Leaf senescence induced by mild water deficit follows the same sequence of macroscopic, biochemical, and molecular events as monocarpic senescence in pea

Insights into plant salt stress signaling and tolerance

Soil salinization is an essential environmental stressor, threatening agricultural yield and ecological security worldwide. Saline soils accumulate excessive soluble salts which are detrimental to most plants by limiting plant growth and productivity. It is of great necessity for plants to efficiently deal with the adverse effects caused by salt stress for survival and successful reproduction. Multiple determinants of salt tolerance have been identified in plants, and the cellular and physiological mechanisms of plant salt response and adaption have been intensely characterized. Plants respond to salt stress signals and rapidly initiate signaling pathways to re-establish cellular homeostasis with adjusted growth and cellular metabolism. This review summarizes the advances in salt stress perception, signaling, and response in plants. A better understanding of plant salt resistance will contribute to improving crop performance under saline conditions using multiple engineering approaches. The rhizosphere microbiome-mediated plant salt tolerance as well as chemical priming for enhanced plant salt resistance are also discussed in this review.

Morphological and Physiological Mechanisms of Melatonin on Delaying Drought-Induced Leaf Senescence in Cotton

Leaf senescence reduces the photosynthetic capacity of leaves, thus significantly affecting the growth, development, and yield formation of cotton. Melatonin (MT) is a multipotent substance proven to delay leaf senescence. However, its potential mechanism in delaying leaf senescence induced by abiotic stress remains unclear. This study aimed to explore the effect of MT on delaying drought-induced leaf senescence in cotton seedlings and to clarify its morphological and physiological mechanisms. Drought stress upregulated the leaf senescence marker genes, destroyed the photosystem, and led to excessive accumulation of reactive oxygen species (ROS, e.g., H₂O₂ and O₂^-), thus accelerating leaf senescence. However, leaf senescence was significantly delayed when 100 μM MT was sprayed on the leaves of the cotton seedlings. The delay was embodied by the increased chlorophyll content, photosynthetic capacity, and antioxidant enzyme activities, as well as decreased H₂O₂, O₂^-, and abscisic acid (ABA) contents by 34.44%, 37.68%, and 29.32%, respectively. MT significantly down-regulated chlorophyll degradation-related genes and senescence marker genes (GhNAC12 and GhWRKY27/71). In addition, MT reduced the chloroplast damage caused by drought-induced leaf senescence and maintained the integrity of the chloroplast lamellae structure under drought stress. The findings of this study collectively suggest that MT can effectively enhance the antioxidant enzyme system, improve photosynthetic efficiency, reduce chlorophyll degradation and ROS accumulation, and inhibit ABA synthesis, thereby delaying drought-induced leaf senescence in cotton.

Unravelling the Role of Piriformospora indica in Combating Water Deficiency by Modulating Physiological Performance and Chlorophyll Metabolism-Related Genes in Cucumis sativus

Water stress is the most critical aspect restricting the development of agriculture in regions with scarce water resources, which requires enhancing irrigation water-saving strategies. The current work discusses the potential application of the plant-strengthening root endophyte Piriformospora indica against moderate (25% less irrigation water) and severe (50% less irrigation water) water stress in comparison to the optimum irrigation conditions of greenhouse cucumbers. P. indica improved growth, nutrient content, and photosynthesis apparatus under normal or water-stress conditions. On the other hand, moderate and severe water stress reduced yield up to 47% and 83%, respectively, in non-colonized cucumber plants, while up to 28 and 78%, respectively, in P. indica-colonized plants. In terms of water-use efficiency (WUE), P. indica improved the WUE of colonized cucumber plants grown under moderate (26 L/kg) or severe stress (73 L/kg) by supporting colonized plants in producing higher yield per unit volume of water consumed by the crop in comparison to non-colonized plants under the same level of moderate (43 L/kg) or severe (81 L/kg) water stress. Furthermore, P. indica increased the indole-3-acetic acid (IAA) content, activity levels of catalase (CAT) and peroxidase (POD) with an apparent clear reduction in the abscisic acid (ABA), ethylene, malondialdehyde (MDA), proline contents and stomatal closure compared to non-stressed plants under both water-stress levels. In addition, chlorophyll a, b, a + b contents were increased in the leaves of the colonized plants under water-stress conditions. This improvement in chlorophyll content could be correlated with a significant increment in the transcripts of chlorophyll biosynthesis genes (protochlorophyllide oxidoreductase [POR], chlorophyll a oxygenase [CAO]) and a reduction in the chlorophyll degradation genes (PPH, pheophorbide a oxygenase [PAO], and red chlorophyll catabolite reductase [RCCR]). In conclusion, P. indica has the potential to enhance the cucumber yield grown under moderate water stress rather than severe water stress by improving WUE and altering the activity levels of antioxidant enzymes and chlorophyll metabolism-related genes.

Enhancement of Nicotiana tabacum Resistance Against Dehydration-Induced Leaf Senescence via Metabolite/Phytohormone-Gene Regulatory Networks Modulated by Melatonin

Melatonin (MEL) is a pleiotropic agent with crucial functions reported in a variety of stress responses and developmental processes. Although MEL involvement in plant defense against natural leaf senescence has been widely reported, the precise regulatory mechanisms by which it delays stress-induced senescence remain unclear. In this study, we found that foliar spraying of melatonin markedly ameliorated dehydration-induced leaf senescence in Nicotiana tabacum, accompanied by attenuated oxidative damage, expression of senescence-related genes, and reduced endogenous ABA production. Metabolite profiling indicated that melatonin-treated plants accumulated higher concentrations of sugars, sugar alcohol, and organic acids, but fewer concentrations of amino acids in the leaves, than untreated plants after exposure to dehydration. Gene expression analysis revealed that the delayed senescence of stressed plants achieved by melatonin treatment might be partially ascribed to the upregulated expression of genes involved in ROS scavenging, chlorophyll biosynthesis, photosynthesis, and carbon/nitrogen balances, and downregulated expression of senescence-associated genes. Furthermore, hormone responses showed an extensively modulated expression, complemented by carotenoid biosynthesis regulation to achieve growth acceleration in melatonin-treated plants upon exposure to dehydration stress. These findings may provide more comprehensive insights into the role of melatonin in alleviating leaf senescence and enhancing dehydration resistance.

