Early leaf senescence is associated with an altered cellular redox balance in Arabidopsis cpr5/old1 mutants

Genetic Analysis and Fine Mapping of a New Rice Mutant, Leaf Tip Senescence 2

Premature leaf senescence significantly reduces rice yields. Despite identifying numerous factors influencing these processes, the intricate genetic regulatory networks governing leaf senescence demand further exploration. We report the characterization of a stably inherited, ethyl methanesulfonate(EMS)-induced rice mutant with wilted leaf tips from seedling till harvesting, designated lts2. This mutant exhibits dwarfism and early senescence at the leaf tips and margins from the seedling stage when compared to the wild type. Furthermore, lts2 displays a substantial decline in both photosynthetic activity and chlorophyll content. Transmission electron microscopy revealed the presence of numerous osmiophilic granules in chloroplast cells near the senescent leaf tips, indicative of advanced cellular senescence. There was also a significant accumulation of H2O2, alongside the up-regulation of senescence-associated genes within the leaf tissues. Genetic mapping situated lts2 between SSR markers Q1 and L12, covering a physical distance of approximately 212 kb in chr.1. No similar genes controlling a premature senescence leaf phenotype have been identified in the region, and subsequent DNA and bulk segregant analysis (BSA) sequencing analyses only identified a single nucleotide substitution (C-T) in the exon of LOC_Os01g35860. These findings position the lts2 mutant as a valuable genetic model for elucidating chlorophyll metabolism and for further functional analysis of the gene in rice.

Function of two splicing variants of RcCPR5 in the resistance of Rosa chinensis to powdery mildew

Rosa chinensis is an important economic and ornamental crop, but powdery mildew greatly reduces its ornamental and economic value. The RcCPR5 gene, encoding a constitutive expressor of pathogenesis-related genes, has two splicing variants in R. chinensis. Compared with RcCPR5-1, RcCPR5-2 has a large C-terminal deletion. During disease development, RcCPR5-2 responded quickly and coordinated with RcCPR5-1 to resist the invasion of the powdery mildew pathogen. In virus-induced gene silencing experiments, down-regulation of RcCPR5 improved the resistance of R. chinensis to powdery mildew. This was confirmed to be broad-spectrum resistance. In the absence of pathogen infection, RcCPR5-1 and RcCPR5-2 formed homodimers and heterodimers to regulate plant growth; but when infected by the powdery mildew pathogen, the RcCPR5-1 and RcCPR5-2 complexes disassociated and released RcSIM/RcSMR to induce effector-triggered immunity, thereby inducing resistance to pathogen infection.

Overexpression of an Antioxidant Enzyme APX1 in cpr5 Mutant Restores its Pleiotropic Growth Phenotype

Breeding crops with enhanced immunity is an effective strategy to reduce yield loss caused by pathogens. The constitutive expresser of pathogenesis-related genes (cpr5) mutant shows enhanced pathogen resistance but retarded growth; thus, it restricts the application of cpr5 in breeding crops with disease resistance. Reactive oxygen species (ROS) play important roles in plant growth and defense. In this study, we determined that the cpr5 mutant exhibited excessive ROS accumulation. However, the mutation of respiratory burst oxidase homolog D (RBOHD), a reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase responsible for the production of ROS signaling in plant immunity, did not suppress excessive ROS levels in cpr5. Furthermore, the cpr5 mutant showed low levels of ascorbate peroxidase 1 (APX1), an important cytosolic ROS-scavenging enzyme. APX1 overexpression in the cpr5 background removed excessive ROS and restored the pleiotropic growth phenotype. Notably, APX1 overexpression did not reduce the resistance of cpr5 mutant to virulent strain Pseudomonas syringae pv. tomato (Pst) DC3000 and avirulent strain Pst DC3000 (avrRpt2). These results suggest that the removal of excessive ROS by APX1 overexpression restored the cpr5 growth phenotype while conserving pathogen resistance. Hence, our study provides a theoretical and empirical basis for utilizing CPR5 in the breeding of crops with disease resistance by effective oxidative stress management via APX1 expression.

Effects of plant age on antioxidant activity and endogenous hormones in Alpine Elymus sibiricus of the Tibetan Plateau

Elymus sibiricus L. is a perennial forage species that has potential to serve as a forage source in livestock grazing systems. However, E. sibiricus has been shown to have a rapid and substantial reduction of aboveground biomass and seed yield after 3 or 4 years and an accelerated aging process. To determine possible aging mechanisms, we planted E. sibiricus seeds in triplicate blocks in 2012, 2015, and 2016, respectively, and harvested samples of leaves and roots at the jointing and heading stages in 2018 and 2019 to determine oxidative indices and endogenous hormones. The fresh aboveground biomass of 4- and 5-year old plants declined by 34.2% and 52.4% respectively compared with 3-year old plants, and the seed yield declined by 12.7% and 34.1%, respectively. The water content in leaves was 51.7%, 43.3%, and 35.6%, and net photosynthesis was 7.73, 6.35, and 2.08 µmol/m²·s in 3-, 4-, and 5-year old plants, respectively. The superoxide anion radical generation rate in leaves and roots did not show any aging pattern. There was a non-significant increase in malondialdehyde concentration with plant age, particularly in leaves and roots at the heading stage in 2019. The superoxide dismutase activity showed a declining trend with age of plant roots at the jointing stage in both 2018 and 2019. The peroxidase activity declined with plant age in both leaves and roots, for example, and the catalase activity in roots 4- and 7-year old plants declined by 13.8% and 0.85%, respectively, compared to 3-year old plants at the heading stage in 2018. Therefore, the reduced capacity of the antioxidant system may lead to oxidative stress during plant aging process. Overall, the concentrations of plant hormones, auxin (IAA), gibberellin (GA), zeatin (ZT), and abscisic acid (ABA) were significantly lower in roots than in leaves. The IAA concentration in leaves and roots exhibited different patterns with plant age. The ZT concentrations in leaves of 3-year old plants was 2.39- and 2.62-fold of those in 4- and 7-year old plants, respectively at the jointing stage, and in roots, the concentration declined with plant age. The changes in the GA concentration with plant age varied between the physiological stages and between years. The ABA concentrations appeared to increase with plant age, particularly in leaves. In conclusion, the aging process of E. sibiricus was apparently associated with an increase in oxidative stress, a decrease of ZT and an increase of ABA, particularly in roots. These findings highlight the effects of plant age on the antioxidant and endogenous hormone activity of E. sibiricus. However, these plant age-related trends showed variations between plant physiological stages and between harvest years that needs to be researched in the future to develop strategies to manage this forage species.

Optimization of immune receptor-related hypersensitive cell death response assay using agrobacterium-mediated transient expression in tobacco plants

BACKGROUND

The study of the regulatory mechanisms of evolutionarily conserved Nucleotide-binding leucine-rich repeat (NLR) resistance (R) proteins in animals and plants is of increasing importance due to understanding basic immunity and the value of various crop engineering applications of NLR immune receptors. The importance of temperature is also emerging when applying NLR to crops responding to global climate change. In particular, studies of pathogen effector recognition and autoimmune activity of NLRs in plants can quickly and easily determine their function in tobacco using agro-mediated transient assay. However, there are conditions that should not be overlooked in these cell death-related assays in tobacco.

