Regulation and execution of molecular disassembly and catabolism during senescence

Essential oils prolonged the cut carnation longevity by limiting the xylem blockage and enhancing the physiological and biochemical levels

Postharvest characteristics, such as vase life and antimicrobial preservation of commercial cut flowers are some of the major determinants of their market value worldwide. Extending vase life while restricting microbial proliferation in cut flowers is an important challenge faced by floricultural researchers. This study evaluates the preservative efficiency of different essential oils used as additive solutions in prolonging the longevity of carnation cv. Madam Collette cut flowers and restricting microbial growth in them. Cut carnations were treated with four essential oils: geranium, thyme, marjoram, and anise at concentrations of 0, 25, 50, and 75 mg/L. While treatment with all the essential oils prolonged the longevity of the cut flowers, thyme and marjoram oils were most effective at concentrations of 50 mg/L each. The vase life of thyme-treated and marjoram-treated carnations almost doubled to 18.5 days and 18.25 days, respectively, as compared to untreated flowers. Treatment with essential oils also led to an increase in water uptake by the cut flowers enhancing their relative water content (RWC). It also restricted the sharp decline of chlorophyll and total carbohydrates content of the flowers during their vase life period. Morphological features of the stem bases of treated and untreated carnations were analyzed using scanning electron microscopy (SEM). The stem ends of geranium and anise-treated carnations showed less bacterial growth than untreated flowers and no apparent xylem blockage was observed even after nine days of treatment. Furthermore, the presence of essential oils also reduced lipid peroxidation and free radical generation as observed by malondialdehyde (MDA) and H2O2 quantification, respectively. It also led to increased production of total phenols leading to enhanced membrane stability. The use of thyme and marjoram essential oils as antimicrobial preservatives and green antioxidants appears to have promising applications in both the industrial and scientific sectors.

Lipid transporter LSR1 positively regulates leaf senescence in Arabidopsis

Senescence is the final stage in the life history of a leaf, whereby plants relocate nutrients from leaves to other developing organs. Recent efforts have begun to focus on understanding the network-based molecular mechanism that incorporates various environmental signals and leaf age information and involves a complex process with the coordinated actions of multiple pathways. Here, we identified a novel participant, named LSR1 (Leaf Senescence Related 1), that involved in the regulation of leaf senescence. Loss-of-function lsr1-1 mutant showed delayed leaf senescence whereas the overexpression of LSR1 accelerated senescence. LSR1 encodes a lipid transfer protein, and the results show that the protein is located in chloroplast and intercellular space. The LSR1 may be involved in the regulation of leaf senescence by transporting lipids in plants.

A conserved clathrin-coated vesicle component, OsSCYL2, regulates plant innate immunity in rice

Clathrin-mediated vesicle trafficking (CMVT) is a fundamental process in all eukaryotic species, and indispensable to organism's growth and development. Recently, it has been suggested that CMVT also plays important roles in the regulation of plant immunity. However, the molecular link between CMVT and plant immunity is largely unknown. SCY1-LIKE2 (SCYL2) is evolutionally conserved among the eukaryote species. Loss-of-function of SCYL2 in Arabidopsis led to severe growth defects. Here, we show that mutation of OsSCYL2 in rice gave rise to a novel phenotype-hypersensitive response-like (HR) cell death in a light-dependent manner. Although mutants of OsSCYL2 showed additional defects in the photosynthetic system, they exhibited enhanced resistance to bacterial pathogens. Subcellular localisation showed that OsSCYL2 localized at Golgi, trans-Golgi network and prevacuolar compartment. OsSCYL2 interacted with OsSPL28, subunit of a clathrin-associated adaptor protein that is known to regulate HR-like cell death in rice. We further showed that OsSCYL2-OsSPL28 interaction is mediated by OsCHC1. Collectively, we characterized a novel component of the CMVT pathway in the regulation of plant immunity. Our work also revealed unidentified new functions of the very conserved SCYL2. It thus may provide new breeding targets to achieve both high yield and enhanced resistance in crops.

Ureides are accumulated similarly in response to UV-C irradiation and wounding in Arabidopsis leaves but are remobilized differently during recovery

Purine degradation products have been shown to play roles in plant response to stresses such as drought, salinity, extended dark, nitrogen deficiency, and pathogen infection. In this study, we used Arabidopsis wild-type (WT) and an Atxdh1-knockout mutant defective in xanthine dehydrogenase1 (XDH1) to examine the role of degraded purine metabolites in the responses to wounding or UV-C stress applied to the middle leaves of the plant. Wounding or UV-C stress in the mutant resulted in lower fresh-weight, increased senescence symptoms, and increased cell death compared to WT plants. In addition, WT plants exhibited lower levels of oxidative stress indicators, reactive oxygen species, and malondialdehyde in their leaves than the mutant. Notably, transcripts and proteins functioning in the purine degradation pathway were regulated in such a way that it led to enhanced ureide levels in WT leaves 24h after applying the UV-C or wound stress. However, different remobilization of the accumulated ureides was observed after 72h of stress. In plants treated with UV-C, the concentration of allantoin was highest in young leaves, whereas in wounded plants it was lowest in these leaves and instead accumulated mainly in the middle leaves that had been wounded. These results indicated that in WT plants treated with UV-C, ureides were remobilized from the lower older and damaged leaves to support young leaf growth during the recovery period from stress. After wounding, however, whilst some ureides were remobilized to the young leaves, more remained in the wounded middle leaves to function as antioxidants and/or healing agents.

AtWAKL10, a Cell Wall Associated Receptor-Like Kinase, Negatively Regulates Leaf Senescence in Arabidopsis thaliana

Receptor-like kinases (RLKs) constitute a large group of cell surface receptors that play crucial roles in multiple biological processes. However, the function of most RLKs in plants has not been extensively explored, and much less for the class of cell wall associated kinases (WAKs) and WAK-like kinases (WAKLs). In this study, analyses of developmental expression patterns uncovered a putative role of AtWAKL10 in modulating leaf senescence, which was further investigated at physiological and molecular levels. The expression level of AtWAKL10 increased with the developmental progression and was rapidly upregulated in senescing leaf tissues. The promoter of AtWAKL10 contains various defense and hormone responsive elements, and its expression could be significantly induced by exogenous ABA, JA and SA. Moreover, the loss-of-function atwakl10 mutant showed earlier senescence along the course of natural development and accelerated leaf senescence under darkness and hormonal stresses, while plants overexpressing AtWAKL10 showed an opposite trend. Additionally, some defense and senescence related WRKY transcription factors could bind to the promoter of AtWAKL10. In addition, deletion and overexpression of AtWAKL10 caused several specific transcriptional alterations, including genes involved in cell extension, cell wall modification, defense response and senescence related WRKYs, which may be implicated in regulatory mechanisms adopted by AtWAKL10 in controlling leaf senescence. Taken together, these results revealed that AtWAKL10 negatively regulated leaf senescence.