Ear photosynthesis in C3 cereals and its contribution to grain yield: methodologies, controversies, and perspectives

In C3 cereals such as wheat and barley, grain filling was traditionally explained as being sustained by assimilates from concurrent leaf photosynthesis and remobilization from the stem. In recent decades, a role for ear photosynthesis as a contributor to grain filling has emerged. This review analyzes several aspects of this topic: (i) methodological approaches for estimation of ear photosynthetic contribution to grain filling; (ii) the existence of genetic variability in the contribution of the ear, and evidence of genetic gains in the past; (iii) the controversy of the existence of C4 metabolism in the ear; (iv) the response of ear photosynthesis to water deficit; and (v) morphological and physiological traits possibly related to ear temperature and thermal balance of the ear. The main conclusions are: (i) there are a number of methodologies to quantify ear photosynthetic activity (e.g. gas exchange and chlorophyll fluorescence) and the contribution of the ear to grain filling (individual ear shading, ear emergence in shaded canopies, and isotope composition); (ii) the contribution of ear photosynthesis seems to have increased in modern wheat germplasm; (iii) the contribution of the ear to grain filling increases under resource-limitation (water deficit, defoliation, or pathogen infection); (iv) there is genetic variability in the contribution of the ear in wheat, opening up the possibility to use this trait to ameliorate grain yield; (v) current evidence supports the existence of C3 metabolism rather than C4 metabolism; (vi) the ear is a 'dehydration avoider organ' under drought; and (vii) thermal balance in the ear is a relevant issue to explore, and more research is needed to clarify the underlying morphological and physiological traits.

Nutrients and water availability constrain the seasonality of vegetation activity in a Mediterranean ecosystem

Anthropogenic nitrogen (N) deposition and resulting differences in ecosystem N and phosphorus (P) ratios are expected to impact photosynthetic capacity, that is, maximum gross primary productivity (GPP_max ). However, the interplay between N and P availability with other critical resources on seasonal dynamics of ecosystem productivity remains largely unknown. In a Mediterranean tree-grass ecosystem, we established three landscape-level (24 ha) nutrient addition treatments: N addition (NT), N and P addition (NPT), and a control site (CT). We analyzed the response of ecosystem to altered nutrient stoichiometry using eddy covariance fluxes measurements, satellite observations, and digital repeat photography. A set of metrics, including phenological transition dates (PTDs; timing of green-up and dry-down), slopes during green-up and dry-down period, and seasonal amplitude, were extracted from time series of GPP_max and used to represent the seasonality of vegetation activity. The seasonal amplitude of GPP_max was higher for NT and NPT than CT, which was attributed to changes in structure and physiology induced by fertilization. PTDs were mainly driven by rainfall and exhibited no significant differences among treatments during the green-up period. Yet, both fertilized sites senesced earlier during the dry-down period (17-19 days), which was more pronounced in the NT due to larger evapotranspiration and water usage. Fertilization also resulted in a faster increase in GPP_max during the green-up period and a sharper decline in GPP_max during the dry-down period, with less prominent decline response in NPT. Overall, we demonstrated seasonality of vegetation activity was altered after fertilization and the importance of nutrient-water interaction in such water-limited ecosystems. With the projected warming-drying trend, the positive effects of N fertilization induced by N deposition on GPP_max may be counteracted by an earlier and faster dry-down in particular in areas where the N:P ratio increases, with potential impact on the carbon cycle of water-limited ecosystems.

Epigenetic Landmarks of Leaf Senescence and Crop Improvement

This review synthesizes knowledge on epigenetic regulation of leaf senescence and discusses the possibility of using this knowledge to improve crop quality. This control level is implemented by different but interacting epigenetic mechanisms, including DNA methylation, covalent histone modifications, and non-covalent chromatin remodeling. The genetic and epigenetic changes may act alone or together and regulate the gene expression, which may result in heritable (stress memory) changes and may lead to crop survival. In the review, the question also arises whether the mitotically stable epigenetic information can be used for crop improvement. The barley crop model for early and late events of dark-induced leaf senescence (DILS), where the point of no return was defined, revealed differences in DNA and RNA modifications active in DILS compared to developmental leaf senescence. This suggests the possibility of a yet-to-be-discovered epigenetic-based switch between cell survival and cell death. Conclusions from the analyzed research contributed to the hypothesis that chromatin-remodeling mechanisms play a role in the control of induced leaf senescence. Understanding this mechanism in crops might provide a tool for further exploitation toward sustainable agriculture: so-called epibreeding.

Micromanagement of Developmental and Stress-Induced Senescence: The Emerging Role of MicroRNAs

MicroRNAs are short (19⁻24-nucleotide-long), non-coding RNA molecules. They downregulate gene expression by triggering the cleavage or translational inhibition of complementary mRNAs. Senescence is a stage of development following growth completion and is dependent on the expression of specific genes. MicroRNAs control the gene expression responsible for plant competence to answer senescence signals. Therefore, they coordinate the juvenile-to-adult phase transition of the whole plant, the growth and senescence phase of each leaf, age-related cellular structure changes during vessel formation, and remobilization of resources occurring during senescence. MicroRNAs are also engaged in the ripening and postharvest senescence of agronomically important fruits. Moreover, the hormonal regulation of senescence requires microRNA contribution. Environmental cues, such as darkness or drought, induce senescence-like processes in which microRNAs also play regulatory roles. In this review, we discuss recent findings concerning the role of microRNAs in the senescence of various plant species.

