Remobilization of leaf S compounds and senescence in response to restricted sulphate supply during the vegetative stage of oilseed rape are affected by mineral N availability

Multi-omics analyses of sid2 mutant reflect the need of isochorismate synthase ICS1 to cope with sulfur limitation in Arabidopsis thaliana

The SID2 (SA INDUCTION-DEFICIENT2) gene that encodes ICS1 (isochorismate synthase), plays a central role in salicylic acid biosynthesis in Arabidopsis. The sid2 and NahG (encoding a bacterial SA hydroxylase) overexpressing mutants (NahG-OE) have currently been shown to outperform wild type, presenting delayed leaf senescence, higher plant biomass and better seed yield. When grown under sulfate-limited conditions (low-S), sid2 mutants exhibited early leaf yellowing compared to the NahG-OE, the npr1 mutant affected in SA signaling pathway, and WT. This indicated that the hypersensitivity of sid2 to sulfate limitation was independent of the canonical npr1 SA-signaling pathway. Transcriptomic and proteomic analyses revealed that major changes occurred in sid2 when cultivated under low-S, changes that were in good accordance with early senescence phenotype and showed the exacerbation of stress responses. The sid2 mutants displayed a lower sulfate uptake capacity when cultivated under low-S and lower S concentrations in their rosettes. Higher glutathione concentrations in sid2 rosettes under low-S were in good accordance with the higher abundance of proteins involved in glutathione and ascorbate redox metabolism. Amino acid and lipid metabolisms were also strongly modified in sid2 under low-S. Depletion of total fatty acids in sid2 under low-S was consistent with the fact that S-metabolism plays a central role in lipid synthesis. Altogether, our results show that functional ICS1 is important for plants to cope with S limiting conditions.

Know when and how to die: gaining insights into the molecular regulation of leaf senescence

Senescence is the ultimate phase in the life cycle of leaves which is crucial for recycling of nutrients to maintain plant fitness and reproductive success. The earliest visible manifestation of leaf senescence is their yellowing, which usually commences with the breakdown of chlorophyll. The degradation process involves a gradual and highly coordinated disassembly of macromolecules resulting in the accumulation of nutrients, which are subsequently mobilized from the senescing leaves to the developing organs. Leaf senescence progresses under overly tight genetic and molecular control involving a well-orchestrated and intricate network of regulators that coordinate spatio-temporally with the influence of both internal and external cues. Owing to the advancements in omics technologies, the availability of mutant resources, scalability of molecular analyses methodologies and the advanced capacity to integrate multidimensional data, our understanding of the genetic and molecular basis of leaf ageing has greatly expanded. The review provides a compilation of the multitier regulation of senescence process and the interrelation between the environment and the terminal phase of leaf development. The knowledge gained would benefit in devising the strategies for manipulation of leaf senescence process to improve crop quality and productivity.

Sulfur in Seeds: An Overview

Sulfur is a growth-limiting and secondary macronutrient as well as an indispensable component for several cellular components of crop plants. Over the years various scientists have conducted several experiments on sulfur metabolism based on different aspects of plants. Sulfur metabolism in seeds has immense importance in terms of the different sulfur-containing seed storage proteins, the significance of transporters in seeds, the role of sulfur during the time of seed germination, etc. The present review article is based on an overview of sulfur metabolism in seeds, in respect to source to sink relationships, S transporters present in the seeds, S-regulated seed storage proteins and the importance of sulfur at the time of seed germination. Sulfur is an essential component and a decidable factor for seed yield and the quality of seeds in terms of oil content in oilseeds, storage of qualitative proteins in legumes and has a significant role in carbohydrate metabolism in cereals. In conclusion, a few future perspectives towards a more comprehensive knowledge on S metabolism/mechanism during seed development, storage and germination have also been stated.

The molecular-physiological functions of mineral macronutrients and their consequences for deficiency symptoms in plants

The visual deficiency symptoms developing on plants constitute the ultimate manifestation of suboptimal nutrient supply. In classical plant nutrition, these symptoms have been extensively used as a tool to characterise the nutritional status of plants and to optimise fertilisation. Here we expand this concept by bridging the typical deficiency symptoms for each of the six essential macronutrients to their molecular and physiological functionalities in higher plants. We focus on the most recent insights obtained during the last decade, which now allow us to better understand the links between symptom and function for each element. A deep understanding of the mechanisms underlying the visual deficiency symptoms enables us to thoroughly understand how plants react to nutrient limitations and how these disturbances may affect the productivity and biodiversity of terrestrial ecosystems. A proper interpretation of visual deficiency symptoms will support the potential for sustainable crop intensification through the development of new technologies that facilitate automatised management practices based on imaging technologies, remote sensing and in-field sensors, thereby providing the basis for timely application of nutrients via smart and more efficient fertilisation.

Metabolic Changes in Different Tissues of Garlic Plant during Growth

The accumulation, distribution, and transportation of nutrients in different tissues of garlic during growth are unclear. Thereby, five tissues (leaf, pseudostem, bulb wrapper, clove skin, and clove) collected at 7 weeks were subjected to metabolomics analysis. A total of 84 biomarkers were identified during garlic plant growth. Most organosulfur compounds, amino acids, and dipeptides were upregulated in the clove, while a reversed trend was observed in other tissues. In addition, nucleotides and alkaloids increased because of senescence in the last 2 weeks except for the clove. The results also indicated that the garlic plant at an early stage is an ideal vegetable that is rich in nutrients. When the leaves began to wither, most nutrients were transported from other tissues to cloves, and the content of 7 total flavor precursors and 20 total amino acids in the clove increased by 113% and 65% after week 5, respectively. Therefore, delayed harvest may improve the nutritional quality of garlic bulbs.

