ORE9, an F-box protein that regulates leaf senescence in Arabidopsis

Age-triggered and dark-induced leaf senescence require the bHLH transcription factors PIF3, 4, and 5

Leaf senescence can be triggered and promoted by a large number of developmental and environmental factors. Numerous lines of evidence have suggested an involvement of phytochromes in the regulation of leaf senescence, but the related signaling pathways and physiological mechanisms are poorly understood. In this study, we initially identified phytochrome-interacting factors (PIFs) 3, 4, and 5 as putative mediators of leaf senescence. Mutations of the PIF genes resulted in a significantly enhanced leaf longevity in age-triggered and dark-induced senescence, whereas overexpressions of these genes accelerated age-triggered and dark-induced senescence in Arabidopsis. Consistently, loss-of-function of PIF4 attenuated dark-induced transcriptional changes associated with chloroplast deterioration and reactive oxygen species (ROS) generation. ChIP-PCR and Dual-Luciferase assays demonstrated that PIF4 can activate chlorophyll degradation regulatory gene NYE1 and repress chloroplast activity maintainer gene GLK2 by binding to their promoter regions. Finally, dark-induced ethylene biosynthesis and ethylene-induced senescence were both dampened in pif4, suggesting the involvement of PIF4 in both ethylene biosynthesis and signaling pathway. Our study provides evidence that PIF3, 4, and 5 are novel positive senescence mediators and gains an insight into the mechanism of light signaling involved in the regulation of leaf senescence.

Redox regulation of plant development

SIGNIFICANCE

We provide a conceptual framework for the interactions between the cellular redox signaling hub and the phytohormone signaling network that controls plant growth and development to maximize plant productivity under stress-free situations, while limiting growth and altering development on exposure to stress.

RECENT ADVANCES

Enhanced cellular oxidation plays a key role in the regulation of plant growth and stress responses. Oxidative signals or cycles of oxidation and reduction are crucial for the alleviation of dormancy and quiescence, activating the cell cycle and triggering genetic and epigenetic control that underpin growth and differentiation responses to changing environmental conditions.

CRITICAL ISSUES

The redox signaling hub interfaces directly with the phytohormone network in the synergistic control of growth and its modulation in response to environmental stress, but a few components have been identified. Accumulating evidence points to a complex interplay of phytohormone and redox controls that operate at multiple levels. For simplicity, we focus here on redox-dependent processes that control root growth and development and bud burst.

FUTURE DIRECTIONS

The multiple roles of reactive oxygen species in the control of plant growth and development have been identified, but increasing emphasis should now be placed on the functions of redox-regulated proteins, along with the central roles of reductants such as NAD(P)H, thioredoxins, glutathione, glutaredoxins, peroxiredoxins, ascorbate, and reduced ferredoxin in the regulation of the genetic and epigenetic factors that modulate the growth and vigor of crop plants, particularly within an agricultural context.

Redox regulation of plant development

SIGNIFICANCE

We provide a conceptual framework for the interactions between the cellular redox signaling hub and the phytohormone signaling network that controls plant growth and development to maximize plant productivity under stress-free situations, while limiting growth and altering development on exposure to stress.

RECENT ADVANCES

Enhanced cellular oxidation plays a key role in the regulation of plant growth and stress responses. Oxidative signals or cycles of oxidation and reduction are crucial for the alleviation of dormancy and quiescence, activating the cell cycle and triggering genetic and epigenetic control that underpin growth and differentiation responses to changing environmental conditions.

CRITICAL ISSUES

The redox signaling hub interfaces directly with the phytohormone network in the synergistic control of growth and its modulation in response to environmental stress, but a few components have been identified. Accumulating evidence points to a complex interplay of phytohormone and redox controls that operate at multiple levels. For simplicity, we focus here on redox-dependent processes that control root growth and development and bud burst.

FUTURE DIRECTIONS

The multiple roles of reactive oxygen species in the control of plant growth and development have been identified, but increasing emphasis should now be placed on the functions of redox-regulated proteins, along with the central roles of reductants such as NAD(P)H, thioredoxins, glutathione, glutaredoxins, peroxiredoxins, ascorbate, and reduced ferredoxin in the regulation of the genetic and epigenetic factors that modulate the growth and vigor of crop plants, particularly within an agricultural context.

Strigolactone signaling regulates rice leaf senescence in response to a phosphate deficiency

Strigolactones (SLs) act as plant hormones that inhibit shoot branching and stimulate secondary growth of the stem, primary root growth, and root hair elongation. In the moss Physcomitrella patens, SLs regulate branching of chloronemata and colony extension. In addition, SL-deficient and SL-insensitive mutants show delayed leaf senescence. To explore the effects of SLs on leaf senescence in rice (Oryza sativa L.), we treated leaf segments of rice dwarf mutants with a synthetic SL analogue, GR24, and evaluated their chlorophyll contents, ion leakage, and expression levels of senescence-associated genes. Exogenously applied GR24 restored normal leaf senescence in SL-deficient mutants, but not in SL-insensitive mutants. Most plants highly produce endogenous SLs in response to phosphate deficiency. Thus, we evaluated effects of GR24 under phosphate deficiency. Chlorophyll levels did not differ of in the wild-type between the sufficient and deficient phosphate conditions, but increased in the SL-deficient mutants under phosphate deficiency, leading in the strong promotion of leaf senescence by GR24 treatment. These results indicate that the mutants exhibited increased responsiveness to GR24 under phosphate deficiency. In addition, GR24 accelerated leaf senescence in the intact SL-deficient mutants under phosphate deficiency as well as dark-induced leaf senescence. The effects of GR24 were stronger in d10 compared to d17. Based on these results, we suggest that SLs regulate leaf senescence in response to phosphate deficiency.

Strigolactone signaling regulates rice leaf senescence in response to a phosphate deficiency

Strigolactones (SLs) act as plant hormones that inhibit shoot branching and stimulate secondary growth of the stem, primary root growth, and root hair elongation. In the moss Physcomitrella patens, SLs regulate branching of chloronemata and colony extension. In addition, SL-deficient and SL-insensitive mutants show delayed leaf senescence. To explore the effects of SLs on leaf senescence in rice (Oryza sativa L.), we treated leaf segments of rice dwarf mutants with a synthetic SL analogue, GR24, and evaluated their chlorophyll contents, ion leakage, and expression levels of senescence-associated genes. Exogenously applied GR24 restored normal leaf senescence in SL-deficient mutants, but not in SL-insensitive mutants. Most plants highly produce endogenous SLs in response to phosphate deficiency. Thus, we evaluated effects of GR24 under phosphate deficiency. Chlorophyll levels did not differ of in the wild-type between the sufficient and deficient phosphate conditions, but increased in the SL-deficient mutants under phosphate deficiency, leading in the strong promotion of leaf senescence by GR24 treatment. These results indicate that the mutants exhibited increased responsiveness to GR24 under phosphate deficiency. In addition, GR24 accelerated leaf senescence in the intact SL-deficient mutants under phosphate deficiency as well as dark-induced leaf senescence. The effects of GR24 were stronger in d10 compared to d17. Based on these results, we suggest that SLs regulate leaf senescence in response to phosphate deficiency.

Gene regulatory cascade of senescence-associated NAC transcription factors activated by ETHYLENE-INSENSITIVE2-mediated leaf senescence signalling in Arabidopsis

Leaf senescence is a finely tuned and genetically programmed degeneration process, which is critical to maximize plant fitness by remobilizing nutrients from senescing leaves to newly developing organs. Leaf senescence is a complex process that is driven by extensive reprogramming of global gene expression in a highly coordinated manner. Understanding how gene regulatory networks involved in controlling leaf senescence are organized and operated is essential to decipher the mechanisms of leaf senescence. It was previously reported that the trifurcate feed-forward pathway involving EIN2, ORE1, and miR164 in Arabidopsis regulates age-dependent leaf senescence and cell death. Here, new components of this pathway have been identified, which enhances knowledge of the gene regulatory networks governing leaf senescence. Comparative gene expression analysis revealed six senescence-associated NAC transcription factors (TFs) (ANAC019, AtNAP, ANAC047, ANAC055, ORS1, and ORE1) as candidate downstream components of ETHYLENE-INSENSITIVE2 (EIN2). EIN3, a downstream signalling molecule of EIN2, directly bound the ORE1 and AtNAP promoters and induced their transcription. This suggests that EIN3 positively regulates leaf senescence by activating ORE1 and AtNAP, previously reported as key regulators of leaf senescence. Genetic and gene expression analyses in the ore1 atnap double mutant revealed that ORE1 and AtNAP act in distinct and overlapping signalling pathways. Transient transactivation assays further demonstrated that ORE1 and AtNAP could activate common as well as differential NAC TF targets. Collectively, the data provide insight into an EIN2-mediated senescence signalling pathway that coordinates global gene expression during leaf senescence via a gene regulatory network involving EIN3 and senescence-associated NAC TFs.