Reproductive sink enhanced drought induced senescence in wheat fertile line is associated with loss of antioxidant competence compared to its CMS line

Reproductive sinks regulate monocarpic senescence in wheat as desinking delayed flag leaf senescence under irrigated condition. In this study, wheat cv. HW 2041 and its isonuclear male sterile line (CMS) were subjected to post-anthesis water deficit stress to understand the association between sink strength, senescence and drought response in relation to oxidative stress and antioxidant defense at cellular and sub-cellular level. CMS plants maintained better water relations and exhibited delayed onset and progression of flag leaf senescence in terms of green leaf area, chlorophyll and protein content than fertile plants under water deficit stress (WDS). Delayed senescence in CMS plants under water deficit stress was associated with less reactive oxygen species generation, lower damage to membranes and better antioxidant defense both in terms of antioxidant enzyme activities and metabolite content compared to fertile plants. Expression of some senescence associated genes (SAGs) such as WRKY transcription factor (WRKY53), glutamine synthetase1 (GS1), wheat cysteine protease (WCP2) and wheat serine protease (WSP) was lower while catalse 2 (CAT2) transcript levels were higher in the CMS plants compared to HW2041 during senescence under water deficit stress. Antioxidant defense in chloroplasts was better in CMS line under water deficit stress compared to HW2041. This is the first report showing that reproductive sink enhanced drought induced senescence in flag leaf of wheat fertile line is associated with higher oxidative stress and damage and loss of antioxidant competence compared to its sterile line under water deficit stress. Higher expression of some SAGs and decline in superoxide dismutase and ascorbate peroxidase activity in the chloroplasts also contributed to the accelerated senescence in fertile line compared to its CMS line under WDS.

Elucidating the molecular mechanisms mediating plant salt-stress responses

Contents Summary 523 I. Introduction 523 II. Sensing salt stress 524 III. Ion homeostasis regulation 524 IV. Metabolite and cell activity responses to salt stress 527 V. Conclusions and perspectives 532 Acknowledgements 533 References 533 SUMMARY: Excess soluble salts in soil (saline soils) are harmful to most plants. Salt imposes osmotic, ionic, and secondary stresses on plants. Over the past two decades, many determinants of salt tolerance and their regulatory mechanisms have been identified and characterized using molecular genetics and genomics approaches. This review describes recent progress in deciphering the mechanisms controlling ion homeostasis, cell activity responses, and epigenetic regulation in plants under salt stress. Finally, we highlight research areas that require further research to reveal new determinants of salt tolerance in plants.

Integrating transcriptomic and metabolomic analysis to understand natural leaf senescence in sunflower

Leaf senescence is a complex process, which has dramatic consequences on crop yield. In sunflower, gap between potential and actual yields reveals the economic impact of senescence. Indeed, sunflower plants are incapable of maintaining their green leaf area over sustained periods. This study characterizes the leaf senescence process in sunflower through a systems biology approach integrating transcriptomic and metabolomic analyses: plants being grown under both glasshouse and field conditions. Our results revealed a correspondence between profile changes detected at the molecular, biochemical and physiological level throughout the progression of leaf senescence measured at different plant developmental stages. Early metabolic changes were detected prior to anthesis and before the onset of the first senescence symptoms, with more pronounced changes observed when physiological and molecular variables were assessed under field conditions. During leaf development, photosynthetic activity and cell growth processes decreased, whereas sucrose, fatty acid, nucleotide and amino acid metabolisms increased. Pathways related to nutrient recycling processes were also up-regulated. Members of the NAC, AP2-EREBP, HB, bZIP and MYB transcription factor families showed high expression levels, and their expression level was highly correlated, suggesting their involvement in sunflower senescence. The results of this study thus contribute to the elucidation of the molecular mechanisms involved in the onset and progression of leaf senescence in sunflower leaves as well as to the identification of candidate genes involved in this process.

Combined effects of girdling and leaf removal on fluorescence characteristic of Alhagi sparsifolia leaf senescence

Plant senescence is largely influenced by carbohydrate content. In order to investigate the impact of carbohydrate content on leaf senescence and photosystem II (PSII) during the senescence process, phloem girdling (PG), leaf removal (LR) and a combination of phloem girdling and leaf removal (GR) were performed on Alhagi sparsifolia (Fabaceae) at the end of the growing season. The results showed that during senescence, leaf soluble sugar content, starch content, the energy absorbed by the unit reaction centre (ABS/RC) increased; whereas, leaf photosynthetic rate, photosynthetic pigment content, maximum photochemical efficiency (φPo ) and energy used by the acceptor site in electron transfer (ETo/RC) decreased. The degree of change was PG > GR > CK (control) > LR. The results of the present work implied that phloem girdling (PG) significantly accelerated leaf senescence, and that single leaf removal (LR) slightly delayed leaf senescence; although leaf removal significantly delayed the senescence process on the girdled leaf (GR). Natural or delayed senescence only slightly inhibited the acceptor site of PSII and did not damage the donor site of PSII. On the other hand, induced senescence not only damaged the donor site of PSII (e.g. oxygen-evolving complex), but also significantly inhibited the acceptor site of PSII. In addition, leaf senescence led to an increase in the energy absorbed by the unit reaction centre (ABS/RC), which subsequently resulted in increasing excitation pressure in the reaction centre (DIo/RC), as well as additional saved Car for absorbing residual light energy and quenching reactive oxygen species during senescence.

Salt stress and senescence: identification of cross-talk regulatory components

Leaf senescence is an active process with a pivotal impact on plant productivity. It results from extensive signalling cross-talk coordinating environmental factors with intrinsic age-related mechanisms. Although many studies have shown that leaf senescence is affected by a range of external parameters, knowledge about the regulatory systems that govern the interplay between developmental programmes and environmental stress is still vague. Salinity is one of the most important environmental stresses that promote leaf senescence and thus affect crop yield. Improving salt tolerance by avoiding or delaying senescence under stress will therefore play an important role in maintaining high agricultural productivity. Experimental evidence suggests that hydrogen peroxide (H2O2) functions as a common signalling molecule in both developmental and salt-induced leaf senescence. In this study, microarray-based gene expression profiling on Arabidopsis thaliana plants subjected to long-term salinity stress to induce leaf senescence was performed, together with co-expression network analysis for H2O2-responsive genes that are mutually up-regulated by salt induced- and developmental leaf senescence. Promoter analysis of tightly co-expressed genes led to the identification of seven cis-regulatory motifs, three of which were known previously, namely CACGTGT and AAGTCAA, which are associated with reactive oxygen species (ROS)-responsive genes, and CCGCGT, described as a stress-responsive regulatory motif, while the others, namely ACGCGGT, AGCMGNC, GMCACGT, and TCSTYGACG were not characterized previously. These motifs are proposed to be novel elements involved in the H2O2-mediated control of gene expression during salinity stress-triggered and developmental senescence, acting through upstream transcription factors that bind to these sites.