Signal Transduction in Leaf Senescence: Progress and Perspective

Leaf senescence is a degenerative process that is genetically controlled and involves nutrient remobilization prior to the death of leaf tissues. Age is a key developmental determinant of the process along with other senescence inducing factors. At the cellular level, different hormones, signaling molecules, and transcription factors contribute to the regulation of senescence. This review summarizes the recent progress in understanding the complexity of the senescence process with primary focuses on perception and transduction of senescence signals as well as downstream regulatory events. Future directions in this field and potential applications of related techniques in crop improvement will be discussed.

Transcriptome profiling of Gerbera hybrida reveals that stem bending is caused by water stress and regulation of abscisic acid

Background

Gerbera hybrida is one of the most popular cut flowers in the world; however, stem bending, which always happens when gerbera flower harvested from the field, greatly limits its vase life. To date the molecular mechanisms underlying stem bending remain poorly understood.

Results

In this study, we performed high-throughput transcriptome sequencing of gerbera during stem bending using the Illumina sequencing technology. Three cDNA libraries constructed from mRNAs of gerbera stem at stem bending stage 0, 2 and 4 were sequenced. More than 300 million high-quality reads were generated and assembled into 96,492 unigenes. Among them, 34,166 unigenes were functionally annotated based on similarity search with known protein. Sequences derived from plants at different stem bending stages were mapped to the assembled transcriptome, and 9,406 differentially expressed genes (DEGs) were identified. Through Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis, specific pathways were identified during the stem bending process, such as phenylpropanoid biosynthesis pathway, phenylalanine metabolism pathway, starch and sucrose metabolism pathway, and plant hormone signal transduction pathway. A total of 211 transcription factors (TFs), including TF families involved in plant senescence, such as NAC, MYB, WRKY, and AP2/ERF members, as well as TFs related to water stress tolerance, were shown to be regulated during stem bending. Gene Onotology (GO) functional enrichment analysis indicated that key genes involved in responses to osmotic and oxidative stresses were also varied in expression during this process. Furthermore, analysis of DEGs involved in the hormone signaling pathways and determination of endogenous abscisic acid (ABA) content showed that stem bending may be an ethylene-independent process, but regulated by ABA. In short, our findings suggested that the stem bending of cut gerbera may be caused by the involvement of water stress and regulation of ABA during the postharvest life.

Conclusions

The transcriptome sequences provide a valuable resource in revealing the molecular mechanism underlying stem bending of cut flower and offer novel genes that can be used to guide future studies for ornamental plant breeding.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5961-1) contains supplementary material, which is available to authorized users.

The impact of photosynthesis on initiation of leaf senescence

Senescence is the last stage of leaf development preceding the death of the organ, and it is important for nutrient remobilization and for feeding sink tissues. There are many reports on leaf senescence, but the mechanisms initiating leaf senescence are still poorly understood. Leaf senescence is affected by many environmental factors and seems to vary in different species and even varieties of plants, which makes it difficult to generalize the mechanism. Here, we give an overview on studies reporting about alterations in the composition of the photosynthetic electron transport chain in chloroplasts during senescence. We hypothesize that alternative electron flow and related generation of the proton motive force required for ATP synthesis become increasingly important during progression of senescence. We address the generation of reactive oxygen species (ROS) in chloroplasts in the initiation of senescence, retrograde signaling from the chloroplast to the nucleus and ROS-dependent signaling associated with leaf senescence. Finally, a few ideas for increasing crop yields by increasing the chloroplast lifespan are presented.

PhDHS Is Involved in Chloroplast Development in Petunia

Deoxyhypusine synthase (DHS) is encoded by a nuclear gene and is the key enzyme involved in the post-translational activation of the eukaryotic translation initiation factor eIF5A. DHS plays important roles in plant growth and development. To gain a better understanding of DHS, the petunia (Petunia hybrida) PhDHS gene was isolated, and the role of PhDHS in plant growth was analyzed. PhDHS protein was localized to the nucleus and cytoplasm. Virus-mediated PhDHS silencing caused a sectored chlorotic leaf phenotype. Chlorophyll levels and photosystem II activity were reduced, and chloroplast development was abnormal in PhDHS-silenced leaves. In addition, PhDHS silencing resulted in extended leaf longevity and thick leaves. A proteome assay revealed that 308 proteins are upregulated and 266 proteins are downregulated in PhDHS-silenced plants compared with control, among the latter, 21 proteins of photosystem I and photosystem II and 12 thylakoid (thylakoid lumen and thylakoid membrane) proteins. In addition, the mRNA level of PheIF5A-1 significantly decreased in PhDHS-silenced plants, while that of another three PheIF5As were not significantly affected in PhDHS-silenced plants. Thus, silencing of PhDHS affects photosynthesis presumably as an indirect effect due to reduced expression of PheIF5A-1 in petunia. Significance: PhDHS-silenced plants develop yellow leaves and exhibit a reduced level of photosynthetic pigment in mesophyll cells. In addition, arrested development of chloroplasts is observed in the yellow leaves.

Degradation of chlorophyll and synthesis of flavonols during autumn senescence-the story told by individual leaves

Autumn senescence of deciduous trees is characterized by chlorophyll degradation and flavonoid synthesis. In the present study, chlorophyll and flavonol contents were measured every morning and evening during the whole autumn with a non-destructive method from individual leaves of Sorbus aucuparia, Acer platanoides, Betula pendula and Prunus padus. In most of the studied trees, the chlorophyll content of each individual leaf remained constant until a phase of rapid degradation commenced. The fast phase lasted only ~1 week and ended with abscission. In S. aucuparia, contrary to the other species, the chlorophyll content of leaflets slowly but steadily decreased during the whole autumn, but rapid chlorophyll degradation commenced only prior to leaflet abscission also in this species. An increase in flavonols commonly accompanied the rapid degradation of chlorophyll. The results may suggest that each individual tree leaf retains its photosynthetic activity, reflected by a high chlorophyll content, until a rapid phase of chlorophyll degradation and flavonoid synthesis begins. Therefore, in studies of autumn senescence, leaves whose chlorophyll content is decreasing and leaves with summertime chlorophyll content (i.e. the leaves that have not yet started to degrade chlorophyll) should be treated separately.