OsNAP connects abscisic acid and leaf senescence by fine-tuning abscisic acid biosynthesis and directly targeting senescence-associated genes in rice

It has long been established that premature leaf senescence negatively impacts the yield stability of rice, but the underlying molecular mechanism driving this relationship remains largely unknown. Here, we identified a dominant premature leaf senescence mutant, prematurely senile 1 (ps1-D). PS1 encodes a plant-specific NAC (no apical meristem, Arabidopsis ATAF1/2, and cup-shaped cotyledon2) transcriptional activator, Oryza sativa NAC-like, activated by apetala3/pistillata (OsNAP). Overexpression of OsNAP significantly promoted senescence, whereas knockdown of OsNAP produced a marked delay of senescence, confirming the role of this gene in the development of rice senescence. OsNAP expression was tightly linked with the onset of leaf senescence in an age-dependent manner. Similarly, ChIP-PCR and yeast one-hybrid assays demonstrated that OsNAP positively regulates leaf senescence by directly targeting genes related to chlorophyll degradation and nutrient transport and other genes associated with senescence, suggesting that OsNAP is an ideal marker of senescence onset in rice. Further analysis determined that OsNAP is induced specifically by abscisic acid (ABA), whereas its expression is repressed in both aba1 and aba2, two ABA biosynthetic mutants. Moreover, ABA content is reduced significantly in ps1-D mutants, indicating a feedback repression of OsNAP on ABA biosynthesis. Our data suggest that OsNAP serves as an important link between ABA and leaf senescence. Additionally, reduced OsNAP expression leads to delayed leaf senescence and an extended grain-filling period, resulting in a 6.3% and 10.3% increase in the grain yield of two independent representative RNAi lines, respectively. Thus, fine-tuning OsNAP expression should be a useful strategy for improving rice yield in the future.

Removal of DELLA repression promotes leaf senescence in Arabidopsis

Leaf senescence is an integrated response of leaf cells to developmental age and various internal and environmental signals. However, the role of gibberellins (GA) in leaf senescence is not clear. In the current study, we investigated the effect of DELLA on leaf senescence. Compared with the wild type (WT), leaf senescence occurred earlier in the mutant ga1-3 gai-t6 rga-t2 rgl1-1 rgl2-1 (abbreviated as Q-DELLA/ga1-3) whose DELLA repression was removed, whereas leaf senescence was retarded in the mutant ga1-3 whose GA biosynthesis was blocked and whose DELLA proteins accumulated abnormally. During leaf senescence, SAG12 and SAG29 were upregulated in Q-DELLA/ga1-3 and downregulated in ga1-3 plants. The Q-DELLA/ga1-3 senescent leaves contained more sugar but less chlorophyll and fatty acids (FAs) than those of ga1-3 and WT. Both absolute and relative contents of C18:3 in Q-DELLA/ga1-3 senescent leaves were lower compared with those of the WT and ga1-3 leaves. The genes regulating FA β-oxidation in Q-DELLA/ga1-3, such as KAT2, LACS6, LACS7, ACX1, ACX2 and MAP2, were significantly upregulated. The removal of DELLA repression highly upregulated certain genes on various hormone pathways, suggesting that GA signaling acts upstream of the jasmonic acid, salicylic acid, and ethylene pathways in regulating leaf senescence.

Drying without senescence in resurrection plants

Research into extreme drought tolerance in resurrection plants using species such as Craterostigma plantagineum, C. wilmsii, Xerophyta humilis, Tortula ruralis, and Sporobolus stapfianus has provided some insight into the desiccation tolerance mechanisms utilized by these plants to allow them to persist under extremely adverse environmental conditions. Some of the mechanisms used to ensure cellular preservation during severe dehydration appear to be peculiar to resurrection plants. Apart from the ability to preserve vital cellular components during drying and rehydration, such mechanisms include the ability to down-regulate growth-related metabolism rapidly in response to changes in water availability, and the ability to inhibit dehydration-induced senescence programs enabling reconstitution of photosynthetic capacity quickly following a rainfall event. Extensive research on the molecular mechanism of leaf senescence in non-resurrection plants has revealed a multi-layered regulatory network operates to control programed cell death pathways. However, very little is known about the molecular mechanisms that resurrection plants employ to avoid undergoing drought-related senescence during the desiccation process. To survive desiccation, dehydration in the perennial resurrection grass S. stapfianus must proceed slowly over a period of 7 days or more. Leaves detached from the plant before 60% relative water content (RWC) is attained are desiccation-sensitive indicating that desiccation tolerance is conferred in vegetative tissue of S. stapfianus when the leaf RWC has declined to 60%. Whilst some older leaves remaining attached to the plant during dehydration will senesce, suggesting dehydration-induced senescence may be influenced by leaf age or the rate of dehydration in individual leaves, the majority of leaves do not senesce. Rather these leaves dehydrate to air-dryness and revive fully following rehydration. Hence it seems likely that there are genes expressed in younger leaf tissues of resurrection plants that enable suppression of drought-related senescence pathways. As very few studies have directly addressed this phenomenon, this review aims to discuss current literature surrounding the activation and suppression of senescence pathways and how these pathways may differ in resurrection plants.

A novel F-box protein CaF-box is involved in responses to plant hormones and abiotic stress in pepper (Capsicum annuum L.)

The F-box protein family is characterized by an F-box motif that has been shown to play an important role in regulating various developmental processes and stress responses. In this study, a novel F-box-containing gene was isolated from leaves of pepper cultivar P70 (Capsicum annuum L.) and designated CaF-box. The full-length cDNA is 2088 bp and contains an open reading frame of 1914 bp encoding a putative polypeptide of 638 amino acids with a mass of 67.8 kDa. CaF-box was expressed predominantly in stems and seeds, and the transcript was markedly upregulated in response to cold stress, abscisic acid (ABA) and salicylic acid (SA) treatment, and downregulated under osmotic and heavy metal stress. CaF-box expression was dramatically affected by salt stress, and was rapidly increased for the first hour, then sharply decreased thereafter. In order to further assess the role of CaF-box in the defense response to abiotic stress, a loss-of-function experiment in pepper plants was performed using a virus-induced gene silencing (VIGS) technique. Measurement of thiobarbituric acid reactive substances (TBARS) and electrolyte leakage revealed stronger lipid peroxidation and cell death in the CaF-box-silenced plants than in control plants, suggesting CaF-box plays an important role in regulating the defense response to abiotic stress resistance in pepper plants.

Regulation of drought tolerance by the F-box protein MAX2 in Arabidopsis

MAX2 (for MORE AXILLARY GROWTH2) has been shown to regulate diverse biological processes, including plant architecture, photomorphogenesis, senescence, and karrikin signaling. Although karrikin is a smoke-derived abiotic signal, a role for MAX2 in abiotic stress response pathways is least investigated. Here, we show that the max2 mutant is strongly hypersensitive to drought stress compared with wild-type Arabidopsis (Arabidopsis thaliana). Stomatal closure of max2 was less sensitive to abscisic acid (ABA) than that of the wild type. Cuticle thickness of max2 was significantly thinner than that of the wild type. Both of these phenotypes of max2 mutant plants correlate with the increased water loss and drought-sensitive phenotype. Quantitative real-time reverse transcription-polymerase chain reaction analyses showed that the expression of stress-responsive genes and ABA biosynthesis, catabolism, transport, and signaling genes was impaired in max2 compared with wild-type seedlings in response to drought stress. Double mutant analysis of max2 with the ABA-insensitive mutants abi3 and abi5 indicated that MAX2 may function upstream of these genes. The expression of ABA-regulated genes was enhanced in imbibed max2 seeds. In addition, max2 mutant seedlings were hypersensitive to ABA and osmotic stress, including NaCl, mannitol, and glucose. Interestingly, ABA, osmotic stress, and drought-sensitive phenotypes were restricted to max2, and the strigolactone biosynthetic pathway mutants max1, max3, and max4 did not display any defects in these responses. Taken together, these results uncover an important role for MAX2 in plant responses to abiotic stress conditions.