Salicylic acid determines differential senescence produced by two Turnip mosaic virus strains involving reactive oxygen species and early transcriptomic changes

Losses produced by virus diseases depend mostly on symptom severity. Turnip mosaic virus (TuMV) is one of the most damaging and widespread potyvirus infecting members of the family Brassicaceae, including Arabidopsis thaliana. We used JPN1 and UK1 TuMV strains to characterize viral infections regarding symptom development, senescence progression, antioxidant response, reactive oxygen species (ROS) accumulation, and transcriptional profiling. Both isolates, despite accumulating similar viral titers, induced different symptomatology and strong differences in oxidative status. Early differences in several senescence-associated genes linked to the ORE1 and ORS1 regulatory networks as well as persistent divergence in key ROS production and scavenging systems of the plant were detected. However, at a later stage, both strains induced nutrient competition, indicating that senescence rates are influenced by different mechanisms upon viral infections. Analyses of ORE1 and ORS1 levels in infected Brassica juncea plants showed a similar pattern, suggesting a conserved differential response to both strains in Brassicaceae spp. Transcriptional analysis of the ORE1 and ORS1 regulons showed similarities between salicylic acid (SA) response and the early induction triggered by UK1, the most severe strain. By means of SA-defective NahG transgenic plants, we found that differential senescence progression and ROS accumulation between strains rely on an intact SA pathway.

Metabolic indicators of drought stress tolerance in wheat: glutamine synthetase isoenzymes and Rubisco

Drought stress has a considerable impact on the ecosystem and agriculture. Continuous water deficit induces early leaf senescence in plants. During this process, chloroplasts are degraded and photosynthesis drastically drops. The objective of this investigation was to look into the regulation of nitrogen and carbon metabolism during water deficit. Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase; EC 4.1.1.39) and the total protein contents inform us of the sink-source relation in plants. Glutamine synthetase (GS, EC 6.3.1.2) isoenzymes are good markers of plastid status (GS2) and the nitrogen metabolism (GS1). Tolerant and sensitive wheat (Triticum aestivum L.) genotypes were tested, which are widely used in agriculture. The amount of protein, Rubisco and GS isoforms in leaves were measured during the grain filling period, as indicative traits that ultimately determine the onset and stage of senescence. The symptoms of senescence first appeared on the oldest and finally on the youngest leaves. Drought stress disrupted the sequentiality of senescence in the sensitive varieties. An untimely senescence appeared in flag leaves, earlier than in the older leaves. Total protein and Rubisco contents decreased and the GS2 isoenzyme declined considerably in the youngest leaves. In the tolerant varieties, however, these physiological parameters did not change under drought, only the sequential senescence of leaf levels accelerated in some cases compared to the control, well-watered plants. Our results revealed that GS is a good indicator of drought stress, which can be applied for the characterization of wheat cultivars in terms of drought stress tolerance.

Long-term exogenous application of melatonin delays drought-induced leaf senescence in apple

To examine the potential roles of melatonin in drought tolerance, we tested the effects of its long-term exogenous application on 'Hanfu' apple (Malus domestica Borkh.). When 100 μm melatonin was added to soils under drought conditions, the resultant oxidative stress was eased and leaf senescence was delayed. This molecule significantly reduced chlorophyll degradation and suppressed the up-regulation of senescence-associated gene 12 (SAG12) and pheophorbide a oxygenase (PAO). Such treatment also alleviated the inhibition of photosynthesis brought on by drought stress. We also investigated quenching and the efficiency of Photosystem II (PSII) photochemistry under dark and light conditions and found that melatonin helped to maintain better function of PSII under drought. The addition of melatonin also controlled the burst of hydrogen peroxide, possibly through direct scavenging and by enhancing the activities of antioxidative enzymes and the capacity of the ascorbate-glutathione cycle. Thus, understanding this effect of melatonin on drought tolerance introduces new possibilities to use this compound for agricultural purposes.

Plant response to environmental conditions: assessing potential production, water demand, and negative effects of water deficit

This paper reviews methods for analyzing plant performance and its genetic variability under a range of environmental conditions. Biomass accumulation is linked every day to available light in the photosynthetically active radiation (PAR) domain, multiplied by the proportion of light intercepted by plants and by the radiation use efficiency. Total biomass is cumulated over the duration of the considered phase (e.g., plant cycle or vegetative phase). These durations are essentially constant for a given genotype provided that time is corrected for temperature (thermal time). Several ways of expressing thermal time are reviewed. Two alternative equations are presented, based either on the effect of transpiration, or on yield components. Their comparative interests and drawbacks are discussed. The genetic variability of each term of considered equations affects yield under water deficit, via mechanisms at different scales of plant organization and time. The effect of any physiological mechanism on yield of stressed plants acts via one of these terms, although the link is not always straightforward. Finally, I propose practical ways to compare the productivity of genotypes in field environments, and a "minimum dataset" of environmental data and traits that should be recorded for that.

Preservation of integrity and activity of Haberlea rhodopensis photosynthetic apparatus during prolonged light deprivation

The effect of prolonged light deprivation on ultrastructure, pigment composition and functions of photosynthetic apparatus of the resurrection plant Haberlea rhodopensis Friv. (Gesneriaceae) was studied. For this purpose, intact plants were kept in darkness for up to 6 months. Haberlea rhodopensis demonstrated extraordinary ability to preserve its photosynthetic machinery intact despite complete absence of light. During the first 4 weeks of light deprivation, we observed preservation of pigment content, chloroplast ultrastructure and a decrease in the rate of CO(2) assimilation. The signs of dark-induced senescence were observed only after the fourth week. This phase was characterized by decrease of pigment content, partial disintegration of chloroplast ultrastructure and by the development of photosystem II down regulation that includes the increases in non-photochemical fluorescence quenching, qN. In comparison with other plants like common bean and Arabidopsis, the processes of dark-induced senescence were very slow and the plants still can recover even after 6 months of light deprivation. We think these findings can open new opportunities for studying not only dark-induced senescence but also to investigate mechanisms determining tolerance to multiple stresses affecting integrity of photosynthetic apparatus.