Regulation of senescence-associated protease genes by sulphur availability according to barley (Hordeum vulgare L.) phenological stage

BACKGROUND AND AIMS

Proteases are responsible for protein degradation during leaf senescence, allowing nutrients to be redirected to sink tissues. In a previous work, we reported that sulphur deficiency produced a delay in the leaf senescence of barley (Hordeum vulgare L.) plants, at both vegetative and reproductive stages. In this work, we analyse the effect of sulphur deficiency on the expression of several genes coding for proteases of different catalytic groups, which have been strongly associated with leaf senescence.

METHODS

Four independent experiments were performed in order to impose low sulphur availability conditions: one of steady-state sulphur deficiency during the vegetative stage and three of sulphur starvation during vegetative and reproductive stages.

KEY RESULTS

Sulphur deficiency inhibited or reduced the senescence-associated induction of seven of the eight proteases analysed. Their induction, as well as senescence and phloem amino acid remobilization, could be achieved with senescence inducers such as methyl-jasmonate (a hormonal stimulus) and darkness, but with different rates of induction dependent on each gene. Sulphur deficiency also exerted an opposite effect on the expression of two cysteine-protease genes (HvSAG12 and HvLEGU) as well as on one serine-protease gene (HvSUBT) according to leaf age and plant phenological stages. All three genes were induced in green leaves but were repressed in senescent leaves of sulphur-deficient plants at the vegetative stage. At the reproductive stage, both cysteine-proteases were only repressed in senescent leaves, while the serine-protease was induced in green and senescent leaves by sulphur deficiency.

CONCLUSIONS

Our results highlight the relevance of adequate sulphur nutrition in order to ensure leaf senescence onset and induction of protease genes, which will consequently impact on grain protein composition and quality. In addition, our results provide evidence that leaf age, plant developmental stage and the nature of the stress modulate the sulphur responses.

Do nitrogen- and sulphur-remobilization-related parameters measured at the onset of the reproductive stage provide early indicators to adjust N and S fertilization in oilseed rape (Brassica napus L.) grown under N- and/or S-limiting supplies?

Specific combinations of physiological and molecular parameters associated with N and S remobilization measured at the onset of flowering were predictive of final crop performances in oilseed rape. Oilseed rape (Brassica napus L.) is a high nitrogen (N) and sulphur (S) demanding crop. Nitrogen- and S-remobilization processes allow N and S requirements to reproductive organs to be satisfied when natural uptake is reduced, thus ensuring high yield and seed quality. The quantification of physiological and molecular indicators of early N and S remobilization could be used as management tools to correct N and S fertilization. However, the major limit of this corrective strategy is to ensure the correlation between final performances-related variables and early measured parameters. In our study, four genotypes of winter oilseed rape (OSR) were grown until seed maturity under four nutritional modalities combining high and/or low N and S supplies. Plant final performances, i.e., seed production, N- and S-harvest indexes, seed N and S use efficiencies, and early parameters related to N- or S-remobilization processes, i.e., photosynthetic leaf area, N and S leaf concentrations, leaf soluble protein and leaf sulphate concentrations, and leaf RuBisCO abundance at flowering, were measured. We demonstrated that contrasting final performances existed according to the N and S supplies. An optimal N:S ratio supply could explain the treatment-specific crop performances, thus justifying N and S concurrent managements. Specific combinations of early measured plant parameters could be used to predict final performances irrespective of the nutritional supply and the genotype. This work demonstrates the potential of physiological and molecular indicators measured at flowering to reflect the functioning of N- and S-compound remobilization and to predict yield and quality penalties. However, because the predictive models are N and S independent, instant N and S leaf analyses are required to further adjust the adequate fertilization. This study is a proof of a concept which opens prospects regarding instant diagnostic tools in the context of N and S mineral fertilization management.

Remobilization and fate of sulphur in mustard

BACKGROUND AND AIMS

Sulphur (S) is an essential macronutrient involved in numerous metabolic pathways required for plant growth. Crops of the plant family Brassicaceae require more S compared with other crops for optimum growth and yield, with most S ultimately sequestered in the mature seeds as the storage proteins cruciferin and napin, along with the unique S-rich secondary metabolite glucosinolate (GSL). It is well established that S assimilation primarily takes place in the shoots rather than roots, and that sulphate is the major form in which S is transported and stored in plants. We carried out a developmental S audit to establish the net fluxes of S in two lines of Brassica juncea mustard where seed GSL content differed but resulted in no yield penalty.

METHODS

We quantified S pools (sulphate, GSL and total S) in different organs at multiple growth stages until maturity, which also allowed us to test the hypothesis that leaf S, accumulated as a primary S sink, becomes remobilized as a secondary source to meet the requirements of GSL as the dominant seed S sink.

KEY RESULTS

Maximum plant sulphate accumulation had occurred by floral initiation in both lines, at which time most of the sulphate was found in the leaves, confirming its role as the primary S sink. Up to 52 % of total sulphate accumulated by the low-GSL plants was lost through senesced leaves. In contrast, S from senescing leaves of the high-GSL line was remobilized to other tissues, with GSL accumulating in the seed from commencement of silique filling until maturity.

CONCLUSION

We have established that leaf S compounds that accumulated as primary S sinks at early developmental stages in condiment type B. juncea become remobilized as a secondary S source to meet the demand for GSL as the dominant seed S sink at maturity.

Sulfate transport systems in plants: functional diversity and molecular mechanisms underlying regulatory coordination

Sulfate transporters are integral membrane proteins controlling the flux of sulfate (SO42-) entering the cells and subcellular compartments across the membrane lipid bilayers. Sulfate uptake is a dynamic biological process that occurs in multiple cell layers and organs in plants. In vascular plants, sulfate ions are taken up from the soil environment to the outermost cell layers of roots and horizontally transferred to the vascular tissues for further distribution to distant organs. The amount of sulfate ions being metabolized in the cytosol and chloroplast/plastid or temporarily stored in the vacuole depends on expression levels and functionalities of sulfate transporters bound specifically to the plasma membrane, chloroplast/plastid envelopes, and tonoplast membrane. The entire system for sulfate homeostasis, therefore, requires different types of sulfate transporters to be expressed and coordinately regulated in specific organs, cell types, and subcellular compartments. Transcriptional and post-transcriptional regulatory mechanisms control the expression levels and functions of sulfate transporters to optimize sulfate uptake and internal distribution in response to sulfate availability and demands for synthesis of organic sulfur metabolites. This review article provides an overview of sulfate transport systems and discusses their regulatory aspects investigated in the model plant species Arabidopsis thaliana.