Initiation, Progression, and Genetic Manipulation of Leaf Senescence

As a representative form of plant senescence, leaf senescence has received the most attention during the last two decades. In this chapter we summarize the initiation of leaf senescence by various internal and external signals, the progression of senescence including switches in gene expression, as well as changes at the biochemical and cellular levels during leaf senescence. Impacts of leaf senescence in agriculture and genetic approaches that have been used in manipulating leaf senescence of crop plants are discussed.

iTRAQ-Based Quantitative Proteomic Analysis Reveals Cold Responsive Proteins Involved in Leaf Senescence in Upland Cotton (Gossypium hirsutum L.)

Premature leaf senescence occurs in the ultimate phase of the plant, and it occurs through a complex series of actions regulated by stress, hormones and genes. In this study, a proteomic analysis was performed to analyze the factors that could induce premature leaf senescence in two cotton cultivars. We successfully identified 443 differential abundant proteins (DAPs) from 7388 high-confidence proteins at four stages between non-premature senescence (NS) and premature senescence (PS), among which 158 proteins were over-accumulated, 238 proteins were down-accumulated at four stages, and 47 proteins displayed overlapped accumulation. All the DAPs were mapped onto 21 different categories on the basis of a Clusters of Orthologous Groups (COG) analysis, and 9 clusters were based on accumulation. Gene Ontology (GO) enrichment results show that processes related to stress responses, including responses to cold temperatures and responses to hormones, are significantly differentially accumulated. More importantly, the enriched proteins were mapped in The Arabidopsis Information Resource (TAIR), showing that 58 proteins play an active role in abiotic stress, hormone signaling and leaf senescence. Among these proteins, 26 cold-responsive proteins (CRPs) are significantly differentially accumulated. The meteorological data showed that the median temperatures declined at approximately 15 days before the onset of aging, suggesting that a decrease in temperature is tightly linked to an onset of cotton leaf senescence. Because accumulations of H₂O₂ and increased jasmonic acid (JA) were detected during PS, we speculate that two pathways associated with JA and H₂O₂ are closely related to premature leaf senescence in cotton.

CaM/BAG5/Hsc70 signaling complex dynamically regulates leaf senescence

Calcium signaling plays an essential role in plant cell physiology, and chaperone-mediated protein folding directly regulates plant programmed cell death. The Arabidopsis thaliana protein AtBAG5 (Bcl-2-associated athanogene 5) is unique in that it contains both a BAG domain capable of binding Hsc70 (Heat shock cognate protein 70) and a characteristic IQ motif that is specific for Ca(2+)-free CaM (Calmodulin) binding and hence acts as a hub linking calcium signaling and the chaperone system. Here, we determined crystal structures of AtBAG5 alone and in complex with Ca(2+)-free CaM. Structural and biochemical studies revealed that Ca(2+)-free CaM and Hsc70 bind AtBAG5 independently, whereas Ca(2+)-saturated CaM and Hsc70 bind AtBAG5 with negative cooperativity. Further in vivo studies confirmed that AtBAG5 localizes to mitochondria and that its overexpression leads to leaf senescence symptoms including decreased chlorophyll retention and massive ROS production in dark-induced plants. Mutants interfering the CaM/AtBAG5/Hsc70 complex formation leads to different phenotype of leaf senescence. Collectively, we propose that the CaM/AtBAG5/Hsc70 signaling complex plays an important role in regulating plant senescence.

The stay-green phenotype of TaNAM-RNAi wheat plants is associated with maintenance of chloroplast structure and high enzymatic antioxidant activity

TaNAM transcription factors play an important role in controlling senescence, which in turn, influences the delivery of nitrogen, iron and other elements to the grain of wheat (Triticum aestivum) plants, thus contributing to grain nutritional value. While lack or diminished expression of TaNAMs determines a stay-green phenotype, the precise effect of these factors on chloroplast structure has not been studied. In this work we focused on the events undergone by chloroplasts in two wheat lines having either control or diminished TaNAM expression due to RNA interference (RNAi). It was found that in RNAi plants maintenance of chlorophyll levels and maximal photochemical efficiency of photosystem II were associated with lack of chloroplast dismantling. Flow cytometer studies and electron microscope analysis showed that RNAi plants conserved organelle ultrastructure and complexity. It was also found that senescence in control plants was accompanied by a low leaf enzymatic antioxidant activity. Lack of chloroplast dismantling in RNAi plants was associated with maintenance of protein and iron concentration in the flag leaf, the opposite being observed in control plants. These data provide a structural basis for the observation that down regulation of TaNAMs confers a functional stay-green phenotype and indicate that the low export of iron and nitrogen from the flag leaf of these plants is concomitant, within the developmental window studied, with lack of chloroplast degradation and high enzymatic antioxidant activity.

AtCOX10, a protein involved in haem o synthesis during cytochrome c oxidase biogenesis, is essential for plant embryogenesis and modulates the progression of senescence

Cytochrome c oxidase (CcO) biogenesis requires several accessory proteins implicated, among other processes, in copper and haem a insertion. In yeast, the farnesyltransferase Cox10p that catalyses the conversion of haem b to haem o is the limiting factor in haem a biosynthesis and is essential for haem a insertion in CcO. In this work, we characterized AtCOX10, a putative Cox10p homologue from Arabidopsis thaliana. AtCOX10 was localized in mitochondria and was able to restore growth of a yeast Δcox10 null mutant on non-fermentable carbon sources, suggesting that it also participates in haem o synthesis. Plants with T-DNA insertions in the coding region of both copies of AtCOX10 could not be recovered, and heterozygous mutant plants showed seeds with embryos arrested at early developmental stages that lacked CcO activity. Heterozygous mutant plants exhibited lower levels of CcO activity and cyanide-sensitive respiration but normal levels of total respiration at the expense of an increase in alternative respiration. AtCOX10 seems to be implicated in the onset and progression of senescence, since heterozygous mutant plants showed a faster decrease in chlorophyll content and photosynthetic performance than wild-type plants after natural and dark-induced senescence. Furthermore, complementation of mutants by expressing AtCOX10 under its own promoter allowed us to obtain plants with T-DNA insertions in both AtCOX10 copies, which showed phenotypic characteristics comparable to those of wild type. Our results highlight the relevance of haem o synthesis in plants and suggest that this process is a limiting factor that influences CcO activity levels, mitochondrial respiration, and plant senescence.