WRKY54 and WRKY70 co-operate as negative regulators of leaf senescence in Arabidopsis thaliana

The plant-specific WRKY transcription factor (TF) family with 74 members in Arabidopsis thaliana appears to be involved in the regulation of various physiological processes including plant defence and senescence. WRKY53 and WRKY70 were previously implicated as positive and negative regulators of senescence, respectively. Here the putative function of other WRKY group III proteins in Arabidopsis leaf senescence has been explored and the results suggest the involvement of two additional WRKY TFs, WRKY 54 and WRKY30, in this process. The structurally related WRKY54 and WRKY70 exhibit a similar expression pattern during leaf development and appear to have co-operative and partly redundant functions in senescence, as revealed by single and double mutant studies. These two negative senescence regulators and the positive regulator WRKY53 were shown by yeast two-hydrid analysis to interact independently with WRKY30. WRKY30 was expressed during developmental leaf senescence and consequently it is hypothesized that the corresponding protein could participate in a senescence regulatory network with the other WRKYs. Expression in wild-type and salicylic acid-deficient mutants suggests a common but not exclusive role for SA in induction of WRKY30, 53, 54, and 70 during senescence. WRKY30 and WRKY53 but not WRKY54 and WRKY70 are also responsive to additional signals such as reactive oxygen species. The results suggest that WRKY53, WRKY54, and WRKY70 may participate in a regulatory network that integrates internal and environmental cues to modulate the onset and the progression of leaf senescence, possibly through an interaction with WRKY30.

Convergence and divergence in gene expression profiles induced by leaf senescence and 27 senescence-promoting hormonal, pathological and environmental stress treatments

In addition to age and developmental progress, leaf senescence and senescence-associated genes (SAGs) can be induced by other factors such as plant hormones, pathogen infection and environmental stresses. The relationship is not clear, however, between these induced senescence processes and developmental leaf senescence, and to what extent these senescence-promoting signals mimic age and developmental senescence in terms of gene expression profiles. By analysing microarray expression data from 27 different treatments (that are known to promote senescence) and comparing them with that from developmental leaf senescence, we were able to show that at early stages of treatments, different hormones and stresses showed limited similarity in the induction of gene expression to that of developmental leaf senescence. Once the senescence process is initiated, as evidenced by visible yellowing, generally after a prolonged period of treatments, a great proportion of SAGs of developmental leaf senescence are shared by gene expression profiles in response to different treatments. This indicates that although different signals that lead to initiation of senescence may do so through distinct signal transduction pathways, senescence processes induced either developmentally or by different senescence-promoting treatments may share common execution events.

Rapid and sensitive hormonal profiling of complex plant samples by liquid chromatography coupled to electrospray ionization tandem mass spectrometry

BACKGROUND

Plant hormones play a pivotal role in several physiological processes during a plant's life cycle, from germination to senescence, and the determination of endogenous concentrations of hormones is essential to elucidate the role of a particular hormone in any physiological process. Availability of a sensitive and rapid method to quantify multiple classes of hormones simultaneously will greatly facilitate the investigation of signaling networks in controlling specific developmental pathways and physiological responses. Due to the presence of hormones at very low concentrations in plant tissues (10-9 M to 10-6 M) and their different chemistries, the development of a high-throughput and comprehensive method for the determination of hormones is challenging.

RESULTS

The present work reports a rapid, specific and sensitive method using ultrahigh-performance liquid chromatography coupled to electrospray ionization tandem spectrometry (UPLC/ESI-MS/MS) to analyze quantitatively the major hormones found in plant tissues within six minutes, including auxins, cytokinins, gibberellins, abscisic acid, 1-amino-cyclopropane-1-carboxyic acid (the ethylene precursor), jasmonic acid and salicylic acid. Sample preparation, extraction procedures and UPLC-MS/MS conditions were optimized for the determination of all plant hormones and are summarized in a schematic extraction diagram for the analysis of small amounts of plant material without time-consuming additional steps such as purification, sample drying or re-suspension.

CONCLUSIONS

This new method is applicable to the analysis of dynamic changes in endogenous concentrations of hormones to study plant developmental processes or plant responses to biotic and abiotic stresses in complex tissues. An example is shown in which a hormone profiling is obtained from leaves of plants exposed to salt stress in the aromatic plant, Rosmarinus officinalis.

Comparison of predictive methods and biological validation for qPCR reference genes in sunflower leaf senescence transcript analysis

The selection and validation of reference genes constitute a key point for gene expression analysis based on qPCR, requiring efficient normalization approaches. In this work, the expression profiles of eight genes were evaluated to identify novel reference genes for transcriptional studies associated to the senescence process in sunflower. Three alternative strategies were applied for the evaluation of gene expression stability in leaves of different ages and exposed to different treatments affecting the senescence process: algorithms implemented in geNorm, BestKeeper software, and the fitting of a statistical linear mixed model (LMModel). The results show that geNorm suggested the use of all combined genes, although identifying α-TUB1 as the most stable expressing gene. BestKeeper revealed α-TUB and β-TUB as stable genes, scoring β-TUB as the most stable one. The statistical LMModel identified α-TUB, actin, PEP, and EF-1α as stable genes in this order. The model-based approximation allows not only the estimation of systematic changes in gene expression, but also the identification of sources of random variation through the estimation of variance components, considering the experimental design applied. Validation of α-TUB and EF-1α as reference genes for expression studies of three sunflower senescence associated genes showed that the first one was more stable for the assayed conditions. We conclude that, when biological replicates are available, LMModel allows a more reliable selection under the assayed conditions. This study represents the first analysis of identification and validation of genuine reference genes for use as internal control in qPCR expression studies in sunflower, experimentally validated throughout six different controlled leaf senescence conditions.