Increased biomass, seed yield and stress tolerance is conferred in Arabidopsis by a novel enzyme from the resurrection grass Sporobolus stapfianus that glycosylates the strigolactone analogue GR24

Isolation of gene transcripts from desiccated leaf tissues of the resurrection grass, Sporobolus stapfianus, resulted in the identification of a gene, SDG8i, encoding a Group 1 glycosyltransferase (UGT). Here, we examine the effects of introducing this gene, under control of the CaMV35S promoter, into the model plant Arabidopsis thaliana. Results show that Arabidopsis plants constitutively over-expressing SDG8i exhibit enhanced growth, reduced senescence, cold tolerance and a substantial improvement in protoplasmic drought tolerance. We hypothesise that expression of SDG8i in Arabidopsis negatively affects the bioactivity of metabolite/s that mediate/s environmentally-induced repression of cell division and expansion, both during normal development and in response to stress. The phenotype of transgenic plants over-expressing SDG8i suggests modulation in activities of both growth- and stress-related hormones. Plants overexpressing the UGT show evidence of elevated auxin levels, with the enzyme acting downstream of ABA to reduce drought-induced senescence. Analysis of the in vitro activity of the UGT recombinant protein product demonstrates that SDG8i can glycosylate the synthetic strigolactone analogue GR24, evoking a link with strigolactone-related processes in vivo. The large improvements observed in survival of transgenic Arabidopsis plants under cold-, salt- and drought-stress, as well as the substantial increases in growth rate and seed yield under non-stress conditions, indicates that overexpression of SDG8i in crop plants may provide a novel means of increasing plant productivity.

Stay-green plants: what do they tell us about the molecular mechanism of leaf senescence

A practical approach to increasing crop yields is to extend the duration of active photosynthesis. Stay-green is a term that is used to describe mutant and transgenic plants or cultivars with the trait of maintaining their leaves for a longer period of time than the wild-type or crosses from which they are derived. Analyzing stay-green genotypes contributes to our understanding of the molecular mechanism regulating leaf senescence which may allow us to extend the duration of active photosynthesis in crop plants. This article summarizes recent studies on stay-green plants and the insights they provide on the mechanism of leaf senescence. Briefly, mutations suppressing ethylene, abscisic acid, brassinosteroid, and strigolactone signal transduction or those activating cytokinin signaling often lead to stay-green phenotypes indicating a complex signaling network regulating leaf senescence. Developmentally regulated transcription factors, including NAC or WRKY family members, play key roles in the induction of leaf senescence and thus alteration in the activity of these transcription factors also result in stay-green phenotypes. Impairment in the enzymatic steps responsible for chlorophyll breakdown also leads to stay-green phenotypes. Some of these genotypes die in the middle of the process of chlorophyll breakdown due to the accumulation of toxic intermediates, while others appear to stay-green but their photosynthetic activity declines in a manner similar to wild-type plants. Alterations in certain metabolic pathways in chloroplasts (e.g., photosynthesis) can lead to a delayed onset of leaf senescence with maintenance of photosynthetic activity longer than wild-type plants, indicating that chloroplast metabolism can also affect the regulatory mechanism of leaf senescence.

Plant leaf senescence and death - regulation by multiple layers of control and implications for aging in general

How do organisms, organs, tissues and cells change their fate when they age towards senescence and death? Plant leaves provide a unique window to explore this question because they show reproducible life history and are readily accessible for experimental assays. Throughout their lifespan, leaves undergo a series of developmental, physiological and metabolic transitions that culminate in senescence and death. Leaf senescence is an 'altruistic death' that allows for the degradation of the nutrients that are produced during the growth phase of the leaf and their redistribution to developing seeds or other parts of the plant, and thus is a strategy that has evolved to maximize the fitness of the plant. During the past decade, there has been significant progress towards understanding the key molecular principles of leaf senescence using genetic and molecular studies, as well as 'omics' analyses. It is now apparent that leaf senescence is a highly complex genetic program that is tightly controlled by multiple layers of regulation, including at the level of chromatin and transcription, as well as by post-transcriptional, translational and post-translational regulation. This Commentary discusses the latest understandings and insights into the underlying molecular mechanisms, and presents the perspectives necessary to enable our system-level understanding of leaf senescence, together with their possible implications for aging in general.

Ethylene-insensitive3 is a senescence-associated gene that accelerates age-dependent leaf senescence by directly repressing miR164 transcription in Arabidopsis

Numerous endogenous and environmental signals regulate the intricate and highly orchestrated process of plant senescence. Ethylene is a well-known inducer of senescence, including fruit ripening and flower and leaf senescence. However, the underlying molecular mechanism of ethylene-induced leaf senescence remains to be elucidated. Here, we examine ethylene-insensitive3 (EIN3), a key transcription factor in ethylene signaling, and find that EIN3 is a functional senescence-associated gene. Constitutive overexpression or temporary activation of EIN3 is sufficient to accelerate leaf senescence symptoms. Conversely, loss of EIN3 and EIN3-Like1 (its close homolog) function leads to a delay in age-dependent and ethylene-, jasmonic acid-, or dark-induced leaf senescence. We further found that EIN3 acts downstream of ORESARA2 (ORE2)/ORE3/EIN2 to repress miR164 transcription and upregulate the transcript levels of ORE1/NAC2, a target gene of miR164. EIN3 directly binds to the promoters of microRNA164 (miR164), and this binding activity progressively increases during leaf ageing. Genetic analysis revealed that overexpression of miR164 or knockout of ORE1/NAC2 represses EIN3-induced early-senescence phenotypes. Collectively, our study defines a continuation of the signaling pathway involving EIN2-EIN3-miR164-NAC2 in regulating leaf senescence and provides a mechanistic insight into how ethylene promotes the progression of leaf senescence in Arabidopsis thaliana.

SUPPRESSOR OF MORE AXILLARY GROWTH2 1 controls seed germination and seedling development in Arabidopsis

Abiotic chemical signals discovered in smoke that are known as karrikins (KARs) and the endogenous hormone strigolactone (SL) control plant growth through a shared MORE AXILLARY GROWTH2 (MAX2)-dependent pathway. A SL biosynthetic pathway and candidate KAR/SL receptors have been characterized, but signaling downstream of MAX2 is poorly defined. A screen for genetic suppressors of the enhanced seed dormancy phenotype of max2 in Arabidopsis (Arabidopsis thaliana) led to identification of a suppressor of max2 1 (smax1) mutant. smax1 restores the seed germination and seedling photomorphogenesis phenotypes of max2 but does not affect the lateral root formation, axillary shoot growth, or senescence phenotypes of max2. Expression of three transcriptional markers of KAR/SL signaling, D14-LIKE2, KAR-UP F-BOX1, and INDOLE-3-ACETIC ACID INDUCIBLE1, is rescued in smax1 max2 seedlings. SMAX1 is a member of an eight-gene family in Arabidopsis that has weak similarity to HEAT SHOCK PROTEIN 101, which encodes a caseinolytic peptidase B chaperonin required for thermotolerance. SMAX1 and the SMAX1-like (SMXL) homologs are differentially expressed in Arabidopsis tissues. SMAX1 transcripts are most abundant in dry seed, consistent with its function in seed germination control. Several SMXL genes are up-regulated in seedlings treated with the synthetic SL GR24. SMAX1 and SMXL2 transcripts are reduced in max2 seedlings, which could indicate negative feedback regulation by KAR/SL signaling. smax1 seed and seedling growth mimics the wild type treated with KAR/SL, but smax1 seedlings are still responsive to 2H-furo[2,3-c]pyran-2-one (KAR2) or GR24. We conclude that SMAX1 is an important component of KAR/SL signaling during seed germination and seedling growth but is not necessary for all MAX2-dependent responses. We hypothesize that one or more SMXL proteins may also act downstream of MAX2 to control the diverse developmental responses to KARs and SLs.

Towards systems biological understanding of leaf senescence

The application of systems biology approaches has greatly facilitated the process of deciphering the molecular mechanisms underlying leaf senescence. Analyses of the leaf senescence transcriptome have identified some of the major biochemical events during senescence including protein degradation and nutrient remobilization. Proteomic studies have confirmed these findings and have suggested up-regulated energy metabolism during leaf senescence which might be important for cell viability maintenance. As a critical part of systems biology, studies involving transcription regulation networking and senescence-inducing signaling have deepened our understanding on the molecular regulation of leaf senescence. The important next steps towards a systems biological understanding of leaf senescence will be discussed.