RESULTS

Environmental conditions play an important role in the immune response of plants. The system used in this study was to establish conditions for optimal hypertensive response (HR) cell death analysis by using the paired NLR RPS4/RRS1 autoimmune and AvrRps4 effector recognition system. The most suitable greenhouse temperature for growing plants was fixed at 22 °C. In this study, RPS4/RRS1-mediated autoimmune activity, RPS4 TIR domain-dependent cell death, and RPS4/RRS1-mediated HR cell death upon AvrRps4 perception significantly inhibited under conditions of 65% humidity. The HR is strongly activated when the humidity is below 10%. Besides, the leaf position of tobacco is important for HR cell death. Position #4 of the leaf from the top in 4-5 weeks old tobacco plants showed the most effective HR cell death.

CONCLUSIONS

As whole genome sequencing (WGS) or resistance gene enrichment sequencing (RenSeq) of various crops continues, different types of NLRs and their functions will be studied. At this time, if we optimize the conditions for evaluating NLR-mediated HR cell death, it will help to more accurately identify the function of NLRs. In addition, it will be possible to contribute to crop development in response to global climate change through NLR engineering.

Arabidopsis CPR5 plays a role in regulating nucleocytoplasmic transport of mRNAs in ethylene signaling pathway

Arabidopsis CPR5 is involved in regulation of ethylene signaling via two different ways: interacting with the ETR1 N-terminal domains, and controlling nucleocytoplasmic transport of ethylene-related mRNAs. The ETR1 receptor plays a predominant role in ethylene signaling in Arabidopsis thaliana. Previous studies showed that both RTE1 and CPR5 can directly bind to the ETR1 receptor and regulate ethylene signaling. RTE1 was suggested to promote the ETR1 receptor signaling by influencing its conformation, but little is known about the regulatory mechanism of CPR5 in ethylene signaling. In this study, we presented the data showing that both RTE1 and CPR5 bound to the N-terminal domains of ETR1, and regulated ethylene signaling via the ethylene receptor. On the other hand, the research provided evidence indicating that CPR5 could act as a nucleoporin to regulate the ethylene-related mRNAs export out of the nucleus, while RTE1 or its homolog (RTH) had no effect on the nucleocytoplasmic transport of mRNAs. Nuclear qRT-PCR analysis and poly(A)-mRNA in situ hybridization showed that defect of CPR5 restricted nucleocytoplasmic transport of mRNAs. These results advance our understanding of the regulatory mechanism of CPR5 in ethylene signaling.

A mutation in Arabidopsis SAL1 alters its in vitro activity against IP3 and delays developmental leaf senescence in association with lower ROS levels

Our manuscript is the first to find a link between activity of SAL1/OLD101 against IP₃ and plant leaf senescence regulation and ROS levels assigning a potential biological role for IP₃. Leaf senescence is a genetically programmed process that limits the longevity of a leaf. We identified and analyzed the recessive Arabidopsis stay-green mutation onset of leaf death 101 (old101). Developmental leaf longevity is extended in old101 plants, which coincided with higher peroxidase activity and decreased H₂O₂ levels in young 10-day-old, but not 25-day-old plants. The old101 phenotype is caused by a point mutation in SAL1, which encodes a bifunctional enzyme with inositol polyphosphate-1-phosphatase and 3' (2'), 5'-bisphosphate nucleotidase activity. SAL1 activity is highly specific for its substrates 3-polyadenosine 5-phosphate (PAP) and inositol 1, 4, 5-trisphosphate (IP₃), where it removes the 1-phosphate group from the IP₃ second messenger. The in vitro activity of recombinant old101 protein against its substrate IP₃ was 2.5-fold lower than that of wild type SAL1 protein. However, the in vitro activity of recombinant old101 mutant protein against PAP remained the same as that of the wild type SAL1 protein. The results open the possibility that the activity of SAL1 against IP₃ may affect the redox balance of young seedlings and that this delays the onset of leaf senescence.

The circadian-controlled PIF8-BBX28 module regulates petal senescence in rose flowers by governing mitochondrial ROS homeostasis at night

Reactive oxygen species (ROS) are unstable reactive molecules that are toxic to cells. Regulation of ROS homeostasis is crucial to protect cells from dysfunction, senescence, and death. In plant leaves, ROS are mainly generated from chloroplasts and are tightly temporally restricted by the circadian clock. However, little is known about how ROS homeostasis is regulated in nonphotosynthetic organs, such as petals. Here, we showed that hydrogen peroxide (H2O2) levels exhibit typical circadian rhythmicity in rose (Rosa hybrida) petals, consistent with the measured respiratory rate. RNA-seq and functional screening identified a B-box gene, RhBBX28, whose expression was associated with H2O2 rhythms. Silencing RhBBX28 accelerated flower senescence and promoted H2O2 accumulation at night in petals, while overexpression of RhBBX28 had the opposite effects. RhBBX28 influenced the expression of various genes related to respiratory metabolism, including the TCA cycle and glycolysis, and directly repressed the expression of SUCCINATE DEHYDROGENASE 1, which plays a central role in mitochondrial ROS (mtROS) homeostasis. We also found that PHYTOCHROME-INTERACTING FACTOR8 (RhPIF8) could activate RhBBX28 expression to control H2O2 levels in petals and thus flower senescence. Our results indicate that the circadian-controlled RhPIF8-RhBBX28 module is a critical player that controls flower senescence by governing mtROS homeostasis in rose.

Nitric oxide, crosstalk with stress regulators and plant abiotic stress tolerance

Nitric oxide is a dynamic gaseous molecule involved in signalling, crosstalk with stress regulators, and plant abiotic-stress responses. It has great exploratory potentials for engineering abiotic stress tolerance in crops. Nitric oxide (NO), a redox-active gaseous signalling molecule, though present uniformly through the eukaryotes, maintain its specificity in plants with respect to its formation, signalling, and functions. Its cellular concentrations are decisive for its function, as a signalling molecule at lower concentrations, but triggers nitro-oxidative stress and cellular damage when produced at higher concentrations. Besides, it also acts as a potent stress alleviator. Discovered in animals as neurotransmitter, NO has come a long way to being a stress radical and growth regulator in plants. As a key redox molecule, it exhibits several key cellular and molecular interactions including with reactive chemical species, hydrogen sulphide, and calcium. Apart from being a signalling molecule, it is emerging as a key player involved in regulations of plant growth, development and plant-environment interactions. It is involved in crosstalk with stress regulators and is thus pivotal in these stress regulatory mechanisms. NO is getting an unprecedented attention from research community, being investigated and explored for its multifaceted roles in plant abiotic stress tolerance. Through this review, we intend to present the current knowledge and updates on NO biosynthesis and signalling, crosstalk with stress regulators, and how biotechnological manipulations of NO pathway are leading towards developing transgenic crop plants that can withstand environmental stresses and climate change. The targets of various stress responsive miRNA signalling have also been discussed besides giving an account of current approaches used to characterise and detect the NO.