A combined 15N tracing/proteomics study in Brassica napus reveals the chronology of proteomics events associated with N remobilisation during leaf senescence induced by nitrate limitation or starvation

Our goal was to identify the leaf proteomic changes which appeared during N remobilisation that were associated or not associated with senescence of oilseed rape in response to contrasting nitrate availability. Remobilisation of N and leaf senescence status were followed using (15)N tracing, patterns of chlorophyll level, total protein content and a molecular indicator based on expression of senescence-associated gene 12/Cab genes. Three phases associated with N remobilisation were distinguished. Proteomics revealed that 55 proteins involved in metabolism, energy, detoxification, stress response, proteolysis and protein folding, were significantly induced during N remobilisation. Four proteases were specifically identified. FtsH, a chloroplastic protease, was induced transiently during the early stages of N remobilisation. Considering the dynamics of N remobilisation, chlorophyll and protein content, the pattern of FtsH expression indicated that this protease could be involved in the degradation of chloroplastic proteins. Aspartic protease increased at the beginning of senescence and was maintained at a high level, implicating this protease in proteolysis during the course of leaf senescence. Two proteases, proteasome beta subunit A1 and senescence-associated gene 12, were induced and continued to increase during the later phase of senescence, suggesting that these proteases are more specifically involved in the proteolysis processes occurring at the final stages of leaf senescence.

An arabidopsis mitogen-activated protein kinase cascade, MKK9-MPK6, plays a role in leaf senescence

Leaf senescence is a developmentally programmed cell death process that constitutes the final step of leaf development, and it can be regulated by multiple environmental cues and endogenous signals. The mitogen-activated protein kinase (MAPK) cascades play diverse roles in intracellular and extracellular signaling in plants. Roles of the MAPK signaling module in leaf senescence are unknown. Here, a MAPK cascade involving MKK9-MPK6 is shown to play an important role in regulating leaf senescence in Arabidopsis (Arabidopsis thaliana). Both MKK9 and MPK6 possess kinase activities, with MPK6 an immediate target of MKK9, as revealed by in vitro, in vivo, and in planta assays. The constitutive and inducible overexpression of MKK9 causes premature senescence in leaves and in whole Arabidopsis plants. The premature senescence phenotype is suppressed when MKK9 is overexpressed in the mpk6 null background. When either MKK9 or MPK6 is knocked out, leaf senescence is delayed.

Comparison of the expression profiles of susceptible and resistant Eucalyptus grandis exposed to Puccinia psidii Winter using SAGE

Eucalyptus grandis Hill ex Maiden and its hybrids are commonly planted by the Brazilian pulp and paper industry, but they are the most susceptible to the neotropical rust disease caused by Puccinia psidii Winter. In an initial attempt to understand the mechanisms of resistance, we constructed two contrasting Serial Analysis of Gene Expression (SAGE) libraries using susceptible and resistant individuals from a segregating half-sibling E. grandis population. Using the Z-test we identified tags differentially expressed between the libraries, preferentially 239 in the susceptible and 232 in the resistant type individuals. Using public (Expressed Sequence Tags) EST databases, 40 of the susceptible and 70 of the resistant tags matched ESTs and were annotated. By comparing the type of genes and their expression levels, distinct differences between the libraries were observed. Susceptible plants showed gene expression linked to leaf senescence, generalised stress responses and detoxification, and are apparently incapable of inducing a competent host defence response. On the other hand, resistant plants showed genes upregulated for cellular polarisation, cytoskeleton restructuring, vesicle transport, and cellulose and lignin biosynthesis. In the resistant individuals, evidence for systemic resistance, anti-oxidative responses and a hypersensitive response was also observed, although no R gene was identified.

Developmental genes have pleiotropic effects on plant morphology and source capacity, eventually impacting on seed protein content and productivity in pea

Increasing pea (Pisum sativum) seed nutritional value and particularly seed protein content, while maintaining yield, is an important challenge for further development of this crop. Seed protein content and yield are complex and unstable traits, integrating all the processes occurring during the plant life cycle. During filling, seeds are the main sink to which assimilates are preferentially allocated at the expense of vegetative organs. Nitrogen seed demand is satisfied partly by nitrogen acquired by the roots, but also by nitrogen remobilized from vegetative organs. In this study, we evaluated the respective roles of nitrogen source capacity and sink strength in the genetic variability of seed protein content and yield. We showed in eight genotypes of diverse origins that both the maximal rate of nitrogen accumulation in the seeds and nitrogen source capacity varied among genotypes. Then, to identify the genetic factors responsible for seed protein content and yield variation, we searched for quantitative trait loci (QTL) for seed traits and for indicators of sink strength and source nitrogen capacity. We detected 261 QTL across five environments for all traits measured. Most QTL for seed and plant traits mapped in clusters, raising the possibility of common underlying processes and candidate genes. In most environments, the genes Le and Afila, which control internode length and the switch between leaflets and tendrils, respectively, determined plant nitrogen status. Depending on the environment, these genes were linked to QTL of seed protein content and yield, suggesting that source-sink adjustments depend on growing conditions.

Regulation and execution of molecular disassembly and catabolism during senescence

Senescence is a highly orchestrated developmental stage in the life cycle of plants. The onset of senescence is tightly controlled by signaling cascades that initiate changes in gene expression and the synthesis of new proteins. This complement of new proteins includes hydrolytic enzymes capable of executing catabolism of macromolecules, which in turn sets in motion disassembly of membrane molecular matrices, leading to loss of cell function and, ultimately, complete breakdown of cellular ultrastructure. A distinguishing feature of senescence that sets it apart from other types of programmed cell death is the recovery of carbon and nitrogen from the dying tissue and their translocation to growing parts of the plant such as developing seeds. For this to be accomplished, the initiation of senescence and its execution have to be meticulously regulated. For example, the initiation of membrane disassembly has to be intricately linked with the recruitment of nutrients because their ensuing translocation out of the senescing tissue requires functional membranes. Molecular mechanisms underlying this linkage and its integration with the catabolism of macromolecules in senescing tissues are addressed.