Characterization and fine mapping of the rice premature senescence mutant ospse1

Premature senescence can limit crop productivity by limiting the growth phase. In the present study, a spontaneous premature senescence mutant was identified in rice (Oryza sativa L.). Genetic analysis revealed that the premature senescence phenotype was controlled by a recessive mutation, which we named Oryza sativa premature senescence1 (ospse1). The ospse1 mutants showed premature leaf senescence from the booting stage and exhibited more severe symptoms during reproductive and ripening stages. Key yield-related agronomic traits such as 1,000-grain weight and seed-setting rate, but not panicle grain number, were significantly reduced in ospse1 plants. Chlorophyll content, net photosynthetic rate, and transpiration rate of ospse1 flag leaves were similar to the wild-type plants in vegetative stages, but these parameters decreased steeply in the mutant after the heading stage. Consistent with this, the senescence-associated genes OsNYC1 and OsSgr were up-regulated in ospse1 mutant during premature leaf senescence. The ospse1 locus was mapped to a 38-kb region on chromosome 1 and sequence analysis of this region identified a single-nucleotide deletion in the 3' region of an open reading frame (ORF) encoding a putative pectate lyase, leading to a frame shift and a longer ORF. Our results suggested that the premature senescence of the ospse1 may be regulated by a novel mechanism mediated by pectate lyase.

A regulatory cascade involving class II ETHYLENE RESPONSE FACTOR transcriptional repressors operates in the progression of leaf senescence

Leaf senescence is the final process of leaf development that involves the mobilization of nutrients from old leaves to newly growing tissues. Despite the identification of several transcription factors involved in the regulation of this process, the mechanisms underlying the progression of leaf senescence are largely unknown. Herein, we describe the proteasome-mediated regulation of class II ETHYLENE RESPONSE FACTOR (ERF) transcriptional repressors and involvement of these factors in the progression of leaf senescence in Arabidopsis (Arabidopsis thaliana). Based on previous results showing that the tobacco (Nicotiana tabacum) ERF3 (NtERF3) specifically interacts with a ubiquitin-conjugating enzyme, we examined the stability of NtERF3 in vitro and confirmed its rapid degradation by plant protein extracts. Furthermore, NtERF3 accumulated in plants treated with a proteasome inhibitor. The Arabidopsis class II ERFs AtERF4 and AtERF8 were also regulated by the proteasome and increased with plant aging. Transgenic Arabidopsis plants with enhanced expression of NtERF3, AtERF4, or AtERF8 showed precocious leaf senescence. Our gene expression and chromatin immunoprecipitation analyses suggest that AtERF4 and AtERF8 targeted the EPITHIOSPECIFIER PROTEIN/EPITHIOSPECIFYING SENESCENCE REGULATOR gene and regulated the expression of many genes involved in the progression of leaf senescence. By contrast, an aterf4 aterf8 double mutant exhibited delayed leaf senescence. Our results provide insight into the important role of class II ERFs in the progression of leaf senescence.

Jasmonates: biosynthesis, perception, signal transduction and action in plant stress response, growth and development. An update to the 2007 review in Annals of Botany

BACKGROUND

Jasmonates are important regulators in plant responses to biotic and abiotic stresses as well as in development. Synthesized from lipid-constituents, the initially formed jasmonic acid is converted to different metabolites including the conjugate with isoleucine. Important new components of jasmonate signalling including its receptor were identified, providing deeper insight into the role of jasmonate signalling pathways in stress responses and development.

SCOPE

The present review is an update of the review on jasmonates published in this journal in 2007. New data of the last five years are described with emphasis on metabolites of jasmonates, on jasmonate perception and signalling, on cross-talk to other plant hormones and on jasmonate signalling in response to herbivores and pathogens, in symbiotic interactions, in flower development, in root growth and in light perception.

CONCLUSIONS

The last few years have seen breakthroughs in the identification of JASMONATE ZIM DOMAIN (JAZ) proteins and their interactors such as transcription factors and co-repressors, and the crystallization of the jasmonate receptor as well as of the enzyme conjugating jasmonate to amino acids. Now, the complex nature of networks of jasmonate signalling in stress responses and development including hormone cross-talk can be addressed.

Using Arabidopsis to study shoot branching in biomass willow

The success of the short-rotation coppice system in biomass willow (Salix spp.) relies on the activity of the shoot-producing meristems found on the coppice stool. However, the regulation of the activity of these meristems is poorly understood. In contrast, our knowledge of the mechanisms behind axillary meristem regulation in Arabidopsis (Arabidopsis thaliana) has grown rapidly in the past few years through the exploitation of integrated physiological, genetic, and molecular assays. Here, we demonstrate that these assays can be directly transferred to study the control of bud activation in biomass willow and to assess similarities with the known hormone regulatory system in Arabidopsis. Bud hormone response was found to be qualitatively remarkably similar in Salix spp. and Arabidopsis. These similarities led us to test whether Arabidopsis hormone mutants could be used to assess allelic variation in the cognate Salix spp. hormone genes. Allelic differences in Salix spp. strigolactone genes were observed using this approach. These results demonstrate that both knowledge and assays from Arabidopsis axillary meristem biology can be successfully applied to Salix spp. and can increase our understanding of a fundamental aspect of short-rotation coppice biomass production, allowing more targeted breeding.

Identification and functional analysis of three MAX2 orthologs in chrysanthemum

MORE AXILLARY BRANCHING 2 (MAX2), initially identified in Arabidopsis thaliana, is a key regulatory gene in strigolactone signal transduction. Three orthologs of MAX2 were cloned from Dendranthema grandiflorum (DgMAX2a, b, and c). Each of the genes has an open reading frame of 2,049 bp and encodes 682 amino acid proteins. The predicted amino acid sequences of the three DgMAX2s are most closely related to the MAX2 orthologs identified in petunia (PhMAX2A and PhMAX2B), and display the highest amino acid sequence similarity with PhMAX2A compared to other MAX2s. Expression analysis revealed that DgMAX2s are predominantly expressed in the stem and axillary buds. On a cellular level, we localized the DgMAX2a::GFP fusion protein to the nucleus in onion epidermal cells, which is consistent with the nuclear localization of MAX2 in Arabidopsis. The chrysanthemum DgMAX2a is able to restore the max2-1 mutant branching to wild-type (WT) Arabidopsis, suggesting that it is a functional MAX2 ortholog. These results suggest that DgMAX2s may be candidate genes for reducing the shoot branching of chrysanthemum.

The role of strigolactones in nutrient-stress responses in plants

Strigolactones (SLs) are a new group of plant hormones, which have been intensively investigated during the last few years. The wide spectrum of SLs actions, including the regulation of shoot/root architecture, and the stimulation of the interactions between roots and fungi or bacteria, as well as the stimulation of germination of parasitic plants, indicates that this group of hormones may play an important role in the mechanisms that control soil exploration, and the root-mediated uptake of nutrients. Current studies have shown that SLs might be factors that have an influence on the plant response to a deficiency of macronutrients. Experimental data from the last four years have confirmed that the biosynthesis and exudation of SLs are increased under phosphorus and nitrogen deficiency. All these data suggest that SLs may regulate the complex response to nutrient stress, which include not only the modification of the plant developmental process, but also the cooperation with other organisms in order to minimize the effects of threats. In this paper the results of studies that indicate that SLs play an important role in the response to nutrient stress are reviewed and the consequences of the higher biosynthesis and exudation of SLs in response to phosphorus and nitrogen deficiency are discussed.

A dual role of strigolactones in phosphate acquisition and utilization in plants

Phosphorus, acquired in the form of phosphate (Pi), is one of the primary macronutrients for plants but is least available in the soil. Pi deficiency is a major factor limiting plant growth, development and reproduction. Plants have developed a complex signaling network to respond to Pi deficiency. The recent discovery of strigolactones, a new class of plant hormones, has led to an emerging signaling module illustrating the integrated control of Pi acquisition, plant-microbe symbiotic interactions and plant architecture. This review article focuses on the recent findings of plant responses and roles of strigolactones to Pi deficiency.

Carotenoid cleavage dioxygenase 7 modulates plant growth, reproduction, senescence, and determinate nodulation in the model legume Lotus japonicus

Strigolactones (SLs) are newly identified hormones that regulate multiple aspects of plant development, infection by parasitic weeds, and mutualistic symbiosis in the roots. In this study, the role of SLs was studied for the first time in the model plant Lotus japonicus using transgenic lines silenced for carotenoid cleavage dioxygenase 7 (LjCCD7), the orthologue of Arabidopsis More Axillary Growth 3. Transgenic LjCCD7-silenced plants displayed reduced height due to shorter internodes, and more branched shoots and roots than the controls, and an increase in total plant biomass, while their root:shoot ratio remained unchanged. Moreover, these lines had longer primary roots, delayed senescence, and reduced flower/pod numbers from the third round of flower and pod setting onwards. Only a mild reduction in determinate nodule numbers and hardly any impact on the colonization by arbuscular mycorrhizal fungi were observed. The results show that the impairment of CCD7 activity in L. japonicus leads to a phenotype linked to SL functions, but with specific features possibly due to the peculiar developmental pattern of this plant species. It is believed that the data also link determinate nodulation, plant reproduction, and senescence to CCD7 function for the first time.