Investigating the Role of Root Exudates in Recruiting Streptomyces Bacteria to the Arabidopsis thaliana Microbiome

Streptomyces species are saprophytic soil bacteria that produce a diverse array of specialized metabolites, including half of all known antibiotics. They are also rhizobacteria and plant endophytes that can promote plant growth and protect against disease. Several studies have shown that streptomycetes are enriched in the rhizosphere and endosphere of the model plant Arabidopsis thaliana. Here, we set out to test the hypothesis that they are attracted to plant roots by root exudates, and specifically by the plant phytohormone salicylate, which they might use as a nutrient source. We confirmed a previously published report that salicylate over-producing cpr5 plants are colonized more readily by streptomycetes but found that salicylate-deficient sid2-2 and pad4 plants had the same levels of root colonization by Streptomyces bacteria as the wild-type plants. We then tested eight genome sequenced Streptomyces endophyte strains in vitro and found that none were attracted to or could grow on salicylate as a sole carbon source. We next used ¹³CO₂ DNA stable isotope probing to test whether Streptomyces species can feed off a wider range of plant metabolites but found that Streptomyces bacteria were outcompeted by faster growing proteobacteria and did not incorporate photosynthetically fixed carbon into their DNA. We conclude that, given their saprotrophic nature and under conditions of high competition, streptomycetes most likely feed on more complex organic material shed by growing plant roots. Understanding the factors that impact the competitiveness of strains in the plant root microbiome could have consequences for the effective application of biocontrol strains.

Ascorbate inactivates the oxygen-evolving complex in prolonged darkness

Ascorbate (Asc, vitamin C) is an essential metabolite participating in multiple physiological processes of plants, including environmental stress management and development. In this study, we acquired knowledge on the role of Asc in dark-induced leaf senescence using Arabidopsis thaliana as a model organism. One of the earliest effects of prolonged darkness is the inactivation of oxygen-evolving complexes (OEC) as demonstrated here by fast chlorophyll a fluorescence and thermoluminescence measurements. We found that inactivation of OEC due to prolonged darkness was attenuated in the Asc-deficient vtc2-4 mutant. On the other hand, the severe photosynthetic phenotype of a psbo1 knockout mutant, lacking the major extrinsic OEC subunit PSBO1, was further aggravated upon a 24-h dark treatment. The psbr mutant, devoid of the PSBR subunit of OEC, performed only slightly disturbed photosynthetic activity under normal growth conditions, whereas it showed a strongly diminished B thermoluminescence band upon dark treatment. We have also generated a double psbo1 vtc2 mutant, and it showed a slightly milder photosynthetic phenotype than the single psbo1 mutant. Our results, therefore, suggest that Asc leads to the inactivation of OEC in prolonged darkness by over-reducing the Mn-complex that is probably enabled by a dark-induced dissociation of the extrinsic OEC subunits. Our study is an example that Asc may negatively affect certain cellular processes and thus its concentration and localization need to be highly controlled.

Developmentally controlled changes during Arabidopsis leaf development indicate causes for loss of stress tolerance with age

Leaf senescence is the final stage of leaf development and is induced by the gradual occurrence of age-related changes (ARCs). The process of leaf senescence has been well described, but the cellular events leading to this process are still poorly understood. By analysis of progressively ageing, but not yet senescing, Arabidopsis thaliana rosette leaves, we aimed to better understand processes occurring prior to the onset of senescence. Using gene expression analysis, we found that as leaves mature, genes responding to oxidative stress and genes involved in stress hormone biosynthesis and signalling were up-regulated. A decrease in primary metabolites that provide protection against oxidative stress was a possible explanation for the increased stress signature. The gene expression and metabolomics changes occurred concomitantly to a decrease in drought, salinity, and dark stress tolerance of individual leaves. Importantly, stress-related genes showed elevated expression in the early ageing mutant old5 and decreased expression in the delayed ageing mutant ore9. We propose that the decreased stress tolerance with age results from the occurrence of senescence-inducing ARCs that is integrated into the leaf developmental programme, and that this ensures a timely and certain death.

Jasmonic acid promotes leaf senescence through MYC2-mediated repression of CATALASE2 expression in Arabidopsis

Plants relocate nutrients and energy from aging leaves to developing tissues during leaf senescence, which is important for plant growth, development, and responses to various environmental stimuli. Both jasmonic acid (JA) and H₂O₂ are two crucial signalling molecules positively regulating leaf senescence, whereas whether and how they are coordinated in leaf senescence remains elusive. Here, we report that H₂O₂ accumulates in JA-treated leaves, while scavenging the increased H₂O₂ can significantly suppresses JA-induced leaf senescence and the expression of senescence-associated genes (SAGs). The mutant myc2 with a mutation of MYC2, a master transcription factor in JA signalling pathway, exhibits delayed leaf senescence with increased catalase activity and decreased H₂O₂ accumulation compared with the wild type upon JA treatment. Further study showed that MYC2 downregulates CATALASE 2 (CAT2) expression by binding to its promoter, thus promoting JA-induced H₂O₂ accumulation and leaf senescence. Moreover, the delayed leaf senescence with reduced H₂O₂ accumulation and SAGs expression of the myc2 mutant is significantly reverted by the cat2-1 mutation in myc2 cat2-1 double mutant. Thus, our study reveals that JA represses CAT2 expression to increase H₂O₂ accumulation, thus promoting leaf senescence in a MYC2 dependent manner in Arabidopsis.

A Rapeseed WRKY Transcription Factor Phosphorylated by CPK Modulates Cell Death and Leaf Senescence by Regulating the Expression of ROS and SA-Synthesis-Related Genes

Salicylic acid (SA) and reactive oxygen species (ROS) are two well-defined inducers of leaf senescence. Here, we identified a novel WRKY transcription factor gene WSR1 (WRKY regulating SA and ROS 1) in Brassica napus (rapeseed) in promoting SA and ROS production, which eventually led to leaf senescence thereafter. Its expression increased in senescing leaves. Ca²⁺-dependent protein kinase (CPK) 5 and -6 interacted with and phosphorylated BnaWSR1. Overexpression of phosphomimic BnaWSR1 (BnaWSR1ca) in rapeseed protoplasts elicited ROS production and cell death while its ectopic expression in Arabidopsis enhanced SA and ROS levels and, hence, accelerated leaf senescence. Furthermore, BnaWSR1ca activated the expression of Isochorismate Synthase 1 (ICS1), Respiratory Burst Oxidase Homologue (Rboh) D, and Senescence-Associated Gene 14 (SAG14). Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) assays demonstrated that BnaWSR1ca directly bound to promoter regions of ICS1, RbohD, and SAG14. These data have identified a CPK-WSR1 module that integrates SA and ROS to control cell death and leaf senescence.