Seed Yield Components and Seed Quality of Oilseed Rape Are Impacted by Sulfur Fertilization and Its Interactions With Nitrogen Fertilization

Although the impact of sulfur (S) availability on the seed yield and nutritional quality of seeds has been demonstrated, its impact coupled with nitrogen (N) availability remains poorly studied in oilseed rape. A deeper knowledge of S and N interactions on seed yield components and seed quality could improve S and N fertilization management in a sustainable manner. To address this question, our goals were to determine the effects of nine different S fertilization management strategies (i) in interaction with different levels of N fertilization and (ii) according to the timing of application (by delaying and fractionating the S inputs) on agronomic performances and components of seed yield. The impact of these various managements of S and N fertilizations was also investigated on the seed quality with a focus on the composition of SSPs (mainly represented by napins and cruciferins). Our results highlighted synergetic effects on S and N use efficiencies at optimum rates of S and N inputs and antagonistic effects at excessive rates of one of the two elements. The data indicated that adjustment of S and N fertilization may lead to high seed yield and seed protein quality in a sustainable manner, especially in the context of reductions in N inputs. Delaying S inputs improved the seed protein quality by significantly increasing the relative abundance of napin (a SSP rich in S-containing amino acids) and decreasing the level of a cruciferin at 30 kDa (a SSP with low content of S-amino acids). These observations suggest that fractionated or delayed S fertilizer inputs could provide additional insights into the development of N and S management strategies to maintain or improve seed yield and protein quality. Our results also demonstrated that the S% in seeds and the napin:30 kDa-cruciferin ratio are highly dependent on S/N fertilization in relation to S supply. In addition, we observed a strong relationship between S% in seeds and the abundance of napin as well as the napin:30 kDa-cruciferin ratio, suggesting that S% may be used as a relevant index for the determination of protein quality in seeds in terms of S-containing amino acids.

Sensitivity analyses for improving sulfur management strategies in winter oilseed rape

Because sulfur (S) depletion in soil results in seed yield losses and grain quality degradation, especially in high S-demanding crops such as oilseed rape (Brassica napus L.), monitoring S fertilisation has become a central issue. Crop models can be efficient tools to conduct virtual experiments under different fertilisation management strategies. Using the process-based model SuMoToRI, we aimed to analyse the impact of different S fertilisation strategies coupled with the variablility observed in major plant characteristics in oilseed rape i.e. radiation use efficiency (RUE), carbon (C) allocation to the leaves (β) and specific leaf area (SLA) on plant performance-driven variables encompassing total biomass (TDW), S in the photosynthetic leaves (QS_mobile.GL) and leaf area index (LAI_GL). The contrasting S supply conditions differed in the amount of S (5 levels), and the timing of application (at bolting and/or at flowering, which included a fractioned condition). For this purpose, we performed a global sensitivity analysis (GSA) and calculated two sensitivity indices i.e. the Partial Raw Correlation Coefficient (PRCC) and the Sobol index. The results showed that whatever the timing of S supply, TDW, LAI_GL and QS_mobile.GL increased as S input increased. For a given S supply, there was no difference in TDW, LAI_GL and QS_mobile.GL between a single and a fractioned supply. Moreover, delaying the supply until flowering reduced the TDW and LAI_GL whereas QS_mobile.GL increased. Results showed that RUE had the greatest impact on TDW under all levels of S supply and all application timings, followed by β and SLA. RUE mostly impacted on QS_mobile.GL, depending on S supply conditions, whereas it was the parameter with the least impact on LAI_GL. Ultimately, our results provide strong evidence of optimised S fertilisation timings and plant characteristics that will guide producers in their agricultural practices by using specific varieties under constrained S fertilisation strategies.

Compartmentalization and Regulation of Sulfate Assimilation Pathways in Plants

Plants utilize sulfate to synthesize primary and secondary sulfur-containing metabolites required for growth and survival in the environment. Sulfate is taken up into roots from the soil and distributed to various organs through the functions of membrane-bound sulfate transporters, while it is utilized as the primary substrate for synthesizing sulfur-containing metabolites in the sulfate assimilation pathways. Transporters and enzymes for the assimilative conversion of sulfate are regulated in highly organized manners depending on changes in sulfate supply from the environment and demand for biosynthesis of reduced sulfur compounds in the plant systems. Over the past few decades, the effect of sulfur nutrition on gene expression of sulfate transporters and assimilatory enzymes has been extensively studied with the aim of understanding the full landscape of regulatory networks.

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.