Nitrogen deficiency hinders etioplast development in stems of dark-grown pea (Pisum sativum) shoot cultures

The effects of nitrogen (N) deprivation were studied in etiolated pea plants (Pisum sativum cv. Zsuzsi) grown in shoot cultures. The average shoot lengths decreased and the stems significantly altered considering their pigment contents, 77 K fluorescence spectra and ultrastructural properties. The protochlorophyllide (Pchlide) content and the relative contribution of the 654-655 nm emitting flash-photoactive Pchlide form significantly decreased. The etioplast inner membrane structure characteristically changed: N deprivation correlated with a decrease in the size and number of prolamellar bodies (PLBs). These results show that N deficiency directly hinders the pigment production, as well as the synthesis of other etioplast inner membrane components in etiolated pea stems.

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.

Phosphorus nutrition in Proteaceae and beyond

Proteaceae in southwestern Australia have evolved on some of the most phosphorus-impoverished soils in the world. They exhibit a range of traits that allow them to both acquire and utilize phosphorus highly efficiently. This is in stark contrast with many model plants such as Arabidopsis thaliana and crop species, which evolved on soils where nitrogen is the major limiting nutrient. When exposed to low phosphorus availability, these plants typically exhibit phosphorus-starvation responses, whereas Proteaceae do not. This Review explores the traits that account for the very high efficiency of acquisition and use of phosphorus in Proteaceae, and explores which of these traits are promising for improving the phosphorus efficiency of crop plants.

NAC Transcription Factors in Senescence: From Molecular Structure to Function in Crops

Within the last decade, NAC transcription factors have been shown to play essential roles in senescence, which is the focus of this review. Transcriptome analyses associate approximately one third of Arabidopsis NAC genes and many crop NAC genes with senescence, thereby implicating NAC genes as important regulators of the senescence process. The consensus DNA binding site of the NAC domain is used to predict NAC target genes, and protein interaction sites can be predicted for the intrinsically disordered transcription regulatory domains of NAC proteins. The molecular characteristics of these domains determine the interactions in gene regulatory networks. Emerging local NAC-centered gene regulatory networks reveal complex molecular mechanisms of stress- and hormone-regulated senescence and basic physiological steps of the senescence process. For example, through molecular interactions involving the hormone abscisic acid, Arabidopsis NAP promotes chlorophyll degradation, a hallmark of senescence. Furthermore, studies of the functional rice ortholog, OsNAP, suggest that NAC genes can be targeted to obtain specific changes in lifespan control and nutrient remobilization in crop plants. This is also exemplified by the wheat NAM1 genes which promote senescence and increase grain zinc, iron, and protein content. Thus, NAC genes are promising targets for fine-tuning senescence for increased yield and quality.

Generation of reactive oxygen species in thylakoids from senescing flag leaves of the barley varieties Lomerit and Carina

During senescence, production of reactive oxygen species increased in thylakoids. In two barley varieties, no difference in superoxide production was observed while singlet oxygen production increased only in one variety. During senescence, chlorophyll content decreased and photosynthetic electron transport was inhibited as shown for flag leaves collected from barley varieties Lomerit and Carina grown in the field. Spin trapping electron paramagnetic resonance (EPR) was used to investigate the production of reactive oxygen species in thylakoid membranes during senescence. EPR measurements were performed with specific spin traps to discriminate between singlet oxygen on one hand and reactive oxygen intermediates on the other hand. The results show that the generation of reactive oxygen intermediates increases in both varieties during senescence. Singlet oxygen increased only in the variety cv. Lomerit while it remained constant at a low level in the variety cv. Carina. Measurements in the presence of inhibitors of photosystem II and of the cytochrome b6f complex revealed that in senescing leaves reduction of oxygen at the acceptor side of photosystem I was the major, but not the only source of superoxide anions. This study shows that during senescence the production of individual reactive oxygen species varies in different barley varieties.

Translational researches on leaf senescence for enhancing plant productivity and quality

Leaf senescence is a very important trait that limits yield and biomass accumulation of agronomic crops and reduces post-harvest performance and the nutritional value of horticultural crops. Significant advance in physiological and molecular understanding of leaf senescence has made it possible to devise ways of manipulating leaf senescence for agricultural improvement. There are three major strategies in this regard: (i) plant hormone biology-based leaf senescence manipulation technology, the senescence-specific gene promoter-directed IPT system in particular; (ii) leaf senescence-specific transcription factor biology-based technology; and (iii) translation initiation factor biology-based technology. Among the first strategy, the P SAG12 -IPT autoregulatory senescence inhibition system has been widely explored and successfully used in a variety of plant species for manipulating senescence. The vast majority of the related research articles (more than 2000) showed that crops harbouring the autoregulatory system displayed a significant delay in leaf senescence without any abnormalities in growth and development, a marked increase in grain yield and biomass, dramatic improvement in horticultural performance, and/or enhanced tolerance to drought stress. This technology is approaching commercialization. The transcription factor biology-based and translation initiation factor biology-based technologies have also been shown to be very promising and have great potentials for manipulating leaf senescence in crops. Finally, it is speculated that technologies based on the molecular understanding of nutrient recycling during leaf senescence are highly desirable and are expected to be developed in future translational leaf senescence research.

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.

Lipid turnover during senescence

Rapid turnover of stored triacylglycerol occurs after seed germination, releasing fatty acids that provide carbon and energy for seedling establishment. Glycerolipid and fatty acid turnover that occurs at other times in the plant life cycle, including senescence is less studied. Although the entire pathway of β-oxidation is induced during senescence, Arabidopsis leaf fatty acids turnover at rates 50 fold lower than in seedlings. Major unknowns in lipid turnover include the identity of lipases responsible for degradation of the wide diversity of galactolipid, phospholipid, and other lipid class structures. Also unknown is the relative flux of the acetyl-CoA product of β-oxidation into alternative metabolic pathways. We present an overview of senescence-related glycerolipid turnover and discuss its function(s) and speculate about how it might be controlled to increase the energy density and nutritional content of crops. To better understand regulation of lipid turnover, we developed a database that compiles and plots transcript expression of lipid-related genes during natural leaf senescence of Arabidopsis. The database allowed identification of coordinated patterns of down-regulation of lipid biosynthesis genes and the contrasting groups of genes that increase, including 68 putative lipases.