The submergence tolerance gene SUB1A delays leaf senescence under prolonged darkness through hormonal regulation in rice

Leaf senescence is a natural age-dependent process that is induced prematurely by various environmental stresses. Typical alterations during leaf senescence include breakdown of chlorophyll, a shift to catabolism of energy reserves, and induction of senescence-associated genes, all of which can occur during submergence, drought, and constant darkness. Here, we evaluated the influence of the submergence tolerance regulator, SUBMERGENCE1A (SUB1A), in the acclimation responses during leaf senescence caused by prolonged darkness in rice (Oryza sativa). SUB1A messenger RNA was highly induced by prolonged darkness in a near-isogenic line containing SUB1A. Genotypes with conditional and ectopic overexpression of SUB1A significantly delayed loss of leaf color and enhanced recovery from dark stress. Physiological analysis revealed that SUB1A postpones dark-induced senescence through the maintenance of chlorophyll and carbohydrate reserves in photosynthetic tissue. This delay allowed leaves of SUB1A genotypes to recover photosynthetic activity more quickly upon reexposure to light. SUB1A also restricted the transcript accumulation of representative senescence-associated genes. Jasmonate and salicylic acid are positive regulators of leaf senescence, but ectopic overexpression of SUB1A dampened responsiveness to both hormones in the context of senescence. We found that ethylene accelerated senescence stimulated by darkness and jasmonate, although SUB1A significantly restrained dark-induced ethylene accumulation. Overall, SUB1A genotypes displayed altered responses to prolonged darkness by limiting ethylene production and responsiveness to jasmonate and salicylic acid, thereby dampening the breakdown of chlorophyll, carbohydrates, and the accumulation of senescence-associated messenger RNAs. A delay of leaf senescence conferred by SUB1A can contribute to the enhancement of tolerance to submergence, drought, and oxidative stress.

Regulation of leaf senescence and crop genetic improvement

Leaf senescence can impact crop production by either changing photosynthesis duration, or by modifying the nutrient remobilization efficiency and harvest index. The doubling of the grain yield in major cereals in the last 50 years was primarily achieved through the extension of photosynthesis duration and the increase in crop biomass partitioning, two things that are intrinsically coupled with leaf senescence. In this review, we consider the functionality of a leaf as a function of leaf age, and divide a leaf's life into three phases: the functionality increasing phase at the early growth stage, the full functionality phase, and the senescence and functionality decreasing phase. A genetic framework is proposed to describe gene actions at various checkpoints to regulate leaf development and senescence. Four categories of genes contribute to crop production: those which regulate (I) the speed and transition of early leaf growth, (II) photosynthesis rate, (III) the onset and (IV) the progression of leaf senescence. Current advances in isolating and characterizing senescence regulatory genes are discussed in the leaf aging and crop production context. We argue that the breeding of crops with leaf senescence ideotypes should be an essential part of further crop genetic improvement.

Carbon deprivation-driven transcriptome reprogramming in detached developmentally arresting Arabidopsis inflorescences

Senescence is genetically controlled and activated in mature tissues during aging. However, immature plant tissues also display senescence-like symptoms when continuously exposed to adverse energy-depleting conditions. We used detached dark-held immature inflorescences of Arabidopsis (Arabidopsis thaliana) to understand the metabolic reprogramming occurring in immature tissues transitioning from rapid growth to precocious senescence. Macroscopic growth of the detached inflorescences rapidly ceased upon placement in water in the dark at 21°C. Inflorescences were completely degreened by 120 h of dark incubation and by 24 h had already lost 24% of their chlorophyll and 34% of their protein content. Comparative transcriptome profiling at 24 h revealed that inflorescence response at 24 h had a large carbon-deprivation component. Genes that positively regulate developmental senescence (ARABIDOPSIS NAC DOMAIN CONTAINING PROTEIN92) and shade-avoidance syndrome (PHYTOCHROME INTERACTING FACTOR4 [PIF4] and PIF5) were up-regulated within 24 h. Mutations in these genes delayed degreening of the inflorescences. Their up-regulation was suppressed in dark-held inflorescences by glucose treatment, which promoted macroscopic growth and development and inhibited degreening of the inflorescences. Detached inflorescences held in the dark for 4 d were still able to reinitiate development to produce siliques upon being brought out to the light, indicating that the transcriptional reprogramming at 24 h was adaptive and reversible. Our results suggest that the response of detached immature tissues to dark storage involves interactions between carbohydrate status sensing and light deprivation signaling and that the dark-adaptive response of the tissues appears to utilize some of the same key regulators as developmental senescence.

Wheat F-box protein recruits proteins and regulates their abundance during wheat spike development

F-box proteins, components of the Skp1-Cullin1-F-box (SCF) protein E3 ubiquitin ligase complex, serve as the variable component responsible for substrate recognition and recruitment in SCF-mediated proteolysis. F-box proteins interact with Skp1 through the F-box motif and with ubiquitination substrates through C-terminal protein interaction domains. F-box proteins regulate plant development, various hormonal signal transduction processes, circadian rhythm, and cell cycle control. We isolated an F-box protein gene from wheat spikes at the onset of flowering. The Triticum aestivum cyclin F-box domain (TaCFBD) gene showed elevated expression levels during early inflorescence development and under cold stress treatment. TaCFBD green fluorescent protein signals were localized in the cytoplasm and plasma membrane. We used yeast two-hybrid screening to identify proteins that potentially interact with TaCFBD. Fructose bisphosphate aldolase, aspartic protease, VHS, glycine-rich RNA-binding protein, and the 26S proteasome non-ATPase regulatory subunit were positive candidate proteins. The bimolecular fluorescence complementation assay revealed the interaction of TaCFBD with partner proteins in the plasma membranes of tobacco cells. Our results suggest that the TaCFBD protein acts as an adaptor between target substrates and the SCF complex and provides substrate specificity to the SCF of ubiquitin ligase complexes.

Mutation of the cytosolic ribosomal protein-encoding RPS10B gene affects shoot meristematic function in Arabidopsis

BACKGROUND

Plant cytosolic ribosomal proteins are encoded by small gene families. Mutants affecting these genes are often viable, but show growth and developmental defects, suggesting incomplete functional redundancy within the families. Dormancy to growth transitions, such as the activation of axillary buds in the shoot, are characterised by co-ordinated upregulation of ribosomal protein genes.

RESULTS

A recessive mutation in RPS10B, one of three Arabidopsis genes encoding the eukaryote-specific cytoplasmic ribosomal protein S10e, was found to suppress the excessive shoot branching mutant max2-1. rps10b-1 mildly affects the formation and separation of shoot lateral organs, including the shoot axillary meristems. Axillary meristem defects are enhanced when rps10b-1 is combined with mutations in REVOLUTA, AUXIN-RESISTANT1, PINOID or another suppressor of max2-1, FAR-RED ELONGATED HYPOCOTYL3. In some of these double mutants, the maintenance of the primary shoot meristem is also affected. In contrast, mutation of ALTERED MERISTEM PROGRAMME1 suppresses the rps10b-1axillary shoot defect. Defects in both axillary shoot formation and organ separation were enhanced by combining rps10b-1 with cuc3, a mutation affecting one of three Arabidopsis NAC transcription factor genes with partially redundant roles in these processes. To assess the effect of rps10b-1 on bud activation independently from bud formation, axillary bud outgrowth on excised cauline nodes was analysed. The outgrowth rate of untreated buds was reduced only slightly by rps10b-1 in both wild-type and max2-1 backgrounds. However, rps10b-1 strongly suppressed the auxin resistant outgrowth of max2-1 buds. A developmental phenotype of rps10b-1, reduced stamen number, was complemented by the cDNA of another family member, RPS10C, under the RPS10B promoter.

CONCLUSIONS

RPS10B promotes shoot branching mainly by promoting axillary shoot development. It contributes to organ boundary formation and leaf polarity, and sustains max2-1 bud outgrowth in the presence of auxin. These processes require the auxin response machinery and precise spatial distribution of auxin. The correct dosage of protein(s) involved in auxin-mediated patterning may be RPS10B-dependent. Inability of other RPS10 gene family members to maintain fully S10e levels might cause the rps10b-1 phenotype, as we found no evidence for unique functional specialisation of either RPS10B promoter or RPS10B protein.