Effect of sulphur deprivation on osmotic potential components and nitrogen metabolism in oilseed rape leaves: identification of a new early indicator

Identification of early sulphur (S) deficiency indicators is important for species such as Brassica napus, an S-demanding crop in which yield and the nutritional quality of seeds are negatively affected by S deficiency. Because S is mostly stored as SO4 (2-) in leaf cell vacuoles and can be mobilized during S deficiency, this study investigated the impact of S deprivation on leaf osmotic potential in order to identify compensation processes. Plants were exposed for 28 days to S or to chlorine deprivation in order to differentiate osmotic and metabolic responses. While chlorine deprivation had no significant effects on growth, osmotic potential and nitrogen metabolism, Brassica napus revealed two response periods to S deprivation. The first one occurred during the first 13 days during which plant growth was maintained as a result of vacuolar SO4 (2-) mobilization. In the meantime, leaf osmotic potential of S-deprived plants remained similar to control plants despite a reduction in the SO4 (2-) osmotic contribution, which was fully compensated by an increase in NO3 (-), PO4 (3-) and Cl(-) accumulation. The second response occurred after 13 days of S deprivation with a significant reduction in growth, leaf osmotic potential, NO3 (-) uptake and NO3 (-) reductase activity, whereas amino acids and NO3 (-) were accumulated. This kinetic analysis of S deprivation suggested that a ([Cl(-)]+[NO3 (-)]+[PO4 (3-)]):[SO4 (2-)] ratio could provide a relevant indicator of S deficiency, modified nearly as early as the over-expression of genes encoding SO4 (2-) tonoplastic or plasmalemmal transporters, with the added advantage that it can be easily quantified under field conditions.

Living to Die and Dying to Live: The Survival Strategy behind Leaf Senescence

Senescence represents the final developmental act of the leaf, during which the leaf cell is dismantled in a coordinated manner to remobilize nutrients and to secure reproductive success. The process of senescence provides the plant with phenotypic plasticity to help it adapt to adverse environmental conditions. Here, we provide a comprehensive overview of the factors and mechanisms that control the onset of senescence. We explain how the competence to senesce is established during leaf development, as depicted by the senescence window model. We also discuss the mechanisms by which phytohormones and environmental stresses control senescence as well as the impact of source-sink relationships on plant yield and stress tolerance. In addition, we discuss the role of senescence as a strategy for stress adaptation and how crop production and food quality could benefit from engineering or breeding crops with altered onset of senescence.

Sulfite Oxidase Activity Is Essential for Normal Sulfur, Nitrogen and Carbon Metabolism in Tomato Leaves

Plant sulfite oxidase [SO; E.C.1.8.3.1] has been shown to be a key player in protecting plants against exogenous toxic sulfite. Recently we showed that SO activity is essential to cope with rising dark-induced endogenous sulfite levels in tomato plants (Lycopersicon esculentum/Solanum lycopersicum Mill. cv. Rheinlands Ruhm). Here we uncover the ramifications of SO impairment on carbon, nitrogen and sulfur (S) metabolites. Current analysis of the wild-type and SO-impaired plants revealed that under controlled conditions, the imbalanced sulfite level resulting from SO impairment conferred a metabolic shift towards elevated reduced S-compounds, namely sulfide, S-amino acids (S-AA), Co-A and acetyl-CoA, followed by non-S-AA, nitrogen and carbon metabolite enhancement, including polar lipids. Exposing plants to dark-induced carbon starvation resulted in a higher degradation of S-compounds, total AA, carbohydrates, polar lipids and total RNA in the mutant plants. Significantly, a failure to balance the carbon backbones was evident in the mutants, indicated by an increase in tricarboxylic acid cycle (TCA) cycle intermediates, whereas a decrease was shown in stressed wild-type plants. These results indicate that the role of SO is not limited to a rescue reaction under elevated sulfite, but SO is a key player in maintaining optimal carbon, nitrogen and sulfur metabolism in tomato plants.

Sulfate resupply accentuates protein synthesis in coordination with nitrogen metabolism in sulfur deprived Brassica napus

To investigate the regulatory interactions between S assimilation and N metabolism in Brassica napus, de novo synthesis of amino acids and proteins was quantified by (15)N and (34)S tracing, and the responses of transporter genes, assimilatory enzymes and metabolites pool involving in nitrate and sulfate metabolism were assessed under continuous sulfur supply, sulfur deprivation and sulfate resupply after 3 days of sulfur (S) deprivation. S-deprived plants were characterized by a strong induction of sulfate transporter genes, ATP sulfurylase (ATPS) and adenosine 5'-phosphosulfate reductase (APR), and by a repressed activity of nitrate reductase (NR) and glutamine synthetase (GS). Sulfate resupply to the S-deprived plants strongly increased cysteine, amino acids and proteins concentration. The increase in sulfate and cysteine concentration caused by sulfate resupply was not matched with the expression of sulfate transporters and the activity of ATPS and APR which were rapidly decreased by sulfate resupply. A strong induction of O-acetylserine(thiol)lyase (OASTL), NR and GS upon sulfate resupply was accompanied with the increase in cysteine, amino acids and proteins pool. Sulfate resupply resulted in a strong increase in de novo synthesis of amino acids and proteins, as evidenced by the increases in N and S incorporation into amino acids (1.8- and 2.4-fold increase) and proteins (2.2-and 6.3-fold increase) when compared to S-deprived plants. The results thus indicate that sulfate resupply followed by S-deprivation accelerates nitrate assimilation for protein synthesis.

SuMoToRI, an Ecophysiological Model to Predict Growth and Sulfur Allocation and Partitioning in Oilseed Rape (Brassica napus L.) Until the Onset of Pod Formation

Sulfur (S) nutrition in rapeseed (Brassica napus L.) is a major concern for this high S-demanding crop, especially in the context of soil S oligotrophy. Therefore, predicting plant growth, S plant allocation (between the plant's compartments) and S pool partitioning (repartition of the mobile-S vs. non-mobile-S fractions) until the onset of reproductive phase could help in the diagnosis of S deficiencies during the early stages. For this purpose, a process-based model, SuMoToRI (Sulfur Model Toward Rapeseed Improvement), was developed up to the onset of pod formation. The key features rely on (i) the determination of the S requirements used for growth (structural and metabolic functions) through critical S dilution curves and (ii) the estimation of a mobile pool of S that is regenerated by daily S uptake and remobilization from senescing leaves. This study describes the functioning of the model and presents the model's calibration and evaluation. SuMoToRI was calibrated and evaluated with independent datasets from greenhouse experiments under contrasting S supply conditions. It is run with a small number of parameters with generic values, except in the case of the radiation use efficiency, which was shown to be modulated by S supply. The model gave satisfying predictions of the dynamics of growth, S allocation between compartments and S partitioning, such as the mobile-S fraction in the leaves, which is an indicator of the remobilization potential toward growing sinks. The mechanistic features of SuMoToRI provide a process-based framework that has enabled the description of the S remobilizing process in a species characterized by senescence during the vegetative phase. We believe that this model structure could be useful for modeling S dynamics in other arable crops that have similar senescence-related characteristics.