Overexpression of a LAM domain containing RNA-binding protein LARP1c induces precocious leaf senescence in Arabidopsis

Leaf senescence is the final stage of leaf life history, and it can be regulated by multiple internal and external cues. La-related proteins (LARPs), which contain a well-conserved La motif (LAM) domain and normally a canonical RNA recognition motif (RRM) or noncanonical RRM-like motif, are widely present in eukaryotes. Six LARP genes (LARP1a-1c and LARP6a-6c) are present in Arabidopsis, but their biological functions have not been studied previously. In this study, we investigated the biological roles of LARP1c from the LARP1 family. Constitutive or inducible overexpression of LARP1c caused premature leaf senescence. Expression levels of several senescence-associated genes and defense-related genes were elevated upon overexpression of LARP1c. The LARP1c null mutant 1c-1 impaired ABA-, SA-, and MeJA-induced leaf senescence in detached leaves. Gene expression profiles of LARP1c showed age-dependent expression in rosette leaves. Taken together, our results suggest LARP1c is involved in regulation of leaf senescence.

Over-expression of Arabidopsis Bax inhibitor-1 delays methyl jasmonate-induced leaf senescence by suppressing the activation of MAP kinase 6

Methyl jasmonate (MeJA) is an important signalling molecule that has been reported to be able to promote plant senescence. The cell death suppressor Bax inhibitor-1 (BI1) has been found to suppress stress factor-mediated cell death in yeast and Arabidopsis. However, the effect and the genetic mechanism of Arabidopsis thaliana BI1 (AtBI1) on leaf senescence remain unclear. It was found here that the AtBI1 mutant, atbi1-2 (a gene knock-out), showed accelerated progression of MeJA-induced leaf senescence, while the AtBI1 complementation lines displayed similar symptoms as the WT during the senescence process. In addition, over-expression of the AtBI1 gene delayed the onset of MeJA-induced leaf senescence. Further analyses showed that during the process of MeJA-induced senescence, the activity of MPK6, a mitogen-activated protein kinase (MAPK), increased in WT plants, whereas it was significantly suppressed in AtBI1-overexpressing plants. Under the MeJA treatment, cytosolic calcium ([Ca(2+)](cyt)) functioned upstream of MPK6 activation and the elevation of [Ca(2+)](cyt) was reduced in AtBI1-overexpressing leaves. These results suggested a role of AtBI1 over-expression in delaying MeJA-induced leaf senescence by suppressing the [Ca(2+)](cyt)-dependent activation of MPK6, thus providing a new insight into the function and mechanism of AtBI1 in plant senescence.

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.

Can the capacity for isoprene emission acclimate to environmental modifications during autumn senescence in temperate deciduous tree species Populus tremula?

Changes in isoprene emission (Φ(isoprene)), and foliage photosynthetic (A) rates, isoprene precursor dimethylallyldiphosphate (DMADP), and nitrogen and carbon contents were studied from late summer to intensive leaf fall in Populus tremula to gain insight into the emission controls by temperature and endogenous, senescence-induced, modifications. Methanol emissions, characterizing degradation of cell wall pectins, were also measured. A rapid reduction in Φ(isoprene) and A of 60-70% of the initial value was observed in response to a rapid reduction of ambient temperature by ca. 15°C (cold stress). Later phases of senescence were associated with further reductions in Φ(isoprene) and A, with simultaneous major decrease in nitrogen content. However, during episodes of temperature increase, A and in particular, Φ(isoprene) partly recovered. Variation in Φ(isoprene) during senescence was correlated with average temperature of preceding days, with the highest degree of explained variance observed with average temperature of 6 days. Throughout the study, methanol emissions were small, but a large burst of methanol emission was associated with leaf yellowing and abscission. Overall, these data demonstrate that the capacity for isoprene emission can adjust to environmental conditions in senescing leaves as well, but the responsiveness is low compared with mid-season and is also affected by stress.

The role of transition metal homeostasis in plant seed development

For human health, transition metal accumulation in edible seeds like cereal grains is of worldwide importance, since Fe and Zn deficiencies are among the most prevalent human nutritional disorders in the world. There have been many recent developments in our understanding of the patterns in which transition metals accumulate in the seeds, the identity of some specific transporters that are required for efficient seed metal accumulation, and the central role played by the ubiquitous plant metal chelator nicotianamine (NA). These and other recent discoveries will be reviewed here.

From sunlight to phytomass: on the potential efficiency of converting solar radiation to phyto-energy

The relationship between solar radiation capture and potential plant growth is of theoretical and practical importance. The key processes constraining the transduction of solar radiation into phyto-energy (i.e. free energy in phytomass) were reviewed to estimate potential solar-energy-use efficiency. Specifically, the out-put:input stoichiometries of photosynthesis and photorespiration in C(3) and C(4) systems, mobilization and translocation of photosynthate, and biosynthesis of major plant biochemical constituents were evaluated. The maintenance requirement, an area of important uncertainty, was also considered. For a hypothetical C(3) grain crop with a full canopy at 30°C and 350 ppm atmospheric [CO(2) ], theoretically potential efficiencies (based on extant plant metabolic reactions and pathways) were estimated at c. 0.041 J J(-1) incident total solar radiation, and c. 0.092 J J(-1) absorbed photosynthetically active radiation (PAR). At 20°C, the calculated potential efficiencies increased to 0.053 and 0.118 J J(-1) (incident total radiation and absorbed PAR, respectively). Estimates for a hypothetical C(4) cereal were c. 0.051 and c. 0.114 J J(-1), respectively. These values, which cannot be considered as precise, are less than some previous estimates, and the reasons for the differences are considered. Field-based data indicate that exceptional crops may attain a significant fraction of potential efficiency.

Differential impact of chronic ozone exposure on expanding and fully expanded poplar leaves

Populus tremula L. × Populus alba L. (Populus ×c anescens (Aiton) Smith) - clone INRA 717-1-B4 saplings (50 cm apex to base and carrying 19 leaves on average) - were followed for 28 days. Half of the trees were grown in charcoal-filtered air while the other half were exposed to 120 ppb ozone for 11 h a day during the light period. The expanding leaf number 4 was tagged at the beginning of the experiment and finished expansion between 7 and 14 days. These leaves were harvested weekly for biochemical and proteome analyses using quantitative bidimensional electrophoresis (DiGE). Independent of the ozone treatment, all the analyses allowed a distinction between expanding and adult leaves. The results indicate that during the expansion phase (Days 0-7) the enzymatic machinery of the leaves is set up, and remains dynamically stable in the adult leaves (Days 14-28). Although ozone had no apparent effect on expanding leaves, the metabolic stability in fully expanded leaves observed in ozone-free plants was disturbed after 2 weeks of exposure and a stress-induced response became apparent.