Putative alternative translation start site-encoding nucleotides of CPR5 regulate growth and resistance

BACKGROUND

The Arabidopsis CONSTITUTIVE EXPRESSER of PATHOGENESIS-RELATED GENES 5 (CPR5) has recently been shown to play a role in gating as part of the nuclear pore complex (NPC). Mutations in CPR5 cause multiple defects, including aberrant trichomes, reduced ploidy levels, reduced growth and enhanced resistance to bacterial and fungal pathogens. The pleiotropic nature of cpr5 mutations implicates that the CPR5 protein affects multiple pathways. However, little is known about the structural features that allow CPR5 to affect the different pathways.

RESULTS

Our in silico studies suggest that in addition to three clusters of putative nuclear localization signals and four or five transmembrane domains, CPR5 contains two putative alternative translation start sites. To test the role of the methionine-encoding nucleotides implicated in those sites, metCPR5 cDNAs, in which the relevant nucleotides were changed to encode glutamine, were fused to the CPR5 native promoter and the constructs transformed to cpr5-2 plants to complement cpr5-compromised phenotypes. The control and metCPR5 constructs were able to complement all cpr5 phenotypes, although the extent of complementation depended on the specific complementing plant lines. Remarkably, plants transformed with metCPR5 constructs showed larger leaves and displayed reduced resistance when challenged to Pseudomonas syringae pv Pst DC3000, as compared to control plants. Thus, the methionine-encoding nucleotides regulate growth and resistance. We propose that structural features of the CPR5 N-terminus are implicated in selective gating of proteins involved in regulating the balance between growth and resistance.

CONCLUSION

Plants need to carefully balance the amount of resources used for growth and resistance. The Arabidopsis CPR5 protein regulates plant growth and immunity. Here we show that N-terminal features of CPR5 are involved in the regulation of the balance between growth and resistance. These findings may benefit efforts to improve plant yield, while maintaining optimal levels of disease resistance.

Transcription factor CDF4 promotes leaf senescence and floral organ abscission by regulating abscisic acid and reactive oxygen species pathways in Arabidopsis

Leaf senescence is a highly complex developmental process that is tightly controlled by multiple layers of regulation. Abscisic acid (ABA) and reactive oxygen species (ROS) are two well-known factors that promote leaf senescence. We show here that the transcription factor CDF4 positively regulates leaf senescence. Constitutive and inducible overexpression of CDF4 accelerates leaf senescence, while knockdown of CDF4 delays it. CDF4 increases endogenous ABA levels by upregulating the transcription of the ABA biosynthesis genes 9-cis-epoxycarotenoid dioxygenase 2, 3 (NCED2, 3) and suppresses H₂ O₂ scavenging by repressing expression of the catalase2 (CAT2) gene. NCED2, 3 knockout and CAT2 overexpression partially rescue premature leaf senescence caused by CDF4 overexpression. We also show that CDF4 promotes floral organ abscission by activating the polygalacturonase PGAZAT gene. Based on these results, we propose that the levels of CDF4, ABA, and ROS undergo a gradual increase driven by their interlinking positive feedback loops during the leaf senescence and floral organ abscission processes.

The Arabidopsis SENESCENCE-ASSOCIATED GENE 13 Regulates Dark-Induced Senescence and Plays Contrasting Roles in Defense Against Bacterial and Fungal Pathogens

SENESCENCE-ASSOCIATED GENE 13 (SAG13) of Arabidopsis is a widely conserved gene of unknown function that has been extensively used as a marker of plant senescence. SAG13 induction occurs during plant cell death processes, including senescence and hypersensitive response, a type of programmed cell death that occurs in response to pathogens. This implies that SAG13 expression is regulated through at least two different signaling pathways affecting these two different processes. Our work highlights a contrasting role for SAG13 in regulating resistance against disease-causing biotrophic bacterial and necrotrophic fungal pathogens with contrasting infection strategies. We provide further evidence that SAG13 is not only induced during oxidative stress but also plays a role in protecting the plant against other stresses. SAG13 is also required for normal seed germination, seedling growth, and anthocyanin accumulation. The work presented here provides evidence for the role of SAG13 in regulating multiple plant processes including senescence, defense, seed germination, and abiotic stress responses. SAG13 is a valuable molecular marker for these processes and is conserved in multiple plant species, and this knowledge has important implications for crop improvement.

Identification of Genes Involved in Low Temperature-Induced Senescence of Panicle Leaf in Litchi chinensis

Warm winters and hot springs may promote panicle leaf growing and repress floral development. To identify genes potentially involved in litchi panicle leaf senescence, eight RNA-sequencing (RNA-Seq) libraries of the senescing panicle leaves under low temperature (LT) conditions and the developing panicle leaves under high temperature (HT) conditions were constructed. For each library, 4.78–8.99 × 10⁶ clean reads were generated. Digital expression of the genes was compared between the senescing and developing panicle leaves. A total of 6477 upregulated differentially expressed genes (DEGs) (from developing leaves to senescing leaves), and 6318 downregulated DEGs were identified, 158 abscisic acid (ABA)-, 68 ethylene-, 107 indole-3-acetic acid (IAA)-, 27 gibberellic acid (GA)-, 68 cytokinin (CTK)-, 37 salicylic acid (SA)-, and 23 brassinolide (BR)-related DEGs. Confirmation of the RNA-Seq data by quantitative real-time PCR (qRT-PCR) analysis suggested that expression trends of the 10 candidate genes using qRT-PCR were similar to those revealed by RNA-Seq, and a significantly positive correlation between the obtained data from qRT-PCR and RNA-Seq were found, indicating the reliability of our RNA-Seq data. The present studies provided potential genes for the future molecular breeding of new cultivars that can induce panicle leaf senescence and reduce floral abortion under warm climates.

A hidden link between leaf development and senescence

Leaf senescence is the final step of leaf development and is usually accompanied by visible color changes from green to yellow or brown. Unlike the senescence of the whole body of animals and unicellular organisms, which is often associated with death, leaf senescence in plants requires highly integrative processes towards cell death with nutrient recycling and storage. Since leaf senescence plays pivotal roles in the production of plant biomass and grain yield, the mechanisms of degradation and relocation of macromolecules as well as the regulation of signaling and biosynthetic pathways have received much attention. The importance of the plant hormone ethylene in the onset of leaf senescence has been clearly documented. However, research has increasingly demonstrated that the function of ethylene in the regulation of leaf senescence is dependent on leaf development. This review raises the issue of how ethylene requires developmental regulators and focuses on the developmental aspect of leaf senescence. It also emphasizes the remarkable impact that developmental regulators have on regulating the onset of leaf senescence.