Transcriptional responses of Medicago truncatula upon sulfur deficiency stress and arbuscular mycorrhizal symbiosis

Sulfur plays an essential role in plants' growth and development and in their response to various abiotic and biotic stresses despite its leachability and its very low abundance in the only form that plant roots can uptake (sulfate). It is part of amino acids, glutathione (GSH), thiols of proteins and peptides, membrane sulfolipids, cell walls and secondary products, so reduced availability can drastically alter plant growth and development. The nutritional benefits of symbiotic interactions can help the plant in case of S deficiency. In particular the arbuscular mycorrhizal (AM) interaction improves N, P, and S plant nutrition, but the mechanisms behind these exchanges are not fully known yet. Although the transcriptional changes in the leguminous model plant Medicago truncatula have been already assessed in several biotic and/or abiotic conditions, S deficiency has not been considered so far. The aim of this work is to get a first overview on S-deficiency responses in the leaf and root tissues of plants interacting with the AM fungus Rhizophagus irregularis. Several hundred genes displayed significantly different transcript accumulation levels. Annotation and GO ID association were used to identify biological processes and molecular functions affected by sulfur starvation. Beside the beneficial effects of AM interaction, plants were greatly affected by the nutritional status, showing various differences in their transcriptomic footprints. Several pathways in which S plays an important role appeared to be differentially affected according to mycorrhizal status, with a generally reduced responsiveness to S deficiency in mycorrhized plants.

Leaf senescence and nitrogen remobilization efficiency in oilseed rape (Brassica napus L.)

Despite its worldwide economic importance for food (oil, meal) and non-food (green energy and chemistry) uses, oilseed rape has a low nitrogen (N) use efficiency (NUE), mainly due to the low N remobilization efficiency (NRE) observed during the vegetative phase when sequential leaf senescence occurs. Assuming that improvement of NRE is the main lever for NUE optimization, unravelling the cellular mechanisms responsible for the recycling of proteins (the main N source in leaf) during sequential senescence is a prerequisite for identifying the physiological and molecular determinants that are associated with high NRE. The development of a relevant molecular indicator (SAG12/Cab) of leaf senescence progression in combination with a (15)N-labelling method were used to decipher the N remobilization associated with sequential senescence and to determine modulation of this process by abiotic factors especially N deficiency. Interestingly, in young leaves, N starvation delayed senescence and induced BnD22, a water-soluble chlorophyll-binding protein that acts against oxidative alterations of chlorophylls and exhibits a protease inhibitor activity. Through its dual function, BnD22 may help to sustain sink growth of stressed plants and contribute to a better utilization of N recycled from senescent leaves, a physiological trait that could improve NUE. Proteomics approaches have revealed that proteolysis involves chloroplastic FtsH protease in the early stages of senescence, aspartic protease during the course of leaf senescence, and the proteasome β1 subunit, mitochondria processing protease and SAG12 (cysteine protease) during the later senescence phases. Overall, the results constitute interesting pathways for screening genotypes with high NRE and NUE.

The impact of sulfate restriction on seed yield and quality of winter oilseed rape depends on the ability to remobilize sulfate from vegetative tissues to reproductive organs

Our current knowledge about sulfur (S) management by winter oilseed rape to satisfy the S demand of developing seeds is still scarce, particularly in relation to S restriction. Our goals were to determine the physiological processes related to S use efficiency that led to maintain the seed yield and quality when S limitation occurred at the bolting or early flowering stages. To address these questions, a pulse-chase (34)SO(2-) 4 labeling method was carried out in order to study the S fluxes from uptake and remobilization at the whole plant level. In response of S limitation at the bolting or early flowering stages, the leaves are the most important source organ for S remobilization during reproductive stages. By combining (34)S-tracer with biochemical fractionation in order to separate sulfate from other S-compounds, it appeared that sulfate was the main form of S remobilized in leaves at reproductive stages and that tonoplastic SULTR4-type transporters were specifically involved in the sulfate remobilisation in case of low S availability. In response to S limitation at the bolting stage, the seed yield and quality were dramatically reduced compared to control plants. These data suggest that the increase of both S remobilization from source leaves and the root proliferation in order to maximize sulfate uptake capacities, were not sufficient to maintain the seed yield and quality. When S limitation occurred at the early flowering stage, oilseed rape can optimize the mobilization of sulfate reserves from vegetative organs (leaves and stem) to satisfy the demand of seeds and maintain the seed yield and quality. Our study also revealed that the stem may act as a transient storage organ for remobilized S coming from source leaves before its utilization by seeds. The physiological traits (S remobilization, root proliferation, transient S storage in stem) observed under S limitation could be used in breeding programs to select oilseed rape genotypes with high S use efficiency.

A specific method of 34S labelling provides evidence that sulphate assimilation occurs in developing seeds and pod walls of Brassica napus L. subjected to ample or limited S nutrition

RATIONALE

Seeds from different species actively assimilate sulphur (S) from sulphate. This has never been proved for Brassica napus L., a high S demanding plant, especially with regard to S limitation. The role of pod walls in the assimilation and allocation of S in well-fed and sulphate-limited conditions also needs to be clarified.