Overexpression of Arabidopsis acyl-CoA binding protein ACBP3 promotes starvation-induced and age-dependent leaf senescence

In Arabidopsis thaliana, a family of six genes (ACBP1 to ACBP6) encodes acyl-CoA binding proteins (ACBPs). Investigations on ACBP3 reported here show its upregulation upon dark treatment and in senescing rosettes. Transgenic Arabidopsis overexpressing ACBP3 (ACBP3-OEs) displayed accelerated leaf senescence, whereas an acbp3 T-DNA insertional mutant and ACBP3 RNA interference transgenic Arabidopsis lines were delayed in dark-induced leaf senescence. Acyl-CoA and lipid profiling revealed that the overexpression of ACBP3 led to an increase in acyl-CoA and phosphatidylethanolamine (PE) levels, whereas ACBP3 downregulation reduced PE content. Moreover, significant losses in phosphatidylcholine (PC) and phosphatidylinositol, and gains in phosphatidic acid (PA), lysophospholipids, and oxylipin-containing galactolipids (arabidopsides) were evident in 3-week-old dark-treated and 6-week-old premature senescing ACBP3-OEs. Such accumulation of PA and arabidopsides (A, B, D, E, and G) resulting from lipid peroxidation in ACBP3-OEs likely promoted leaf senescence. The N-terminal signal sequence/transmembrane domain in ACBP3 was shown to be essential in ACBP3-green fluorescent protein targeting and in promoting senescence. Observations that recombinant ACBP3 binds PC, PE, and unsaturated acyl-CoAs in vitro and that ACBP3 overexpression enhances degradation of the autophagy (ATG)-related protein ATG8 and disrupts autophagosome formation suggest a role for ACBP3 as a phospholipid binding protein involved in the regulation of leaf senescence by modulating membrane phospholipid metabolism and ATG8 stability in Arabidopsis. Accelerated senescence in ACBP3-OEs is dependent on salicylic acid but not jasmonic acid signaling.

Senescence-induced loss in photosynthesis enhances cell wall beta-glucosidase activity

A link between senescence-induced decline in photosynthesis and activity of beta-glucosidase is examined in the leaves of Arabidopsis. The enzyme is purified and characterized. The molecular weight of the enzyme is 58 kDa. It shows maximum activity at pH 5.5 and at temperature of 50 degrees C. Photosynthetic measurements and activity of the enzyme are conducted at different developmental stages including senescence of leaves. Senescence causes a significant loss in total chlorophyll, stomatal conductance, rate of evaporation and in the ability of the leaves for carbon dioxide fixation. The process also brings about a decline in oxygen evolution, quantum yield of photosystem II (PS II) and quantum efficiency of PS II photochemistry of thylakoid membrane. The loss in photosynthesis is accompanied by a significant increase in the activity of the cell wall-bound beta-glucosidase that breaks down polysaccharides to soluble sugars. The loss in photosynthesis as a signal for the enhancement in the activity of the enzyme is confirmed from the observation that incubation of excised mature leaves in continuous dark or in light with a photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) that leads to sugar starvation enhances the activity of the enzyme. The work suggests that in the background of photosynthetic decline, the polysaccharides bound to cell wall that remains intact even during late phase of senescence may be the last target of senescing leaves for a possible source of sugar for remobilization and completion of the energy-dependent senescence program.

Proteomic analysis of pollination-induced corolla senescence in petunia

Senescence represents the last phase of petal development during which macromolecules and organelles are degraded and nutrients are recycled to developing tissues. To understand better the post-transcriptional changes regulating petal senescence, a proteomic approach was used to profile protein changes during the senescence of Petuniaxhybrida 'Mitchell Diploid' corollas. Total soluble proteins were extracted from unpollinated petunia corollas at 0, 24, 48, and 72 h after flower opening and at 24, 48, and 72 h after pollination. Two-dimensional gel electrophoresis (2-DE) was used to identify proteins that were differentially expressed in non-senescing (unpollinated) and senescing (pollinated) corollas, and image analysis was used to determine which proteins were up- or down-regulated by the experimentally determined cut-off of 2.1-fold for P <0.05. One hundred and thirty-three differentially expressed protein spots were selected for sequencing. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to determine the identity of these proteins. Searching translated EST databases and the NCBI non-redundant protein database, it was possible to assign a putative identification to greater than 90% of these proteins. Many of the senescence up-regulated proteins were putatively involved in defence and stress responses or macromolecule catabolism. Some proteins, not previously characterized during flower senescence, were identified, including an orthologue of the tomato abscisic acid stress ripening protein 4 (ASR4). Gene expression patterns did not always correlate with protein expression, confirming that both proteomic and genomic approaches will be required to obtain a detailed understanding of the regulation of petal senescence.

Proteomics of the response of Arabidopsis thaliana to infection with Alternaria brassicicola

We have studied the proteome of the model plant Arabidopsis thaliana infected with a necrotrophic fungal pathogen, Alternaria brassicicola. The Arabidopsis-A. brassicicola host-pathogen pair is being developed as a model genetic system for incompatible plant-fungal interactions, in which the spread of disease is limited by plant defense responses. After confirming that a defense response was induced at the transcriptional level, we identified proteins whose abundance on 2-DE gels increased or decreased in infected leaves. At least 11 protein spots showed reproducible differences in abundance, increasing or decreasing during the progress of the infection. The pathogenesis-related protein PR4, a glycosyl hydrolase, and the antifungal protein osmotin are strongly up-regulated. Two members of the Arabidopsis glutathione S-transferase (GST) family increased in abundance in infected leaves. The spots in which these GST proteins were identified contain additional members of the GST family. Representation of GST family members in several protein spots migrating at similar molecular weight suggests post-translational modifications. The signature of GST regulation may be specific for the type of plant-pathogen interaction. The proteomic view of the defense response to A. brassicicola can be compared with other types of plant-pathogen interactions, and to leaf senescence, identifying unique regulatory patterns.

Pyrrolizidine alkaloid biosynthesis, evolution of a pathway in plant secondary metabolism

The system of pyrrolizidine alkaloids has proven to be a powerful system for studying the evolution of a biosynthetic pathway in plant secondary metabolism. Pyrrolizidine alkaloids are typical plant secondary products produced by the plant as a defense against herbivores. The first specific enzyme, homospermidine synthase, has been shown to have evolved by duplication of the gene encoding deoxyhypusine synthase, which is involved in primary metabolism. Despite the identical function of homospermidine synthase for pyrrolizidine alkaloid biosynthesis in the various plant lineages, this gene duplication has occurred several times independently during angiosperm evolution. After duplication, these gene copies diverged with respect to gene function and regulation. In the diverse plant lineages producing pyrrolizidine alkaloids, homospermidine synthase has been shown to be expressed in a variety of tissues, suggesting that the regulatory elements were recruited individually after the duplication of the structural gene. The molecular, kinetic, and expression data of this system are discussed with respect to current models of gene and pathway evolution.