O3-Induced Leaf Senescence in Tomato Plants Is Ethylene Signaling-Dependent and Enhances the Population Abundance of Bemisia tabaci

Elevated ozone (O3) can alter the phenotypes of host plants particularly in induction of leaf senescence, but few reports examine the involvement of phytohormone in O3-induced changes in host phenotypes that influence the foraging quality for insects. Here, we used an ethylene (ET) receptor mutant Nr and its wild-type to determine the function of the ET signaling pathway in O3-induced leaf senescence, and bottom-up effects on the performance of Bemisia tabaci in field open-top chambers (OTCs). Our results showed that elevated O3 reduced photosynthetic efficiency and chlorophyll content and induced leaf senescence of plant regardless of plant genotype. Leaf senescence in Nr plants was alleviated relative to wild-type under elevated O3. Further analyses of foliar quality showed that elevated O3 had little effect on phytohormone-mediated defenses, but significantly increased the concentration of amino acids in two plant genotypes. Furthermore, Nr plants had lower amino acid content relative to wild-type under elevated O3. These results provided an explanation of O3-induced increase in abundance of B. tabaci. We concluded that O3-induced senescence of plant was ET signal-dependent, and positive effects of O3-induced leaf senescence on the performance of B. tabaci largely resulted from changes of nutritional quality of host plants.

Reactive oxygen species and nitric oxide induce senescence of rudimentary leaves and the expression profiles of the related genes in Litchi chinensis

Litchi is one of the most important subtropical evergreen fruit trees in southern Asia. Previous studies indicated that high-temperature conditions encourage growth of rudimentary leaves in panicles and suppress flowering. We have demonstrated that methyl viologen dichloride hydrate (MV) and sodium nitroprusside (SNP) promoted flowering in litchi partially by inhibiting the growth of rudimentary leaves via reactive oxygen species (ROS) and nitric oxide (NO). In the present study, we examined the microstructure and ultrastructure, programmed cell death (PCD) ratio, nuclei morphology of the rudimentary leaves, and the expression of senescence-related genes after the treatment with ROS or NO. The results showed that chromatins of the ROS- or NO-treated cells in the rudimentary leaves were condensed. Fusion of the cytoplasm-digesting vesicles with the vacuole and degradation of cytoplasm forming scattered debris were found in those of the treated cells. Treatment with ROS or NO increased the cell PCD ratio. Morphology of the nuclei stained by propidium iodide (PI) showed that nuclei shape became irregular after the ROS or NO treatment. Further, the expression levels of LcRboh, LcMC-1-like, and LcPirin were higher in the ROS- and NO-treated rudimentary leaves than those in the control ones, suggesting that these genes may be involved in the ROS and NO-induced senescence and abscission of the rudimentary leaves in litchi. Our results suggested that ROS and NO play an important role in inducing the senescence of the rudimentary leaves, and ROS- and NO-induced PCD may be involved in the regulation of the rudimentary leaf growth in litchi.

Arabidopsis CPR5 regulates ethylene signaling via molecular association with the ETR1 receptor

The plant hormone ethylene plays various functions in plant growth, development and response to environmental stress. Ethylene is perceived by membrane-bound ethylene receptors, and among the homologous receptors in Arabidopsis, the ETR1 ethylene receptor plays a major role. The present study provides evidence demonstrating that Arabidopsis CPR5 functions as a novel ETR1 receptor-interacting protein in regulating ethylene response and signaling. Yeast split ubiquitin assays and bi-fluorescence complementation studies in plant cells indicated that CPR5 directly interacts with the ETR1 receptor. Genetic analyses indicated that mutant alleles of cpr5 can suppress ethylene insensitivity in both etr1-1 and etr1-2, but not in other dominant ethylene receptor mutants. Overexpression of Arabidopsis CPR5 either in transgenic Arabidopsis plants, or ectopically in tobacco, significantly enhanced ethylene sensitivity. These findings indicate that CPR5 plays a critical role in regulating ethylene signaling. CPR5 is localized to endomembrane structures and the nucleus, and is involved in various regulatory pathways, including pathogenesis, leaf senescence, and spontaneous cell death. This study provides evidence for a novel regulatory function played by CPR5 in the ethylene receptor signaling pathway in Arabidopsis.

CPR5 modulates salicylic acid and the unfolded protein response to manage tradeoffs between plant growth and stress responses

Completion of a plant's life cycle depends on successful prioritization of signaling favoring either growth or defense. Although hormones are pivotal regulators of growth-defense tradeoffs, the underlying signaling mechanisms remain obscure. The unfolded protein response (UPR) is essential for physiological growth as well as management of endoplasmic reticulum (ER) stress in unfavorable growth conditions. The plant UPR transducers are the kinase and ribonuclease IRE1 and the transcription factors bZIP28 and bZIP60. We analyzed management of the tradeoff between growth and ER stress defense by the stress response hormone salicylic acid (SA) and the UPR, which is modulated by SA via unknown mechanisms. We show that the plant growth and stress regulator CPR5, which represses accumulation of SA, favors growth in physiological conditions through inhibition of the SA-dependent IRE1-bZIP60 arm that antagonizes organ growth; CPR5 also favors growth in stress conditions through repression of ER stress-induced bZIP28/IRE1-bZIP60 arms. By demonstrating a physical interaction of CPR5 with bZIP60 and bZIP28, we provide mechanistic insights into CPR5-mediated modulation of UPR signaling. These findings define a critical surveillance strategy for plant growth-ER stress defense tradeoffs based on CPR5 and SA-modulated UPR signaling, whereby CPR5 acts as a positive modulator of growth in physiological conditions and in stress by antagonizing SA-dependent growth inhibition through UPR modulation.

NORE1/SAUL1 integrates temperature-dependent defense programs involving SGT1b and PAD4 pathways and leaf senescence in Arabidopsis

Leaf senescence is not only primarily governed by developmental age but also influenced by various internal and external factors. Although some genes that control leaf senescence have been identified, the detailed regulatory mechanisms underlying integration of diverse senescence-associated signals into the senescence programs remain to be elucidated. To dissect the regulatory pathways involved in leaf senescence, we isolated the not oresara1-1 (nore1-1) mutant showing accelerated leaf senescence phenotypes from an EMS-mutagenized Arabidopsis thaliana population. We found that altered transcriptional programs in defense response-related processes were associated with the accelerated leaf senescence phenotypes observed in nore1-1 through microarray analysis. The nore1-1 mutation activated defense program, leading to enhanced disease resistance. Intriguingly, high ambient temperature effectively suppresses the early senescence and death phenotypes of nore1-1. The gene responsible for the phenotypes of nore1-1 contains a missense mutation in SENESCENCE-ASSOCIATED E3 UBIQUITIN LIGASE 1 (SAUL1), which was reported as a negative regulator of premature senescence in the light intensity- and PHYTOALEXIN DEFICIENT 4 (PAD4)-dependent manner. Through extensive double mutant analyses, we recently identified suppressor of the G2 Allele of SKP1b (SGT1b), one of the positive regulators for disease resistance conferred by many resistance (R) proteins, as a downstream signaling component in NORE1-mediated senescence and cell death pathways. In conclusion, NORE1/SAUL1 is a key factor integrating signals from temperature-dependent defense programs and leaf senescence in Arabidopsis. These findings provide a new insight that plants might utilize defense response program in regulating leaf senescence process, possibly through recruiting the related genes during the evolution of the leaf senescence program.

