteins, as well as the functional categories of their target genes. These results outline components of the regulatory network controlling plant development under sulfate stress, forming a basis for further investigations to elucidate the complete network. In turn, this will advance our broader understanding of plant responses to a range of other nutrient stresses.

Isolation and characterization of a GS2 gene in melon (Cucumis melo L.) and its expression patterns under the fertilization of different forms of N

We isolated a novel glutamine synthetase (GS, EC 6.3.1.2) gene M-GS2 (accession: AY773090) by the RACE approach from melon. The full-length cDNA of M-GS2 is 1807 bp and contains a 1296 bp open reading frame (ORF) encoding 432 amino acids. The deduced protein contains conserved structural domains among plant GS2 proteins and shares extensive sequence homology with GS2 enzymes from other higher plants. M-GS2 expresses with specificity in leaf, and identification of a chloroplast transit peptide (cTP) in M-GS2 suggests that it localizes to the chloroplast. As shown by real-time quantitative PCR, distinct forms of nitrogen (N) found in fertilizers transcriptionally regulated M-GS2 differently. Ammonium and nitrate feeding only significantly regulated M-GS2 transcripts in leaf; starving (0.75 mM) or moderate (3.75 mM) N levels dramatically increased M-GS2 transcripts for 1 day, decreasing to a constant low level after 2-3 days, while sufficient N level (7.5 mM) had a minor effect throughout 3 days compared to controls. Glutamate feeding, however, not only significantly regulated M-GS2 transcripts in leaf (decreased initially then increased to higher levels than controls), but also in root, where it was up-regulated continuously. Our results suggested that M-GS2 is the first GS gene cloned and characterized in melon and melon responds to the variations in N fertilization by differentially expressing M-GS2.

Leaf senescence and nutrient remobilisation in barley and wheat

Extensive studies have been undertaken on senescence processes in barley and wheat and their importance for the nitrogen use efficiency of these crop plants. During the senescence processes, proteins are degraded and nutrients are re-mobilised from senescing leaves to other organs, especially the developing grain. Most of the proteins degraded reside in the chloroplasts, with Rubisco constituting the most dominant protein fraction. Despite intensive studies, the proteases responsible for Rubisco degradation have not yet been identified. Evidence for degradation of stromal proteins outside of chloroplasts is summarised. Rubisco is thought to be released from chloroplasts into vesicles containing stroma material (RCB = Rubisco-containing bodies). These vesicles may then take different routes for their degradation. Transcriptome analyses on barley and wheat senescence have identified genes involved in degradative, metabolic and regulatory processes that could be used in future strategies aimed at modifying the senescence process. The breeding of crops for characters related to senescence processes, e.g. higher yields and better nutrient use efficiency, is complex. Such breeding has to cope with the dilemma that delayed senescence, which could lead to higher yields, is correlated with a decrease in nutrient use efficiency. Pinpointing regulatory genes involved in senescence might lead to tools that could effectively overcome this dilemma.