Molecular cloning and characterization of a granulin-containing cysteine protease SPCP3 from sweet potato (Ipomoea batatas) senescent leaves

Granulins are a family of evolutionarily ancient proteins that are involved in regulating cell growth and division in animals. In this report a full-length cDNA, SPCP3, was isolated from senescent leaves of sweet potato (Ipomoea batatas). SPCP3 contains 1389 nucleotides (462 amino acids) in its open reading frame, and exhibits high amino acid sequence homologies (ca. 64-73.6%) with several plant granulin-containing cysteine proteases, including potato, tomato, soybean, kidney bean, pea, maize, rice, cabbage, and Arabidopsis. Gene structural analysis shows that SPCP3 encodes a putative precursor protein. Via cleavage of the N-terminal propeptide, it generates a protein with 324 amino acids (from the 139th to the 462nd amino acid residues), which contains two main domains: the conserved catalytic domain with the putative catalytic residues (the 163rd Cys, 299th His and 319th Asn) and the C-terminal granulin domain (from the 375th to the 462nd amino acid residues). Semi-quantitative RT-PCR and protein gel blot hybridization showed that SPCP3 gene expression was enhanced significantly in natural senescent leaves and in dark- and ethephon-induced senescent leaves, but was almost undetectable in mature green leaves, veins, and roots. Phylogenic analysis showed that SPCP3 displayed close association with a group of plant granulin-containing cysteine proteases which have been implied to be involved in programmed cell death. In conclusion, sweet potato SPCP3 is a functional, senescence-associated gene. Its mRNA and protein levels were significantly enhanced in natural and induced senescing leaves. The physiological role and/or function of SPCP3 associated with programmed cell death during leaf senescence were also discussed.

Quantitative trait loci analysis of leaf and plant longevity in Arabidopsis thaliana

The natural variation in leaf and plant longevity in Arabidopsis thaliana was analysed in a set of 45 ecotypes and 155 recombinant inbred lines derived from a Cape Verde Islands (Cvi) x Landsberg erecta (Ler) cross. Post-bolting longevity was inversely related to time to flowering and rosette leaf number in the set of 45 ecotypes, with Cvi having the longest and Ler the shortest post-bolting longevity. The recombinant inbred line population was tested under low or high soil nutrient levels (LN or HN, respectively). Three quantitative trait loci (QTL), one in chromosome 3 and two in chromosomes 1 and 5, were associated with longevity of the 6th rosette leaf under LN and HN, respectively. Four QTL for post-bolting longevity were found in chromosomes 1, 3, 4, and 5, and two in chromosomes 1 and 5 under LN and HN, respectively. An epistatic interaction affecting post-bolting longevity under LN, but not HN, was detected. Ler and Cvi carry a mix of increasing and decreasing alleles for the QTL affecting longevity of the 6th leaf and post-bolting longevity. Longevity of the 6th rosette leaf was associated with different QTL than post-bolting longevity, and it was affected by different QTL depending on nutrient availability. By contrast, the major QTL affecting post-bolting longevity exerted significant effects irrespective of soil nutrient availability.

Grain filling of cereals under soil drying

Monocarpic plants require the initiation of whole-plant senescence to remobilize and transfer assimilates pre-stored in vegetative tissues to grains. Delayed whole-plant senescence caused by either heavy use of nitrogen fertilizer or adoption of lodging-resistant cultivars/hybrids that remain green when the grains are due to ripen results in a low harvest index with much nonstructural carbohydrate (NSC) left in the straw. Usually, water stress during the grain-filling period induces early senescence, reduces photosynthesis, and shortens the grain-filling period; however, it increases the remobilization of NSC from the vegetative tissues to the grain. If mild soil drying is properly controlled during the later grain-filling period in rice (Oryza sativa) and wheat (Triticum aestivum), it can enhance whole-plant senescence, lead to faster and better remobilization of carbon from vegetative tissues to grains, and accelerate the grain-filling rate. In cases where plant senescence is unfavorably delayed, such as by heavy use of nitrogen and the introduction of hybrids with strong heterosis, the gain from the enhanced remobilization and accelerated grain-filling rate can outweigh the loss of reduced photosynthesis and the shortened grain-filling period, leading to an increased grain yield, better harvest index and higher water-use efficiency.

Successive maturation and senescence of individual leaves during barley whole plant ontogeny reveals temporal and spatial regulation of photosynthetic function in conjunction with C and N metabolism

During ontogeny of barley plants (Hordeum vulgare, L. cv. Barke), a continuous developmental gradient of new leaves at the top and lower leaves undergoing senescence is maintained. In the course of senescence, specific recycling processes efficiently transfer valuable resources, e.g. nitrogen and carbon, to the growing young leaves and ears. In order to understand the temporal and spatial sequence of processes underlying this developmental program of leaf formation and degradation, changes in photosynthetic parameters, as well as C and N levels of all individual leaves were determined. During whole plant ontogeny, a strict sequential pattern of incorporation and degradation of C and N resources in the individual leaves, accompanied by a sequential loss of chlorophyll and photosynthetic function, was observed. In addition, protein levels of key enzymes of C and N anabolism AGPase (ADPglucose pyrophosphorylase) and GS (glutamine synthetase; plastidic isoform) also showed a strict pattern of sequential down-regulation in senescing leaves. Their decline preceded the breakdown of chlorophyll, total C and N levels and photosynthetic performance in the leaves. Quantitative real time PCR measurements revealed that the down-regulation of protein content of AGPase and GS correlated with a drastic decrease in their transcript levels. These data elucidated precise temporal and spatial regulation of C and N metabolism and allocation with photosynthetic function in the leaves during whole plant ontogeny of barley.

Effects of senescence-induced alteration in cytokinin metabolism on source-sink relationships and ontogenic and stress-induced transitions in tobacco

Senescence and reserve mobilization are integral components of plant development, are basic strategies in stress mitigation, and regulated at least in part by cytokinin. In the present study the effect of altered cytokinin metabolism caused by senescence-specific autoregulated expression of the Agrobacterium tumefaciens IPT gene under control of the P(SAG12) promoter (P(SAG12)-IPT) on seed germination and the response to a water-deficit stress was studied in tobacco (Nicotiana tabacum L.). Cytokinin levels, sugar content and composition of the leaf strata within the canopy of wild-type and P(SAG12)-IPT plants confirmed the reported altered source-sink relations. No measurable difference in sugar and pigment content of discs harvested from apical and basal leaves was evident 72 h after incubation with (+)-ABA or in darkness, indicating that expression of the transgene was not restricted to senescing leaves. No difference in quantum efficiency, photosynthetic activity, accumulation of ABA, and stomatal conductance was apparent in apical, middle and basal leaves of either wild-type or P(SAG12)-IPT plants after imposition of a mild water stress. However, compared to wild-type plants, P(SAG12)-IPT plants were slower to adjust biomass allocation. A stress-induced increase in root:shoot ratio and specific leaf area (SLA) occurred more rapidly in wild-type than in P(SAG12)-IPT plants reflecting delayed remobilization of leaf reserves to sink organs in the transformant. P(SAG12)-IPT seeds germinated more slowly even though abscisic acid (ABA) content was 50% that of the wild-type seeds confirming cytokinin-induced alterations in reserve remobilization. Thus, senescence is integral to plant growth and development and an increased endogenous cytokinin content impacts source-sink relations to delay ontogenic transitions wherein senescence in a necessary process.