METHODS

Freshly harvested seeds and pod walls from plants well-supplied (HS) or limited with sulphate (LS) from the 'visible buds' stage were subjected to a nutrient solution containing (34)S-sulphate (10 atom% excess) for 24 h. The (34)S labelling of the sulphate and protein fractions was determined with an elemental analyser connected to a continuous flow isotope ratio mass spectrometer. The amino acid profiles of seeds and pod walls were also determined by ion-exchange chromatography.

RESULTS

The 24 h of (34)S-sulphate feeding treatment leads to an important production of proteins in HS and LS seeds, associated with a decrease in numerous amino acid contents. The treatment also leads to an incorporation of (34)S in seeds and pod walls proteins in both HS and LS conditions. The incorporation of (34)S in proteins was not different between HS and LS seeds, but was lower in LS than in HS pod walls, related to a higher incorporation in the other organic S compounds.

CONCLUSIONS

This study provides evidence that Brassica napus seeds and pod walls are able to assimilate sulphate in HS and LS conditions, and that the LS condition leads to enhancement of the sulphate assimilation capacity of pod walls, which may be of crucial importance for the allocation of S to developing seeds.

Legume adaptation to sulfur deficiency revealed by comparing nutrient allocation and seed traits in Medicago truncatula

Reductions in sulfur dioxide emissions and the use of sulfur-free mineral fertilizers are decreasing soil sulfur levels and threaten the adequate fertilization of most crops. To provide knowledge regarding legume adaptation to sulfur restriction, we subjected Medicago truncatula, a model legume species, to sulfur deficiency at various developmental stages, and compared the yield, nutrient allocation and seed traits. This comparative analysis revealed that sulfur deficiency at the mid-vegetative stage decreased yield and altered the allocation of nitrogen and carbon to seeds, leading to reduced levels of major oligosaccharides in mature seeds, whose germination was dramatically affected. In contrast, during the reproductive period, sulfur deficiency had little influence on yield and nutrient allocation, but the seeds germinated slowly and were characterized by low levels of a biotinylated protein, a putative indicator of germination vigor that has not been previously related to sulfur nutrition. Significantly, plants deprived of sulfur at an intermediary stage (flowering) adapted well by remobilizing nutrients from source organs to seeds, ensuring adequate quantities of carbon and nitrogen in seeds. This efficient remobilization of photosynthates may be explained by vacuolar sulfate efflux to maintain leaf metabolism throughout reproductive growth, as suggested by transcript and metabolite profiling. The seeds from these plants, deprived of sulfur at the floral transition, contained normal levels of major oligosaccharides but their germination was delayed, consistent with low levels of sucrose and the glycolytic enzymes required to restart seed metabolism during imbibition. Overall, our findings provide an integrative view of the legume response to sulfur deficiency.

S-deficiency responsive accumulation of amino acids is mainly due to hydrolysis of the previously synthesized proteins - not to de novo synthesis in Brassica napus

To characterize the mechanisms of amino acid accumulation under sulphur (S)-deficiency and its physiological significance in Brassica napus, stable isotopes (15) N and (34) S were employed. The plants were exposed for 9 days to S-deficient conditions (0.05 mM vs 1.5 mM sulphate). After 9 days of S-deficiency, leaf-osmotic potential and total chlorophyll content significantly decreased. S uptake decreased by 94%, whereas N uptake and biomass were not significantly changed. Using (15) N and (34) S labelling, de novo synthesis of amino acids and proteins derived from newly absorbed NO(3) (-) and SO(4) (2) (-) and the content of N and S in the previously synthesized amino acids and proteins were quantified. At the whole plant level, S-deficiency increased the pool of amino acids but resulted in strong decrease of incorporation of newly absorbed NO(3) (-) and SO(4) (2) (-) into amino acids by 22.2 and 76.6%, respectively, compared to the controls. Total amount of N and S incorporated into proteins also decreased by 28.8 and 62.1%, respectively. The levels of (14) N- and (32) S-proteins (previously synthesized proteins) strongly decreased, mainly in mature leaves. The data thus indicate that amino acid accumulation under short-term S-deficiency results from the degradation of previously synthesized proteins rather than from de novo synthesis.

Sulphur limitation provokes physiological and leaf proteome changes in oilseed rape that lead to perturbation of sulphur, carbon and oxidative metabolisms

BACKGROUND

The decline in industrial emissions of sulphur (S) has led to a sulphate depletion in soil resulting in an alteration of crop performance. In oilseed rape, an S deficiency dramatically reduced the seed yield and/or quality. Paradoxically, little is known about the impact of sulphate limitation on oilseed rape leaf metabolism, despite it being a key determinant of growth. In order to identify the metabolic processes involved in the oilseed rape response to S restriction, an analysis of the young leaf proteome combined with a physiological study was carried out at the vegetative stage.

RESULTS

S limitation does not significantly reduce the total shoot biomass but inhibits growth and photosynthesis of young leaves. This photosynthesis decline is not due to a decrease in chlorophyll content, which remains similar to Control. The increase in anthocyanins and H(2)O(2) content in young leaves of S-limited plants suggests that S restriction leads to an oxidative stress. Proteomic analysis at 35 d of S limitation also revealed the induction of 12-oxophitodienoate reductase and ACC synthase, respectively involved in jasmonate and ethylene biosynthesis, two phytohormones that could be implicated in oxidative stress. Proteins involved in photosynthesis and carbon metabolism were also modulated by S restriction. In particular, the decrease in plastocyanin and ferredoxin-NADP reductase suggests that H(2)O(2) accumulation is associated with perturbation of the photosynthetic electron transport chain. The accumulation of chloroplastic Cu-Zn SOD reinforces the idea that an oxidative stress probably occurs in the chloroplast. Proteomic results suggest that the maintenance of chlorophyll in S-limited conditions is related to an accumulation of Water Soluble Chlorophyll binding Proteins, involved in the protection of chlorophyll against ROS. The accumulation of the catalytic α-subunit of chloroplastic ATP synthase suggests that energy production is maintained.