The Thiol Reductase Activity of YUCCA6 Mediates Delayed Leaf Senescence by Regulating Genes Involved in Auxin Redistribution

Auxin, a phytohormone that affects almost every aspect of plant growth and development, is biosynthesized from tryptophan via the tryptamine, indole-3-acetamide, indole-3-pyruvic acid, and indole-3-acetaldoxime pathways. YUCCAs (YUCs), flavin monooxygenase enzymes, catalyze the conversion of indole-3-pyruvic acid (IPA) to the auxin (indole acetic acid). Arabidopsis thaliana YUC6 also exhibits thiol-reductase and chaperone activity in vitro; these activities require the highly conserved Cys-85 and are essential for scavenging of toxic reactive oxygen species (ROS) in the drought tolerance response. Here, we examined whether the YUC6 thiol reductase activity also participates in the delay in senescence observed in YUC6-overexpressing (YUC6-OX) plants. YUC6 overexpression delays leaf senescence in natural and dark-induced senescence conditions by reducing the expression of SENESCENCE-ASSOCIATED GENE 12 (SAG12). ROS accumulation normally occurs during senescence, but was not observed in the leaves of YUC6-OX plants; however, ROS accumulation was observed in YUC6-OX(C85S) plants, which overexpress a mutant YUC6 that lacks thiol reductase activity. We also found that YUC6-OX plants, but not YUC6-OX(C85S) plants, show upregulation of three genes encoding NADPH-dependent thioredoxin reductases (NTRA, NTRB, and NTRC), and GAMMA-GLUTAMYLCYSTEINE SYNTHETASE 1 (GSH1), encoding an enzyme involved in redox signaling. We further determined that excess ROS accumulation caused by methyl viologen treatment or decreased glutathione levels caused by buthionine sulfoximine treatment can decrease the levels of auxin efflux proteins such as PIN2-4. The expression of PINs is also reduced in YUC6-OX plants. These findings suggest that the thiol reductase activity of YUC6 may play an essential role in delaying senescence via the activation of genes involved in redox signaling and auxin availability.

Photosynthetic lesions can trigger accelerated senescence in Arabidopsis thaliana

Senescence is a highly regulated process characterized by the active breakdown of cells, which ultimately leads to the death of plant organs or whole plants. In annual plants such as Arabidopsis thaliana senescence can be observed in each individual leaf. Whether deficiencies in photosynthesis promote the induction of senescence was investigated by monitoring chlorophyll degradation, photosynthetic parameters, and reactive oxygen species accumulation in photosynthetic mutants. Several mutations affecting components of the photosynthetic apparatus, including psal-2, psan-2, and psbs, were found to lead to premature or faster senescence, as did simultaneous inactivation of the STN7 and STN8 kinases. Premature senescence is apparently not directly linked to an overall reduction in photosynthesis but to perturbations in specific aspects of the process. Dark-induced senescence is accelerated in mutants affected in linear electron flow, especially psad2-1, psan-2, and pete2-1, as well as in stn7 and stn8 mutants and STN7 and STN8 overexpressor lines. Interestingly, no direct link with ROS production could be observed.

A novel thiol-reductase activity of Arabidopsis YUC6 confers drought tolerance independently of auxin biosynthesis

YUCCA (YUC) proteins constitute a family of flavin monooxygenases (FMOs), with an important role in auxin (IAA) biosynthesis. Here we report that Arabidopsis plants overexpressing YUC6 display enhanced IAA-related phenotypes and exhibit improved drought stress tolerance, low rate of water loss and controlled ROS accumulation under drought and oxidative stresses. Co-overexpression of an IAA-conjugating enzyme reduces IAA levels but drought stress tolerance is unaffected, indicating that the stress-related phenotype is not based on IAA overproduction. YUC6 contains a previously unrecognized FAD- and NADPH-dependent thiol-reductase activity (TR) that overlaps with the FMO domain involved in IAA biosynthesis. Mutation of a conserved cysteine residue (Cys-85) preserves FMO but suppresses TR activity and stress tolerance, whereas mutating the FAD- and NADPH-binding sites, that are common to TR and FMO domains, abolishes all outputs. We provide a paradigm for a single protein playing a dual role, regulating plant development and conveying stress defence responses.

Fusarium oxysporum triggers tissue-specific transcriptional reprogramming in Arabidopsis thaliana

Some of the most devastating agricultural diseases are caused by root-infecting pathogens, yet the majority of studies on these interactions to date have focused on the host responses of aerial tissues rather than those belowground. Fusarium oxysporum is a root-infecting pathogen that causes wilt disease on several plant species including Arabidopsis thaliana. To investigate and compare transcriptional changes triggered by F. oxysporum in different Arabidopsis tissues, we infected soil-grown plants with F. oxysporum and subjected root and leaf tissue harvested at early and late timepoints to RNA-seq analyses. At least half of the genes induced or repressed by F. oxysporum showed tissue-specific regulation. Regulators of auxin and ABA signalling, mannose binding lectins and peroxidases showed strong differential expression in root tissue. We demonstrate that ARF2 and PRX33, two genes regulated in the roots, promote susceptibility to F. oxysporum. In the leaves, defensins and genes associated with the response to auxin, cold and senescence were strongly regulated while jasmonate biosynthesis and signalling genes were induced throughout the plant.

OsWRKY42 represses OsMT1d and induces reactive oxygen species and leaf senescence in rice

We isolated a rice (Oryza sativa L.) WRKY gene which is highly upregulated in senescent leaves, denoted OsWRKY42. Analysis of OsWRKY42-GFP expression and its effects on transcriptional activation in maize protoplasts suggested that the OsWRKY42 protein functions as a nuclear transcriptional repressor. OsWRKY42-overexpressing (OsWR KY42OX) transgenic rice plants exhibited an early leaf senescence phenotype with accumulation of the reactive oxygen species (ROS) hydrogen peroxide and a reduced chlorophyll content. Expression analysis of ROS producing and scavenging genes revealed that the metallothionein genes clustered on chromosome 12, especially OsMT1d, were strongly repressed in OsWRKY42OX plants. An OsMT1d promoter:LUC construct was found to be repressed by OsWRKY42 overexpression in rice protoplasts. Finally, chromatin immunoprecipitation analysis demonstrated that OsWRKY42 binds to the W-box of the OsMT1d promoter. Our results thus suggest that OsWRKY42 represses OsMT1d-mediated ROS scavenging and thereby promotes leaf senescence in rice.

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

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

Comparative changes in the antioxidant system in the flag leaf of early and normally senescing near-isogenic lines of wheat (Triticum aestivum L.)