The molecular and genetic control of leaf senescence and longevity in Arabidopsis

The life of a leaf initiated from a leaf primordium ends with senescence, the final step of leaf development. Leaf senescence is a developmentally programmed degeneration process that is controlled by multiple developmental and environmental signals. It is a highly regulated and complex process that involves orderly, sequential changes in cellular physiology, biochemistry, and gene expression. Elucidating molecular mechanisms underlying such a complex, yet delicate process of leaf senescence is a challenging and important biological task. For the past decade, impressive progress has been achieved on the molecular processes of leaf senescence through identification of genes that show enhanced expression during senescence. In addition, Arabidopsis has been established as a model plant for genetic analysis of leaf senescence. The progress on the characterization of genetic mutants of leaf senescence in Arabidopsis has firmly shown that leaf senescence is a genetically controlled developmental phenomenon involving numerous regulatory elements. Especially, employment of global expression analysis as well as genomic resources in Arabidopsis has been very fruitful in revealing the molecular genetic nature and mechanisms underlying leaf senescence. This progress, including molecular characterization of some of the genetic regulatory elements, are revealing that senescence is composed of a complex regulatory network. In this review, we will present current understanding of the molecular genetic mechanisms by which leaf senescence is regulated and processed, focusing mostly on the regulatory factors of senescence in Arabidopsis. We also present a potential biotechnological implication of leaf senescence studies on the improvement of important agronomic traits such as crop yield and post-harvest shelf life. We further provide future research prospects to better understand the complex regulatory network of senescence.

Leaf senescence: signals, execution, and regulation

Leaf senescence is a type of postmitotic senescence. The onset and progression of leaf senescence are controlled by an array of external and internal factors including age, levels of plant hormones/growth regulators, and reproductive growth. Many environmental stresses and biological insults such as extreme temperature, drought, nutrient deficiency, insufficient light/shadow/darkness, and pathogen infection can induce senescence. Perception of signals often leads to changes in gene expression, and the upregulation of thousands of senescence-associated genes (SAGs) causes the senescence syndrome: decline in photosynthesis, degradation of macromolecules, mobilization of nutrients, and ultimate cell death. Identification and analysis of SAGs, especially genome-scale investigations on gene expression during leaf senescence, make it possible to decipher the molecular mechanisms of signal perception, execution, and regulation of the leaf senescence process. Biochemical and metabolic changes during senescence have been elucidated, and potential components in signal transduction such as receptor-like kinases and MAP kinase cascade have been identified. Studies on some master regulators such as WRKY transcription factors and the senescence-responsive cis element of the senescence-specific SAG12 have shed some light on transcriptional regulation of leaf senescence.

The developing reproductive ‘sink’ induces oxidative stress to mediate nitrogen mobilization during monocarpic senescence in wheat

Removal of reproductive 'sink,' i.e., spikelets from wheat, after anthesis delays the rate of flag leaf senescence. Oxidative stress and the oxidative damage to proteins were studied in relation to nitrogen mobilization in wheat plants showing normal and delayed senescence. Wheat plants lacking a reproductive sink showed decreased oxidative stress, lower lipid peroxidation and maintained higher protein, oxidatively damaged proteins, and nitrogen levels as compared to plants with reproductive sink during monocarpic senescence. Oxidative damage to the proteins when not followed by high proteolytic activities led to a slower nitrogen mobilization in wheat plants lacking a reproductive sink. Thus, the influence of the reproductive sink was due to its ability to drive forward the nitrogen mobilization process through high ROS levels which mediated both damage to the proteins and influenced proteolytic activities.

Effect of herbicidal application of 2,4-dichlorophenoxyacetic acid in Arabidopsis

The whole genome expression pattern of Arabidopsis in response to the auxinic herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was evaluated using the Affymetrix ATH1-121501 array. Arabidopsis plants were grown in vitro and were exposed to 1 mM 2,4-D for 1 h, after which time gene transcription levels were measured. In response to the treatment 148 genes showed increased levels of transcription and concurrently 85 genes showed decreased levels of transcript. Genes which showed significant change in transcription levels belonged to the following functional categories: transcription, metabolism, cellular communication and signal transduction, subcellular localisation, transport facilitation, protein fate, protein with binding function or cofactor requirement and regulation of/interaction with cellular environment. Interestingly 25.3% of the genes regulated by the treatment could not be classified into a known functional category. The data obtained from these experiments were used to assess the current model of auxinic herbicide action and indicated that 2,4-D not only modulates the expression of auxin, ethylene and abscisic acid (ABA) pathways but also regulates a wide variety of other cellular functions.

The molecular analysis of leaf senescence--a genomics approach

Senescence in green plants is a complex and highly regulated process that occurs as part of plant development or can be prematurely induced by stress. In the last decade, the main focus of research has been on the identification of senescence mutants, as well as on genes that show enhanced expression during senescence. Analysis of these is beginning to expand our understanding of the processes by which senescence functions. Recent rapid advances in genomics resources, especially for the model plant species Arabidopsis, are providing scientists with a dazzling array of tools for the identification and functional analysis of the genes and pathways involved in senescence. In this review, we present the current understanding of the mechanisms by which plants control senescence and the processes that are involved.

Virtual plants: modelling as a tool for the genomics of tolerance to water deficit

Modelling can simulate the responses of virtual plants carrying diverse combinations of alleles under different scenarios of abiotic stress. The main difficulty is mathematically expressing the genetic variability of responses to environmental conditions. Modelling via gene regulatory networks is not feasible for such complex systems, but plants can be modelled using response curves to environmental conditions that are 'meta mechanisms' at plant level. Each genotype is represented by a set of response parameters that are valid under a wide range of conditions. Transgenesis of one function experimentally affected one response parameter only. Transgenic plants or plants carrying any combination of quantitative trait loci might therefore be simulated and tested under different climatic scenarios, before genetic manipulations are performed.