Homospermidine in transgenic tobacco results in considerably reduced spermidine levels but is not converted to pyrrolizidine alkaloid precursors

Homospermidine synthase is the first specific enzyme in the biosynthesis of pyrrolizidine alkaloids. Whereas the substrates putrescine and spermidine are part of the highly dynamic polyamine pool of plants, the product homospermidine is incorporated exclusively into the necine base moiety of pyrrolizidine alkaloids. Recently, the gene encoding homospermidine synthase has been shown to have been recruited several times independently during angiosperm evolution by the duplication of the gene encoding deoxyhypusine synthase. To test whether high levels of homospermidine suffice for conversion, at least in traces, to precursors of pyrrolizidine alkaloids, transgenic tobacco plants were generated expressing homospermidine synthase. Analyses of the polyamine content revealed that, in the transgenic plants, about 80% of spermidine was replaced by homospermidine without any conspicuous modifications of the phenotype. Tracer-feeding experiments and gas chromatographic analyses suggested that these high levels of homospermidine were not sufficient to explain the formation of alkaloid precursors. These results are discussed with respect to current models of pathway evolution.

Evolution of plant senescence

BACKGROUND

Senescence is integral to the flowering plant life-cycle. Senescence-like processes occur also in non-angiosperm land plants, algae and photosynthetic prokaryotes. Increasing numbers of genes have been assigned functions in the regulation and execution of angiosperm senescence. At the same time there has been a large expansion in the number and taxonomic spread of plant sequences in the genome databases. The present paper uses these resources to make a study of the evolutionary origins of angiosperm senescence based on a survey of the distribution, across plant and microbial taxa, and expression of senescence-related genes.

RESULTS

Phylogeny analyses were carried out on protein sequences corresponding to genes with demonstrated functions in angiosperm senescence. They include proteins involved in chlorophyll catabolism and its control, homeoprotein transcription factors, metabolite transporters, enzymes and regulators of carotenoid metabolism and of anthocyanin biosynthesis. Evolutionary timelines for the origins and functions of particular genes were inferred from the taxonomic distribution of sequences homologous to those of angiosperm senescence-related proteins. Turnover of the light energy transduction apparatus is the most ancient element in the senescence syndrome. By contrast, the association of phenylpropanoid metabolism with senescence, and integration of senescence with development and adaptation mediated by transcription factors, are relatively recent innovations of land plants. An extended range of senescence-related genes of Arabidopsis was profiled for coexpression patterns and developmental relationships and revealed a clear carotenoid metabolism grouping, coordinated expression of genes for anthocyanin and flavonoid enzymes and regulators and a cluster pattern of genes for chlorophyll catabolism consistent with functional and evolutionary features of the pathway.

CONCLUSION

The expression and phylogenetic characteristics of senescence-related genes allow a framework to be constructed of decisive events in the evolution of the senescence syndrome of modern land-plants. Combining phylogenetic, comparative sequence, gene expression and morphogenetic information leads to the conclusion that biochemical, cellular, integrative and adaptive systems were progressively added to the ancient primary core process of senescence as the evolving plant encountered new environmental and developmental contexts.

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.

The ATG autophagic conjugation system in maize: ATG transcripts and abundance of the ATG8-lipid adduct are regulated by development and nutrient availability

Plants employ sophisticated mechanisms to recycle intracellular constituents needed for growth, development, and survival under nutrient-limiting conditions. Autophagy is one important route in which cytoplasm and organelles are sequestered in bulk into vesicles and subsequently delivered to the vacuole for breakdown by resident hydrolases. The formation and trafficking of autophagic vesicles are directed in part by associated conjugation cascades that couple the AUTOPHAGY-RELATED8 (ATG8) and ATG12 proteins to their respective targets, phosphatidylethanolamine and the ATG5 protein. To help understand the importance of autophagy to nutrient remobilization in cereals, we describe here the ATG8/12 conjugation cascades in maize (Zea mays) and examine their dynamics during development, leaf senescence, and nitrogen and fixed-carbon starvation. From searches of the maize genomic sequence using Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) counterparts as queries, we identified orthologous loci encoding all components necessary for ATG8/12 conjugation, including a five-member gene family expressing ATG8. Alternative splicing was evident for almost all Atg transcripts, which could have important regulatory consequences. In addition to free ATG8, its membrane-associated, lipidated form was detected in many maize tissues, suggesting that its conjugation cascade is active throughout the plant at most, if not all, developmental stages. Levels of Atg transcripts and/or the ATG8-phosphatidylethanolamine adduct increase during leaf senescence and nitrogen and fixed-carbon limitations, indicating that autophagy plays a key role in nutrient remobilization. The description of the maize ATG system now provides a battery of molecular and biochemical tools to study autophagy in this crop under field conditions.

Wheat (Triticum aestivum) NAM proteins regulate the translocation of iron, zinc, and nitrogen compounds from vegetative tissues to grain

The NAM-B1 gene is a NAC transcription factor that affects grain nutrient concentrations in wheat (Triticum aestivum). An RNAi line with reduced expression of NAM genes has lower grain protein, iron (Fe), and zinc (Zn) concentrations. To determine whether decreased remobilization, lower plant uptake, or decreased partitioning to grain are responsible for this phenotype, mineral dynamics were quantified in wheat tissues throughout grain development. Control and RNAi wheat were grown in potting mix and hydroponics. Mineral (Ca, Cu, Fe, K, Mg, Mn, P, S, and Zn) and nitrogen (N) contents of organs were determined at regular intervals to quantify the net remobilization from vegetative tissues and the accumulation of nutrients in grain. Total nutrient accumulation was similar between lines, but grain Fe, Zn, and N were at lower concentrations in the NAM knockdown line. In potting mix, net remobilization of N, Fe, and Zn from vegetative tissues was impaired in the RNAi line. In hydroponics with ample nutrients, net remobilization was not observed, but grain Fe and Zn contents and concentrations remained lower in the RNAi line. When Fe or Zn was withheld post-anthesis, both lines demonstrated remobilization. These results suggest that a major effect of the NAM genes is an increased efflux of nutrients from the vegetative tissues and a higher partitioning of nutrients to grain.