CONCLUSION

S limitation leads to photosynthesis and carbon metabolism disturbances that could be responsible for the oxidative stress observed in the young leaves of oilseed rape. Despite this, induction of proteins involved in oxidative stress resistance and energy production shows that the leaf capacity to capture and use photosynthetic active radiations for ATP production remains efficient for as long as possible.

Improving growth and productivity of Oleiferous Brassicas under changing environment: significance of nitrogen and sulphur nutrition, and underlying mechanisms

Mineral nutrients are the integral part of the agricultural systems. Among important plant nutrients, nitrogen (N) and sulphur (S) are known essential elements for growth, development, and various physiological functions in plants. Oleiferous brassicas (rapeseed and mustard) require higher amounts of S in addition to N for optimum growth and yield. Therefore, balancing S-N fertilization, optimization of nutrient replenishment, minimization of nutrient losses to the environment, and the concept of coordination in action between S and N could be a significant strategy for improvement of growth and productivity of oleiferous brassicas. Additionally, positive interaction between S and N has been reported to be beneficial for various aspects of oilseed brassicas. The current paper updates readers on the significance of N and S for the improvement of plant growth, development, and productivity in detail. In addition, S-N nutrition-mediated control of major plant antioxidant defense system components involved in the removal and/or metabolism of stress-induced/generated reactive oxygen species in plants (hence, the control of plant growth, development, and productivity) has been overviewed.

Sulphur limitation and early sulphur deficiency responses in poplar: significance of gene expression, metabolites, and plant hormones

The influence of sulphur (S) depletion on the expression of genes related to S metabolism, and on metabolite and plant hormone contents was analysed in young and mature leaves, fine roots, xylem sap, and phloem exudates of poplar (Populus tremula×Populus alba) with special focus on early consequences. S depletion was applied by a gradual decrease of sulphate availability. The observed changes were correlated with sulphate contents. Based on the decrease in sulphate contents, two phases of S depletion could be distinguished that were denominated as 'S limitation' and 'early S deficiency'. S limitation was characterized by improved sulphate uptake (enhanced root-specific sulphate transporter PtaSULTR1;2 expression) and reduction capacities (enhanced adenosine 5'-phosphosulphate (APS) reductase expression) and by enhanced remobilization of sulphate from the vacuole (enhanced putative vacuolar sulphate transporter PtaSULTR4;2 expression). During early S deficiency, whole plant distribution of S was impacted, as indicated by increasing expression of the phloem-localized sulphate transporter PtaSULTR1;1 and by decreasing glutathione contents in fine roots, young leaves, mature leaves, and phloem exudates. Furthermore, at 'early S deficiency', expression of microRNA395 (miR395), which targets transcripts of PtaATPS3/4 (ATP sulphurylase) for cleavage, increased. Changes in plant hormone contents were observed at 'early S deficiency' only. Thus, S depletion affects S and plant hormone metabolism of poplar during 'S limitation' and 'early S deficiency' in a time series of events. Despite these consequences, the impact of S depletion on growth of poplar plants appears to be less severe than in Brassicaceae such as Arabidopsis thaliana or Brassica sp.

Cadmium at high dose perturbs growth, photosynthesis and nitrogen metabolism while at low dose it up regulates sulfur assimilation and antioxidant machinery in garden cress (Lepidium sativum L.)

Metal contamination of soils has become a worldwide problem and great environmental threat, as these metals accumulate in soils and plants in excess, and enter the food chain. Increased cadmium (Cd) uptake from contaminated soils leads to altered plant metabolism and limits the crop productivity. The experimental crop, Lepidium sativum L. (Garden Cress, Family: Brassicaceae) is a medicinally and economically important plant. An experiment was conducted to examine the effect of different concentrations of Cd (0, 25, 50 or 100 mg kg(-1) soil) on the performance of L. sativum. Cd accumulation in roots and leaves (roots>leaves) increased with the increaseing Cd concentration in soil. High Cd concentration (100mg Cd kg(-1) soil) inhibited the leaf area and plant dry mass and significant decline in net photosynthetic rate (P(N)), stomatal conductance (gs), intercellular CO(2) (Ci), chlorophyll (Chl a, Chl b, total Chl) content, carbonic anhydrase (CA; E.C. 4.2.1.1) activity, nitrate reductase (NR; E.C. 1.6.6.1) activity and nitrogen (N) content was also observed. However, ATP-sulfurylase (ATP-S; EC. 2.7.7.4) activity, sulfur (S) content and activities of antioxidant enzymes such as superoxide dismutase (SOD; E.C. 1.15.1.1); catalase (CAT; E.C. 1.11.1.6); ascorbate peroxidase (APX; E.C. 1.11.1.11) and glutathione reductase (GR; E.C. 1.6.4.2) and glutathione (GSH) content were increased. Specifically, the decrease in NR activity and N content showed that Cd affects N metabolism negatively; whereas, the increase in ATP-S activity and S content suggests the up-regulation of S assimilation pathway for possible Cd tolerance in coordination with enhanced activities of antioxidant enzymes and GSH. High Cd concentration (100mg Cd kg(-1) soil) perturbs the L. sativum growth by interfering with the photosynthetic machinery and disrupting the coordination between carbon, N and S metabolism. On the other hand, at low Cd concentration (25mg Cd kg(-1) soil) co-ordination of S and N metabolism complemented to the antioxidant machinery to protect the growth and photosynthesis of L. sativum plants.

Do initial S reserves and mineral S availability alter leaf S-N mobilization and leaf senescence in oilseed rape?