The antioxidant system was significantly inhibited in the early aging line than the near-isogenic normal aging line during senescence. The antioxidant system plays pivotal roles in removal of reactive oxygen species (ROS) produced during leaf senescence. To explore its roles in leaf senescence of wheat (Triticum aestivum L.), the concentrations of antioxidants, activities, and gene expression of antioxidant enzymes were evaluated in flag leaves of the early aging line (EAL) and the near-isogenic normal aging line (NL) during senescence. The results showed that the total chlorophyll and soluble protein in the EAL declined earlier and faster, while more malondialdehyde and ROS accumulated compared with the NL. The activities of superoxide dismutase, catalase, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase were lower in the EAL than in the NL across multiple measuring dates. Additionally, the EAL had less amounts of reduced ascorbate and glutathione as well as lower reduction state with the progression of senescence. Concomitantly, the gene expression of antioxidant enzymes in the EAL was also significantly repressed relative to those in the NL during natural senescence. Taken together, the earlier onset and faster rate of senescence in the EAL could be a result of an imbalance of ROS production and ROS-scavenging antioxidant system, which provided valuable hints toward understanding leaf senescence of wheat.

Stitching together the Multiple Dimensions of Autophagy Using Metabolomics and Transcriptomics Reveals Impacts on Metabolism, Development, and Plant Responses to the Environment in Arabidopsis

Autophagy is a fundamental process in the plant life story, playing a key role in immunity, senescence, nutrient recycling, and adaptation to the environment. Transcriptomics and metabolomics of the rosette leaves of Arabidopsis thaliana autophagy mutants (atg) show that autophagy is essential for cell homeostasis and stress responses and that several metabolic pathways are affected. Depletion of hexoses, quercetins, and anthocyanins parallel the overaccumulation of several amino acids and related compounds, such as glutamate, methionine, glutathione, pipecolate, and 2-aminoadipate. Transcriptomic data show that the pathways for glutathione, methionine, raffinose, galacturonate, and anthocyanin are perturbed. Anthocyanin depletion in atg mutants, which was previously reported as a possible defect in flavonoid trafficking to the vacuole, appears due to the downregulation of the master genes encoding the enzymes and regulatory proteins involved in flavonoid biosynthesis. Overexpression of the PRODUCTION OF ANTHOCYANIN PIGMENT1 transcription factor restores anthocyanin accumulation in vacuoles of atg mutants. Transcriptome analyses reveal connections between autophagy and (1) salicylic acid biosynthesis and response, (2) cytokinin perception, (3) oxidative stress and plant defense, and possible interactions between autophagy and the COP9 signalosome machinery. The metabolic and transcriptomic signatures identified for the autophagy mutants are discussed and show consistencies with the observed phenotypes.

Knockdown of GDCH gene reveals reactive oxygen species-induced leaf senescence in rice

Glycine decarboxylase complex (GDC) is a multi-protein complex, comprising P-, H-, T- and L-protein subunits, which plays a major role in photorespiration in plants. While structural analysis has demonstrated that the H subunit of GDC (GDCH) plays a pivotal role in GDC, research on the role of GDCH in biological processes in plants is seldom reported. Here, the function of GDCH, stresses resulting from GDCH-knockdown and the interactions of these stresses with other cellular processes were studied in rice plants. Under high CO(2), the OsGDCH RNA interference (OsGDCH-RNAi) plants grew normally, but under ambient CO(2), severely suppressed OsGDCH-RNAi plants (SSPs) were non-viable, which displayed a photorespiration-deficient phenotype. Under ambient CO(2), chlorophyll loss, protein degradation, lipid peroxidation and photosynthesis decline occurred in SSPs. Electron microscopy studies showed that chloroplast breakdown and autophagy took place in these plants. Reactive oxygen species (ROS), including O2(-) and H(2)O(2), accumulated and the antioxidant enzyme activities decreased in the leaves of SSPs under ambient CO(2). The expression of transcription factors and senescence-associated genes (SAGs), which was up-regulated in SSPs after transfer to ambient CO(2), was enhanced in wild-type plants treated with H(2)O(2). Evidences demonstrate ROS induce senescence in SSPs, and transcription factors OsWRKY72 may mediate the ROS-induced senescence.

A feedback regulatory loop between G3P and lipid transfer proteins DIR1 and AZI1 mediates azelaic-acid-induced systemic immunity

Systemic acquired resistance (SAR), a highly desirable form of plant defense, provides broad-spectrum immunity against diverse pathogens. The recent identification of seemingly unrelated chemical inducers of SAR warrants an investigation of their mutual interrelationships. We show that SAR induced by the dicarboxylic acid azelaic acid (AA) requires the phosphorylated sugar derivative glycerol-3-phosphate (G3P). Pathogen inoculation induced the release of free unsaturated fatty acids (FAs) and thereby triggered AA accumulation, because these FAs serve as precursors for AA. AA accumulation in turn increased the levels of G3P, which is required for AA-conferred SAR. The lipid transfer proteins DIR1 and AZI1, both of which are required for G3P- and AA-induced SAR, were essential for G3P accumulation. Conversely, reduced G3P resulted in decreased AZI1 and DIR1 transcription. Our results demonstrate that an intricate feedback regulatory loop among G3P, DIR1, and AZI1 regulates SAR and that AA functions upstream of G3P in this pathway.

The MPK6-ERF6-ROS-responsive cis-acting Element7/GCC box complex modulates oxidative gene transcription and the oxidative response in Arabidopsis

Reactive oxygen species (ROS) have been characterized as both important signaling molecules and universal stressors that mediate many developmental and physiological responses. So far, details of the transcriptional mechanism of ROS-responsive genes are largely unknown. In the study reported here, we identified seven potential ROS-responsive cis-acting elements (ROSEs) from the promoters of genes up-regulated by ROS in Arabidopsis (Arabidopsis thaliana). We also found that the APETALA2/ethylene-responsive element binding factor6 (ERF6) could bind specifically to the ROSE7/GCC box. Coexpression of ERF6 enhanced luciferase activity driven by ROSE7. The deficient mutants of ERF6 showed growth retardation and higher sensitivity to photodamage. ERF6 interacted physically with mitogen-activated protein kinase6 (MPK6) and also served as a substrate of MPK6. MPK6-mediated ERF6 phosphorylation at both serine-266 and serine-269 affected the dynamic alternation of the ERF6 protein, which resulted in changes in ROS-responsive gene transcription. These data might provide new insight into the mechanisms that regulate ROS-responsive gene transcription via a complex of MPK6, ERF6, and the ROSE7/GCC box under oxidative stress or a fluctuating light environment.