DAD2 is an α/β hydrolase likely to be involved in the perception of the plant branching hormone, strigolactone

Strigolactones are a recently discovered class of plant hormone involved in branching, leaf senescence, root development, and plant-microbe interactions. They are carotenoid-derived lactones, synthesized in the roots and transported acropetally to modulate axillary bud outgrowth (i.e., branching). However, a receptor for strigolactones has not been identified. We have identified the DAD2 gene from petunia, an ortholog of the rice and Arabidopsis D14 genes, and present evidence for its roles in strigolactone perception and signaling. DAD2 acts in the strigolactone pathway, and the dad2 mutant is insensitive to the strigolactone analog GR24. The crystal structure of DAD2 reveals an α/β hydrolase fold containing a canonical catalytic triad with a large internal cavity capable of accommodating strigolactones. In the presence of GR24 DAD2 interacts with PhMAX2A, a central component of strigolactone signaling, in a GR24 concentration-dependent manner. DAD2 can hydrolyze GR24, with mutants of the catalytic triad abolishing both this activity and the ability of DAD2 to interact with PhMAX2A. The hydrolysis products can neither stimulate the protein-protein interaction nor modulate branching. These observations suggest that DAD2 acts to bind the mobile strigolactone signal and then interacts with PhMAX2A during catalysis to initiate an SCF-mediated signal transduction pathway.

Selection and validation of endogenous reference genes for qRT-PCR analysis in leafy spurge (Euphorbia esula)

Quantitative real-time polymerase chain reaction (qRT-PCR) is the most important tool in measuring levels of gene expression due to its accuracy, specificity, and sensitivity. However, the accuracy of qRT-PCR analysis strongly depends on transcript normalization using stably expressed reference genes. The aim of this study was to find internal reference genes for qRT-PCR analysis in various experimental conditions for seed, adventitious underground bud, and other organs of leafy spurge. Eleven candidate reference genes (BAM4, PU1, TRP-like, FRO1, ORE9, BAM1, SEU, ARF2, KAPP, ZTL, and MPK4) were selected from among 171 genes based on expression stabilities during seed germination and bud growth. The other ten candidate reference genes were selected from three different sources: (1) 3 stably expressed leafy spurge genes (60S, bZIP21, and MD-100) identified from the analyses of leafy spurge microarray data; (2) 3 orthologs of Arabidopsis “general purpose” traditional reference genes (GAPDH_1, GAPDH_2, and UBC); and (3) 4 orthologs of Arabidopsis stably expressed genes (UBC9, SAND, PTB, and F-box) identified from Affymetrix ATH1 whole-genome GeneChip studies. The expression stabilities of these 21 genes were ranked based on the C_T values of 72 samples using four different computation programs including geNorm, Normfinder, BestKeeper, and the comparative ΔC_T method. Our analyses revealed SAND, PTB, ORE9, and ARF2 to be the most appropriate reference genes for accurate normalization of gene expression data. Since SAND and PTB were obtained from 4 orthologs of Arabidopsis, while ORE9 and ARF2 were selected from 171 leafy spurge genes, it was more efficient to identify good reference genes from the orthologs of other plant species that were known to be stably expressed than that of randomly testing endogenous genes. Nevertheless, the two newly identified leafy spurge genes, ORE9 and ARF2, can serve as orthologous candidates in the search for reference genes from other plant species.

Early senescence and cell death in Arabidopsis saul1 mutants involves the PAD4-dependent salicylic acid pathway

Age-dependent leaf senescence and cell death in Arabidopsis (Arabidopsis thaliana) requires activation of the transcription factor ORESARA1 (ORE1) and is not initiated prior to a leaf age of 28 d. Here, we investigate the conditional execution of events that regulate early senescence and cell death in senescence-associated ubiquitin ligase1 (saul1) mutants, deficient in the PLANT U-BOX-ARMADILLO E3 ubiquitin ligase SAUL1. In saul1 mutants challenged with low light, the switch of age-dependent cell death was turned on prematurely, as indicated by the accumulation of ORE1 transcripts, induction of the senescence marker gene SENESCENCE-ASSOCIATED GENE12, and cell death. However, ORE1 accumulation by itself was not sufficient to cause saul1 phenotypes, as demonstrated by double mutant analysis. Exposure of saul1 mutants to low light for only 24 h did not result in visible symptoms of senescence; however, the senescence-promoting transcription factor genes WRKY53, WRKY6, and NAC-LIKE ACTIVATED BY AP3/PI were up-regulated, indicating that senescence in saul1 seedlings was already initiated. To resolve the time course of gene expression, microarray experiments were performed at narrow intervals. Differential expression of the genes involved in salicylic acid and defense mechanisms were the earliest events detected, suggesting a central role for salicylic acid in saul1 senescence and cell death. The salicylic acid content increased in low-light-treated saul1 mutants, and application of exogenous salicylic acid was indeed sufficient to trigger saul1 senescence in permissive light conditions. Double mutant analyses showed that PHYTOALEXIN DEFICIENT4 (PAD4) but not NONEXPRESSER OF PR GENES1 (NPR1) is essential for saul1 phenotypes. Our results indicate that saul1 senescence depends on the PAD4-dependent salicylic acid pathway but does not require NPR1 signaling.

Ectopic expression of Capsicum-specific cell wall protein Capsicum annuum senescence-delaying 1 (CaSD1) delays senescence and induces trichome formation in Nicotiana benthamiana

Secreted proteins are known to have multiple roles in plant development, metabolism, and stress response. In a previous study to understand the roles of secreted proteins, Capsicum annuum secreted proteins (CaS) were isolated by yeast secretion trap. Among the secreted proteins, we further characterized Capsicum annuum senescence-delaying 1 (CaSD1), a gene encoding a novel secreted protein that is present only in the genus Capsicum. The deduced CaSD1 contains multiple repeats of the amino acid sequence KPPIHNHKPTDYDRS. Interestingly, the number of repeats varied among cultivars and species in the Capsicum genus. CaSD1 is constitutively expressed in roots, and Agrobacterium-mediated transient overexpression of CaSD1 in Nicotiana benthamiana leaves resulted in delayed senescence with a dramatically increased number of trichomes and enlarged epidermal cells. Furthermore, senescence- and cell division-related genes were differentially regulated by CaSD1-overexpressing plants. These observations imply that the pepper-specific cell wall protein CaSD1 plays roles in plant growth and development by regulating cell division and differentiation.

Specialisation within the DWARF14 protein family confers distinct responses to karrikins and strigolactones in Arabidopsis

Karrikins are butenolides derived from burnt vegetation that stimulate seed germination and enhance seedling responses to light. Strigolactones are endogenous butenolide hormones that regulate shoot and root architecture, and stimulate the branching of arbuscular mycorrhizal fungi. Thus, karrikins and strigolactones are structurally similar but physiologically distinct plant growth regulators. In Arabidopsis thaliana, responses to both classes of butenolides require the F-box protein MAX2, but it remains unclear how discrete responses to karrikins and strigolactones are achieved. In rice, the DWARF14 protein is required for strigolactone-dependent inhibition of shoot branching. Here, we show that the Arabidopsis DWARF14 orthologue, AtD14, is also necessary for normal strigolactone responses in seedlings and adult plants. However, the AtD14 paralogue KARRIKIN INSENSITIVE 2 (KAI2) is specifically required for responses to karrikins, and not to strigolactones. Phylogenetic analysis indicates that KAI2 is ancestral and that AtD14 functional specialisation has evolved subsequently. Atd14 and kai2 mutants exhibit distinct subsets of max2 phenotypes, and expression patterns of AtD14 and KAI2 are consistent with the capacity to respond to either strigolactones or karrikins at different stages of plant development. We propose that AtD14 and KAI2 define a class of proteins that permit the separate regulation of karrikin and strigolactone signalling by MAX2. Our results support the existence of an endogenous, butenolide-based signalling mechanism that is distinct from the strigolactone pathway, providing a molecular basis for the adaptive response of plants to smoke.

Nitric oxide and protein S-nitrosylation are integral to hydrogen peroxide-induced leaf cell death in rice

Nitric oxide (NO) is a key redox-active, small molecule involved in various aspects of plant growth and development. Here, we report the identification of an NO accumulation mutant, nitric oxide excess1 (noe1), in rice (Oryza sativa), the isolation of the corresponding gene, and the analysis of its role in NO-mediated leaf cell death. Map-based cloning revealed that NOE1 encoded a rice catalase, OsCATC. Furthermore, noe1 resulted in an increase of hydrogen peroxide (H(2)O(2)) in the leaves, which consequently promoted NO production via the activation of nitrate reductase. The removal of excess NO reduced cell death in both leaves and suspension cultures derived from noe1 plants, implicating NO as an important endogenous mediator of H(2)O(2)-induced leaf cell death. Reduction of intracellular S-nitrosothiol (SNO) levels, generated by overexpression of rice S-nitrosoglutathione reductase gene (GSNOR1), which regulates global levels of protein S-nitrosylation, alleviated leaf cell death in noe1 plants. Thus, S-nitrosylation was also involved in light-dependent leaf cell death in noe1. Utilizing the biotin-switch assay, nanoliquid chromatography, and tandem mass spectrometry, S-nitrosylated proteins were identified in both wild-type and noe1 plants. NO targets identified only in noe1 plants included glyceraldehyde 3-phosphate dehydrogenase and thioredoxin, which have been reported to be involved in S-nitrosylation-regulated cell death in animals. Collectively, our data suggest that both NO and SNOs are important mediators in the process of H(2)O(2)-induced leaf cell death in rice.