Effects of a barley (Hordeum vulgare) chromosome 6 grain protein content locus on whole-plant nitrogen reallocation under two different fertilisation regimes

A large fraction of protein N harvested with crop seeds is derived from N remobilisation from senescing vegetative plant parts, while a smaller fraction stems from de novo N assimilation occurring after anthesis. This study contrasts near-isogenic barley (Hordeum vulgare L.) germplasm, varying in the allelic state of a major grain protein content (GPC) locus on chromosome 6. Plant material was grown under both low- and high-N fertilisation levels. The analyses indicated that leaf N remobilisation occurred earlier in high-GPC germplasm under both fertilisation regimes, as indicated by an earlier decrease of total leaf N, chlorophylls, soluble- and membrane-proteins. At the same time, kernel free amino acid levels were enhanced, while leaf free amino acid levels were lower in high-GPC barleys, suggesting enhanced retranslocation of organic N to the developing sinks. Enhanced or longer availability of leaf nitrates was detected in high-GPC varieties and lines, at least under high N fertilisation, indicating that the GPC locus profoundly influences whole-plant N allocation and management. Results presented here, together with data from a recent transcriptomic analysis, make a substantial contribution to our understanding of whole-plant N storage, remobilisation and retranslocation to developing sinks.

Nondestructive analysis of senescence in mesophyll cells by spectral resolution of protein synthesis-dependent pigment metabolism

* Over 6 d of dark-induced senescence, leaf segments of wild-type Lolium temulentum lost > 96% chlorophyll a + b; leaves from plants containing a staygreen mutation introgressed from Festuca pratensis, which has a lesion in the senescence-associated fragmentation of pigment-proteolipid complexes, retained over 43% of total chlorophyll over the same period. * Mutant segments preferentially retained thylakoid membrane proteins (exemplified by LHCP II) but lost other cellular proteins at the same rate as wild-type tissue. The protein synthesis inhibitor D-MDMP inhibited chlorophyll degradation and partially prevented protein loss in both genotypes, but tissues treated with the ineffective L-stereoisomer were indistinguishable from water controls. * Principal-components analysis of leaf reflectance spectra distinguished between genotypes, time points and D-MDMP treatments, showing the disruption of pigment metabolism during senescence brought about by the staygreen mutation, by inhibition of protein synthesis and by combinations of the two factors. * The build-up of oxidized, dephytylated and phaeo-derivatives of chl a during senescence of staygreen tissue was prevented by D-MDMP and associated with characteristic difference spectra when senescent mutant tissue was compared with wild-type or inhibitor-treated samples. The suitability of senescence as a subject for systems biology approaches is discussed.

Physiology and molecular biology of petal senescence

Petal senescence is reviewed, with the main emphasis on gene expression in relation to physiological functions. Autophagy seems to be the major mechanism for large-scale degradation of macromolecules, but it is still unclear if it contributes to cell death. Depending on the species, petal senescence is controlled by ethylene or is independent of this hormone. EIN3-like (EIL) transcription factors are crucial in ethylene-regulated senescence. The presence of adequate sugar levels in the cell delays senescence and prevents an increase in the levels of EIL mRNA and the subsequent up-regulation of numerous senescence-associated genes. A range of other transcription factors and regulators are differentially expressed in ethylene-sensitive and ethylene-insensitive petal senescence. Ethylene-independent senescence is often delayed by cytokinins, but it is still unknown whether these are natural regulators. A role for caspase-like enzymes or metacaspases has as yet not been established in petal senescence, and a role for proteins released by organelles such as the mitochondrion has not been shown. The synthesis of sugars, amino acids, and fatty acids, and the degradation of nucleic acids, proteins, lipids, fatty acids, and cell wall components are discussed. It is claimed that there is not enough experimental support for the widely held view that a gradual increase in cell leakiness, resulting from gradual plasma membrane degradation, is an important event in petal senescence. Rather, rupture of the vacuolar membrane and subsequent rapid, complete degradation of the plasma membrane seems to occur. This review recommends that more detailed analysis be carried out at the level of cells and organelles rather than at that of whole petals.

Jasmonate signalling network in Arabidopsis thaliana: crucial regulatory nodes and new physiological scenarios

Plant development and stress responses are regulated by complex signalling networks that mediate specific and dynamic plant responses upon activation by various types of exogenous and endogenous signal. In this review, we focus on the latest published work on jasmonate (JA) signalling components and new regulatory nodes in the transcriptional network that regulates a number of diverse plant responses to developmental and environmental cues. Not surprisingly, the majority of the key revelations in the field have been made in Arabidopsis thaliana. However, for comparative reasons, we integrate information on Arabidopsis with recent reports for other plant species (when available). Recent findings on the regulation of plant responses to pathogens by JAs, as well as new evidence implicating JAs in the regulation of senescence, suggest a common mechanism of JA action in these responses via distinct groups of transcription factors. Moreover, a significant increase in the amount of evidence has allowed placing of specific mitogen-activated protein kinases (MAPKs) as crucial regulatory nodes in the defence signalling network. In addition, we report on new physiological scenarios for JA signalling, such as organogenesis of nitrogen-fixing nodules and anticancer therapy.

Integrated signaling in flower senescence: an overview

Flower senescence is the terminal phase of developmental processes that lead to the death of flower, which include, flower wilting, shedding of flower parts and fading of blossoms. Since it is a rapid process as compared to the senescence of other parts of the plant it therefore provides excellent model system for the study of senescence. During flower senescence, developmental and environmental stimuli enhance the upregulation of catabolic processes causing breakdown and remobilization of cellular constituents. Ethylene is well known to play regulatory role in ethylene-sensitive flowers while in ethylene-insensitive flowers abscisic acid (ABA) is thought to be primary regulator. Subsequent to perception of flower senescence signal, death of petals is accompanied by the loss of membrane permeability, increase in oxidative and decreased level of protective enzymes. The last stages of senescence involve the loss of of nucleic acids (DNA and RNA), proteins and organelles, which is achieved by activation of several nucleases, proteases and wall modifiers. Environmental stimuli such as pollination, drought and other stresses also affect senescence by hormonal imbalance. In this article we have covered the following: perception mechanism and specificity of flower senescence, flower senescence-associated events, like degradation of cell membranes, proteins and nucleic acids, environmental/external factors affecting senescence, like pollination and abiotic stress, hormonal and non-hormonal regulation of flower/petal senescence and finally the senescence associated genes (SAGs) have also been described.