Winter oilseed rape is sensitive to S limitation, however few studies have clearly assessed the impact of initial S reserves on the remobilization of leaf N-S compounds and senescence dynamics within the leaves in S limited plants. As a consequence, the impacts of high or low initial S reserves on these parameters, further cross-combined with either high or low S availabilities, were examined using a ¹⁵N and ³⁴S double-labelling method associated with a study of gene expression of relevant tonoplastic sulphate transporters (BnSultr4;1 and BnSultr4;2) and a molecular indicator of leaf senescence (BnSAG12/BnCab). Plants with high initial S status and S limitation showed an optimal growth comparable to control plants. Moreover, in response to S limitation, leaf soluble protein content, total S, recently assimilated S (i.e., ³⁴S) and the sulphate content in the oldest leaves declined, and the expression of genes encoding tonoplastic sulphate transporters were up-regulated. However, compared to control plants, S limitation delayed leaf senescence. These data suggested that in response to S limitation, plants with high initial S were able to sustain optimized leaf growth by increasing endogenous N and S remobilization independently of the leaf senescence process. In contrast, if these low S plants had no initial S reserves, leaf N-S remobilization was not sufficient to allow optimal growth. As a conclusion, our study supports a model where oilseed rape is able to compensate transiently for S limitation through a fine management of leaf N-S remobilization and a delayed leaf senescence dynamics.

Increased phloem transport of S-methylmethionine positively affects sulfur and nitrogen metabolism and seed development in pea plants

Seeds of grain legumes are important energy and food sources for humans and animals. However, the yield and quality of legume seeds are limited by the amount of sulfur (S) partitioned to the seeds. The amino acid S-methylmethionine (SMM), a methionine derivative, has been proposed to be an important long-distance transport form of reduced S, and we analyzed whether SMM phloem loading and source-sink translocation are important for the metabolism and growth of pea (Pisum sativum) plants. Transgenic plants were produced in which the expression of a yeast SMM transporter, S-Methylmethionine Permease1 (MMP1, YLL061W), was targeted to the phloem and seeds. Phloem exudate analysis showed that concentrations of SMM are elevated in MMP1 plants, suggesting increased phloem loading. Furthermore, expression studies of genes involved in S transport and metabolism in source organs, as well as xylem sap analyses, support that S uptake and assimilation are positively affected in MMP1 roots. Concomitantly, nitrogen (N) assimilation in root and leaf and xylem amino acid profiles were changed, resulting in increased phloem loading of amino acids. When investigating the effects of increased S and N phloem transport on seed metabolism, we found that protein levels were improved in MMP1 seeds. In addition, changes in SMM phloem loading affected plant growth and seed number, leading to an overall increase in seed S, N, and protein content in MMP1 plants. Together, these results suggest that phloem loading and source-sink partitioning of SMM are important for plant S and N metabolism and transport as well as seed set.

Is the remobilization of S and N reserves for seed filling of winter oilseed rape modulated by sulphate restrictions occurring at different growth stages?

How the remobilization of S and N reserves can meet the needs of seeds of oilseed rape subject to limitation of S fertilization remains largely unclear. Thus, this survey aims to determine the incidence of sulphate restriction [low S (LS)] applied at bolting [growth stage (GS) 32], visible bud (GS 53), and start of pod filling (GS 70) on source-sink relationships for S and N, and on the dynamics of endogenous/exogenous S and N contributing to seed yield and quality. Sulphate restrictions applied at GS 32, GS 53, and GS 70 were annotated LS(32), LS(53), and LS(70). Long-term (34)SO(4)(2-) and (15)NO(3)(-) labelling was used to explore S and N partitioning at the whole-plant level. In LS(53), the sulphur remobilization efficiency (SRE) to seeds increased, but not enough to maintain seed quality. In LS(32), an early S remobilization from leaves provided S for root, stem, and pod growth, but the subsequent demand for seed development was not met adequately and the N utilization efficiency (NUtE) was reduced when compared with high S (HS). The highest SRE (65 ± 1.2% of the remobilized S) associated with an efficient foliar S mobilization (with minimal residual S concentrations of 0.1-0.2% dry matter) was observed under LS(70) treatment, which did not affect yield components.

Effect of mineral sulphur availability on nitrogen and sulphur uptake and remobilization during the vegetative growth of Brassica napus L

Because it has a high demand for sulphur (S), oilseed rape is particularly sensitive to S limitation. However, the physiological effects of S limitation remain unclear, especially during the rosette stage. For this reason a study was conducted to determine the effects of mineral S limitation on nitrogen (N) and S uptake and remobilization during vegetative growth of oilseed rape at both the whole-plant and leaf rank level for plants grown during 35 d with 300 microM (34)SO(4)(2-) (control plants; +S) or with 15 microM (34)SO(4)(2-) (S-limited plants; -S). The results highlight that S-limited plants showed no significant differences either in whole-plant and leaf biomass or in N uptake, when compared with control plants. However, total S and (34)S (i.e. deriving from S uptake) contents were greatly reduced for the whole plant and leaf after 35 d, and a greater redistribution of endogenous S from leaves to the benefit of roots was observed. The relative expression of tonoplast and plasmalemma sulphate transporters was also strongly induced in the roots. In conclusion, although S-limited plants had 20 times less mineral S than control plants, their development remained surprisingly unchanged. During S limitation, oilseed rape is able to recycle endogenous S compounds (mostly sulphate) from leaves to roots. However, this physiological adaptation may be effective only over a short time scale (i.e. vegetative growth).

Regulation of sulfate uptake and assimilation--the same or not the same?

Plant take up the essential nutrient sulfur as sulfate from the soil, reduce it, and assimilate into bioorganic compounds, with cysteine being the first product. Both sulfate uptake and assimilation are highly regulated by the demand for the reduced sulfur, by availability of nutrients, and by environmental conditions. In the last decade, great progress has been achieved in dissecting the regulation of sulfur metabolism. Sulfate uptake and reduction of activated sulfate, adenosine 5'-phosphosulfate (APS), to sulfite by APS reductase appear to be the key regulatory steps. Here, we review the current knowledge on regulation of these processes, with special attention given to similarities and differences.