Strigolactone and karrikin signal perception: receptors, enzymes, or both?

The signaling molecules strigolactone (SL) and karrikin are involved in seed germination, development of axillary meristems, senescence of leaves, and interactions with arbuscular mycorrhizal fungi. The signal transduction pathways for both SLs and karrikins require the same F-box protein (MAX2) and closely related α/β hydrolase fold proteins (DAD2 and KAI2). The crystal structure of DAD2 has been solved revealing an α/β hydrolase fold protein with an internal cavity capable of accommodating SLs. DAD2 responds to the SL analog GR24 by changing conformation and binding to MAX2 in a GR24 concentration-dependent manner. DAD2 can also catalyze hydrolysis of GR24. Structure activity relationships of analogs indicate that the butenolide ring common to both SLs and karrikins is essential for biological activity, but the remainder of the molecules can be significantly modified without loss of activity. The combination of data from the study of DAD2, KAI2, and chemical analogs of SLs and karrikins suggests a model for binding that requires nucleophilic attack by the active site serine of the hydrolase at the carbonyl atom of the butenolide ring. A conformational change occurs in the hydrolase that results in interaction with the F-box protein MAX2. Downstream signal transduction is then likely to occur via SCF (Skp-Cullin-F-box) complex-mediated ubiquitination of target proteins and their subsequent degradation. The role of the catalytic activity of the hydrolase is unclear but it may be integral in binding as well as possibly allowing the signal to be cleared from the receptor. The α/β hydrolase fold family consists mostly of active enzymes, with a few notable exceptions. We suggest that DAD2 and KAI2 represent an intermediate stage where some catalytic activity is retained at the same time as a receptor role has evolved.

Strigolactone signaling is required for auxin-dependent stimulation of secondary growth in plants

Long distance cell-to-cell communication is critical for the development of multicellular organisms. In this respect, plants are especially demanding as they constantly integrate environmental inputs to adjust growth processes to different conditions. One example is thickening of shoots and roots, also designated as secondary growth. Secondary growth is mediated by the vascular cambium, a stem cell-like tissue whose cell-proliferating activity is regulated over a long distance by the plant hormone auxin. How auxin signaling is integrated at the level of cambium cells and how cambium activity is coordinated with other growth processes are largely unknown. Here, we provide physiological, genetic, and pharmacological evidence that strigolactones (SLs), a group of plant hormones recently described to be involved in the repression of shoot branching, positively regulate cambial activity and that this function is conserved among species. We show that SL signaling in the vascular cambium itself is sufficient for cambium stimulation and that it interacts strongly with the auxin signaling pathway. Our results provide a model of how auxin-based long-distance signaling is translated into cambium activity and suggest that SLs act as general modulators of plant growth forms linking the control of shoot branching with the thickening of stems and roots.

Alkamides activate jasmonic acid biosynthesis and signaling pathways and confer resistance to Botrytis cinerea in Arabidopsis thaliana

Alkamides are fatty acid amides of wide distribution in plants, structurally related to N-acyl-L-homoserine lactones (AHLs) from Gram-negative bacteria and to N- acylethanolamines (NAEs) from plants and mammals. Global analysis of gene expression changes in Arabidopsis thaliana in response to N-isobutyl decanamide, the most highly active alkamide identified to date, revealed an overrepresentation of defense-responsive transcriptional networks. In particular, genes encoding enzymes for jasmonic acid (JA) biosynthesis increased their expression, which occurred in parallel with JA, nitric oxide (NO) and H₂O₂ accumulation. The activity of the alkamide to confer resistance against the necrotizing fungus Botrytis cinerea was tested by inoculating Arabidopsis detached leaves with conidiospores and evaluating disease symptoms and fungal proliferation. N-isobutyl decanamide application significantly reduced necrosis caused by the pathogen and inhibited fungal proliferation. Arabidopsis mutants jar1 and coi1 altered in JA signaling and a MAP kinase mutant (mpk6), unlike salicylic acid- (SA) related mutant eds16/sid2-1, were unable to defend from fungal attack even when N-isobutyl decanamide was supplied, indicating that alkamides could modulate some necrotrophic-associated defense responses through JA-dependent and MPK6-regulated signaling pathways. Our results suggest a role of alkamides in plant immunity induction.

Protein degradation - an alternative respiratory substrate for stressed plants

In cellular circumstances under which carbohydrates are scarce, plants can metabolize proteins and lipids as alternative respiratory substrates. Respiration of protein is less efficient than that of carbohydrate as assessed by the respiratory quotient; however, under certain adverse conditions, it represents an important alternative energy source for the cell. Significant effort has been invested in understanding the regulation of protein degradation in plants. This has included an investigation of how proteins are targeted to the proteosome, and the processes of senescence and autophagy. Here we review these events with particular reference to amino acid catabolism and its role in supporting the tricarboxylic acid cycle and direct electron supply to the ubiquinone pool of the mitochondrial electron transport chain in plants.

The Arabidopsis aminopeptidase LAP2 regulates plant growth, leaf longevity and stress response

Peptidases are known to play key roles in multiple biological processes in all living organisms. In higher plants, the vast majority of putative aminopeptidases remain uncharacterized. In this study, we performed functional and expression analyses of the Arabidopsis LAP2 through cDNA cloning, isolation of T-DNA insertional mutants, characterization of the enzymatic activity, characterization of gene expression and transcriptomics and metabolomics analyses of the mutants. Loss of function of LAP2, one of the 28 aminopeptidases in Arabidopsis, reduced vegetative growth, accelerated leaf senescence and rendered plants more sensitive to various stresses. LAP2 is highly expressed in the leaf vascular tissue and the quiescent center region. Integration of global gene expression and metabolite analyses suggest that LAP2 controlled intracellular amino acid turnover. The mutant maintained free leucine by up-regulating key genes for leucine biosynthesis. However, this influenced the flux of glutamate strikingly. As a result, γ-aminobutyric acid, a metabolite that is derived from glutamate, was diminished in the mutant. Decrements in these nitrogen-rich compounds are associated with morphological alterations and stress sensitivity of the mutant. The results indicate that LAP2 is indeed an enzymatically active aminopeptidase and plays key roles in senescence, stress response and amino acid turnover.

The e3 ubiquitin ligase gene family in plants: regulation by degradation

The regulation of protein expression and activity has been for long time considered only in terms of transcription/translation efficiency. In the last years, the discovery of post-transcriptional and post-translational regulation mechanisms pointed out that the key factor in determining transcript/protein amount is the synthesis/degradation ratio, together with post-translational modifications of proteins. Polyubiquitinaytion marks target proteins directed to degradation mediated by 26S-proteasome. Recent functional genomics studies pointed out that about 5% of Arabidopsis genome codes for proteins of ubiquitination pathway. The most of them (more than one thousand genes) correspond to E3 ubiquitin ligases that specifically recognise target proteins. The huge size of this gene family, whose members are involved in regulation of a number of biological processes including hormonal control of vegetative growth, plant reproduction, light response, biotic and abiotic stress tolerance and DNA repair, indicates a major role for protein degradation in control of plant life.

F-box protein MAX2 has dual roles in karrikin and strigolactone signaling in Arabidopsis thaliana

Smoke is an important abiotic cue for plant regeneration in postfire landscapes. Karrikins are a class of compounds discovered in smoke that promote seed germination and influence early development of many plants by an unknown mechanism. A genetic screen for karrikin-insensitive mutants in Arabidopsis thaliana revealed that karrikin signaling requires the F-box protein MAX2, which also mediates responses to the structurally-related strigolactone family of phytohormones. Karrikins and the synthetic strigolactone GR24 trigger similar effects on seed germination, seedling photomorphogenesis, and expression of a small set of genes during these developmental stages. Karrikins also repress MAX4 and IAA1 transcripts, which show negative feedback regulation by strigolactone. We demonstrate that all of these common responses are abolished in max2 mutants. Unlike strigolactones, however, karrikins do not inhibit shoot branching in Arabidopsis or pea, indicating that plants can distinguish between these signals. These results suggest that a MAX2-dependent signal transduction mechanism was adapted to mediate responses to two chemical cues with distinct roles in plant ecology and development.

WRKY22 transcription factor mediates dark-induced leaf senescence in Arabidopsis

Arabidopsis WRKY proteins are plant-specific transcription factors, encoded by a large gene family, which contain the highly conserved amino acid sequence WRKYGQK and the zinc-finger-like motifs, Cys(2)His(2) or Cys(2)HisCys. They can recognize and bind the TTGAC(C/T) W-box ciselements found in the promoters of target genes, and are involved in the regulation of gene expression during pathogen defense, wounding, trichome development, and senescence. Here we investigated the physiological function of the Arabidopsis WRKY22 transcription factor during dark-induced senescence. WRKY22 transcription was suppressed by light and promoted by darkness. In addition, AtWRKY22 expression was markedly induced by H(2)O(2). These results indicated that AtWRKY22 was involved in signal pathways in response to abiotic stress. Dark-treated AtWRKY22 over-expression and knockout lines showed accelerated and delayed senescence phenotypes, respectively, and senescence-associated genes exhibited increased and decreased expression levels. Mutual regulation existed between AtWRKY22 and AtWRKY6, AtWRKY53, and AtWRKY70, respectively. Moreover, AtWRKY22 could influence their relative expression levels by feedback regulation or by other, as yet unknown mechanisms in response to dark. These results prove that AtWRKY22 participates in the dark-induced senescence signal transduction pathway.

ORS1, an H₂O₂-responsive NAC transcription factor, controls senescence in Arabidopsis thaliana

We report here that ORS1, a previously uncharacterized member of the NAC transcription factor family, controls leaf senescence in Arabidopsis thaliana. Overexpression of ORS1 accelerates senescence in transgenic plants, whereas its inhibition delays it. Genes acting downstream of ORS1 were identified by global expression analysis using transgenic plants producing dexamethasone-inducible ORS1-GR fusion protein. Of the 42 up-regulated genes, 30 (~70%) were previously shown to be up-regulated during age-dependent senescence. We also observed that 32 (~76%) of the ORS1-dependent genes were induced by long-term (4 d), but not short-term (6 h) salinity stress (150 mM NaCl). Furthermore, expression of 16 and 24 genes, respectively, was induced after 1 and 5 h of treatment with hydrogen peroxide (H₂O₂), a reactive oxygen species known to accumulate during salinity stress. ORS1 itself was found to be rapidly and strongly induced by H₂O₂ treatment in both leaves and roots. Using in vitro binding site selection, we determined the preferred binding motif of ORS1 and found it to be present in half of the ORS1-dependent genes. ORS1 is a paralog of ORE1/ANAC092/AtNAC2, a previously reported regulator of leaf senescence. Phylogenetic footprinting revealed evolutionary conservation of the ORS1 and ORE1 promoter sequences in different Brassicaceae species, indicating strong positive selection acting on both genes. We conclude that ORS1, similarly to ORE1, triggers expression of senescence-associated genes through a regulatory network that may involve cross-talk with salt- and H₂O₂-dependent signaling pathways.

High-resolution temporal profiling of transcripts during Arabidopsis leaf senescence reveals a distinct chronology of processes and regulation

Leaf senescence is an essential developmental process that impacts dramatically on crop yields and involves altered regulation of thousands of genes and many metabolic and signaling pathways, resulting in major changes in the leaf. The regulation of senescence is complex, and although senescence regulatory genes have been characterized, there is little information on how these function in the global control of the process. We used microarray analysis to obtain a high-resolution time-course profile of gene expression during development of a single leaf over a 3-week period to senescence. A complex experimental design approach and a combination of methods were used to extract high-quality replicated data and to identify differentially expressed genes. The multiple time points enable the use of highly informative clustering to reveal distinct time points at which signaling and metabolic pathways change. Analysis of motif enrichment, as well as comparison of transcription factor (TF) families showing altered expression over the time course, identify clear groups of TFs active at different stages of leaf development and senescence. These data enable connection of metabolic processes, signaling pathways, and specific TF activity, which will underpin the development of network models to elucidate the process of senescence.

Molecular evolution and selection patterns of plant F-box proteins with C-terminal kelch repeats

The F-box protein superfamily represents one of the largest families in the plant kingdom. F-box proteins phylogenetically organize into numerous subfamilies characterized by their carboxyl (C)-terminal protein-protein interaction domain. Among the largest F-box protein subfamilies in plant genomes are those with C-terminal kelch repeats. In this study, we analyzed the phylogeny and evolution of F-box kelch proteins/genes (FBKs) in seven completely sequenced land plant genomes including a bryophyte, a lycophyte, monocots, and eudicots. While absent in prokaryotes, F-box kelch proteins are widespread in eukaryotes. Nonplant eukaryotes usually contain only a single FBK gene. In land plant genomes, however, FBKs expanded dramatically. Arabidopsis thaliana, for example, contains at least 103 F-box genes with well-conserved C-terminal kelch repeats. The construction of a phylogenetic tree based on the full-length amino acid sequences of the FBKs that we identified in the seven species enabled us to classify FBK genes into unstable/stable/superstable categories. In contrast to superstable genes, which are conserved across all seven species, kelch domains of unstable genes, which are defined as lineage specific, showed strong signatures of positive selection, indicating adaptational potential. We found evidence for conserved protein features such as binding affinities toward A. thaliana SKP1-like adaptor proteins and subcellular localization among closely related FBKs. Pseudogenization seems to occur only rarely, but differential transcriptional regulation of close relatives may result in subfunctionalization.

Phylogenetic comparison of F-Box (FBX) gene superfamily within the plant kingdom reveals divergent evolutionary histories indicative of genomic drift

The emergence of multigene families has been hypothesized as a major contributor to the evolution of complex traits and speciation. To help understand how such multigene families arose and diverged during plant evolution, we examined the phylogenetic relationships of F-Box (FBX) genes, one of the largest and most polymorphic superfamilies known in the plant kingdom. FBX proteins comprise the target recognition subunit of SCF-type ubiquitin-protein ligases, where they individually recruit specific substrates for ubiquitylation. Through the extensive analysis of 10,811 FBX loci from 18 plant species, ranging from the alga Chlamydomonas reinhardtii to numerous monocots and eudicots, we discovered strikingly diverse evolutionary histories. The number of FBX loci varies widely and appears independent of the growth habit and life cycle of land plants, with a little as 198 predicted for Carica papaya to as many as 1350 predicted for Arabidopsis lyrata. This number differs substantially even among closely related species, with evidence for extensive gains/losses. Despite this extraordinary inter-species variation, one subset of FBX genes was conserved among most species examined. Together with evidence of strong purifying selection and expression, the ligases synthesized from these conserved loci likely direct essential ubiquitylation events. Another subset was much more lineage specific, showed more relaxed purifying selection, and was enriched in loci with little or no evidence of expression, suggesting that they either control more limited, species-specific processes or arose from genomic drift and thus may provide reservoirs for evolutionary innovation. Numerous FBX loci were also predicted to be pseudogenes with their numbers tightly correlated with the total number of FBX genes in each species. Taken together, it appears that the FBX superfamily has independently undergone substantial birth/death in many plant lineages, with its size and rapid evolution potentially reflecting a central role for ubiquitylation in driving plant fitness.

Molecular analysis and expression of a floral organ-relative F-box gene isolated from 'Zigui shatian' pummelo (Citrus grandis Osbeck)

F-box proteins are a large family of eukaryotic proteins that contained a conserved motif of approximately 40 amino acids. They play an important role in the processing of degradation of cellular regulatory proteins. In this study we isolated a full-length of cDNA encoding a putative F-box protein from Citrus grandis Osbeck CV 'Zigui shatian' pummelo and designated as CgF-box. The cDNA sequence of CgF-box was 920 bp containing a 585 bp open reading frame encoding a precursor protein of 194 amino acid residues. The deduced protein comprised a conserved F-box domain at the position from the 40th to 84th amino acids. Cluster analysis suggested that CgF-box was more closely related to the grape F-Box protein. Southern hybridization verified CgF-box existed in the genome as multiple copies. The expression analysis revealed that the expression level of CgF-box gene remarkably increases during the flower developmental process of 'Zigui shatian' pummelo, such as high level of expression was noted in style, petal and anther, on the other hand low level of expression was found in ovary and leaf. For further verifying the different expression in different tissue of this gene, in situ hybridization was conducted, strong expression signal could be observed in the style, stigma and anther, low even no signal was observed in ovary. According to their findings we can conclude that CgF-box was not only involved in flower maturation, but also showed different roles in different tissue.