H2O2-induced leaf cell death and the crosstalk of reactive nitric/oxygen species

In plants, the chloroplast is the main reactive oxygen species (ROS) producing site under high light stress. Catalase (CAT), which decomposes hydrogen peroxide (H2 O2 ), is one of the controlling enzymes that maintains leaf redox homeostasis. The catalase mutants with reduced leaf catalase activity from different plant species exhibit an H2 O2 -induced leaf cell death phenotype. This phenotype was differently affected by light intensity or photoperiod, which may be caused by plant species, leaf redox status or growth conditions. In the rice CAT mutant nitric oxide excess 1 (noe1), higher H2 O2 levels induced the generation of nitric oxide (NO) and higher S-nitrosothiol (SNO) levels, suggesting that NO acts as an important endogenous mediator in H2 O2 -induced leaf cell death. As a free radical, NO could also react with other intracellular and extracellular targets and form a series of related molecules, collectively called reactive nitrogen species (RNS). Recent studies have revealed that both RNS and ROS are important partners in plant leaf cell death. Here, we summarize the recent progress on H2 O2 -induced leaf cell death and the crosstalk of RNS and ROS signals in the plant hypersensitive response (HR), leaf senescence, and other forms of leaf cell death triggered by diverse environmental conditions. [Formula: see text] [ Chengcai Chu (Corresponding author)].

Cross-talk of nitric oxide and reactive oxygen species in plant programed cell death

In plants, programed cell death (PCD) is an important mechanism to regulate multiple aspects of growth and development, as well as to remove damaged or infected cells during responses to environmental stresses and pathogen attacks. Under biotic and abiotic stresses, plant cells exhibit a rapid synthesis of nitric oxide (NO) and a parallel accumulation of reactive oxygen species (ROS). Frequently, these responses trigger a PCD process leading to an intrinsic execution of plant cells. The accumulating evidence suggests that both NO and ROS play key roles in PCD. These redox active small molecules can trigger cell death either independently or synergistically. Here we summarize the recent progress on the cross-talk of NO and ROS signals in the hypersensitive response, leaf senescence, and other kinds of plant PCD caused by diverse cues.

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.

Growth under elevated atmospheric CO(2) concentration accelerates leaf senescence in sunflower (Helianthus annuus L.) plants

Some morphogenetic and metabolic processes were sensitive to a high atmospheric CO(2) concentration during sunflower primary leaf ontogeny. Young leaves of sunflower plants growing under elevated CO(2) concentration exhibited increased growth, as reflected by the high specific leaf mass referred to as dry weight in young leaves (16 days). The content of photosynthetic pigments decreased with leaf development, especially in plants grown under elevated CO(2) concentrations, suggesting that high CO(2) accelerates chlorophyll degradation, and also possibly leaf senescence. Elevated CO(2) concentration increased the oxidative stress in sunflower plants by increasing H(2)O(2) levels and decreasing activity of antioxidant enzymes such as catalase and ascorbate peroxidase. The loss of plant defenses probably increases the concentration of reactive oxygen species in the chloroplast, decreasing the photosynthetic pigment content as a result. Elevated CO(2) concentration was found to boost photosynthetic CO(2) fixation, especially in young leaves. High CO(2) also increased the starch and soluble sugar contents (glucose and fructose) and the C/N ratio during sunflower primary leaf development. At the beginning of senescence, we observed a strong increase in the hexoses to sucrose ratio that was especially marked at high CO(2) concentration. These results indicate that elevated CO(2) concentration could promote leaf senescence in sunflower plants by affecting the soluble sugar levels, the C/N ratio and the oxidative status during leaf ontogeny. It is likely that systemic signals produced in plants grown with elevated CO(2), lead to early senescence and a higher oxidation state of the cells of these plant leaves.

Senescence-specific alteration of hydrogen peroxide levels in Arabidopsis thaliana and oilseed rape spring variety Brassica napus L. cv. Mozart

In order to analyze the signaling function of hydrogen peroxide (H(2)O(2)) production in senescence in more detail, we manipulated intracellular H(2)O(2) levels in Arabidopsis thaliala (L.) Heynh by using the hydrogen-peroxide-sensitive part of the Escherichia coli transcription regulator OxyR, which was directed to the cytoplasm as well as into the peroxisomes. H(2)O(2) levels were lowered and senescence was delayed in both transgenic lines, but OxyR was found to be more effective in the cytoplasm. To transfer this knowledge to crop plants, we analyzed oilseed rape plants Brassica napus L. cv. Mozart for H(2)O(2) and its scavenging enzymes catalase (CAT) and ascorbate peroxidase (APX) during leaf and plant development. H(2)O(2) levels were found to increase during bolting and flowering time, but no increase could be observed in the very late stages of senescence. With increasing H(2)O(2) levels, CAT and APX activities declined, so it is likely that similar mechanisms are used in oilseed rape and Arabidopsis to control H(2)O(2) levels. Under elevated CO(2) conditions, oilseed rape senescence was accelerated and coincided with an earlier increase in H(2)O(2) levels, indicating that H(2)O(2) may be one of the signals to inducing senescence in a broader range of Brassicaceae.

Gene network analysis and functional studies of senescence-associated genes reveal novel regulators of Arabidopsis leaf senescence

Plant leaf senescence has been recognized as the last phase of plant development, a highly ordered process regulated by genes known as senescence associated genes (SAGs). However, the function of most of SAGs in regulating leaf senescence as well as regulators of those functionally known SAGs are still unclear. We have previously developed a curated database of genes potentially associated with leaf senescence, the Leaf Senescence Database (LSD). In this study, we built gene networks to identify common regulators of leaf senescence in Arabidopsis thaliana using promoting or delaying senescence genes in LSD. Our results demonstrated that plant hormones cytokinin, auxin, nitric oxide as well as small molecules, such as Ca(2+), delay leaf senescence. By contrast, ethylene, ABA, SA and JA as well as small molecules, such as oxygen, promote leaf senescence, altogether supporting the idea that phytohormones play a critical role in regulating leaf senescence. Functional analysis of candidate SAGs in LSD revealed that a WRKY transcription factor WRKY75 and a Cys2/His2-type transcription factor AZF2 are positive regulators of leaf senescence and loss-of-function of WRKY75 or AZF2 delayed leaf senescence. We also found that silencing of a protein phosphatase, AtMKP2, promoted early senescence. Collectively, LSD can serve as a comprehensive resource for systematic study of the molecular mechanism of leaf senescence as well as offer candidate genes for functional analyses.

Leaf senescence and abiotic stresses share reactive oxygen species-mediated chloroplast degradation

Leaf senescence is a genetically programmed decline in various cellular processes including photosynthesis and involves the hydrolysis of macromolecules such as proteins, lipids, etc. It is governed by the developmental age and is induced or enhanced by environmental stresses such as drought, heat, salinity and others. Internal factors such as reproductive structures also influence the rate of leaf senescence. Reactive oxygen species (ROS) generation is one of the earliest responses of plant cells under abiotic stresses and senescence. Chloroplasts are the main targets of ROS-linked damage during various environmental stresses and natural senescence as ROS detoxification systems decline with age. Plants adapt to environmental stresses through the process of acclimation, which involves less ROS production coupled with an efficient antioxidant defence. Chloroplasts are a major site of protein degradation, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is rapidly and selectively degraded during senescence and stress. The process of protein degradation is initiated by ROS and involves the action of proteolytic enzymes such as cysteine and serine proteases. The mechanism of Rubisco degradation still remains to be elucidated. The molecular understanding of leaf senescence was achieved through the characterization of senescence-associated genes and various senescence mutants of Arabidopsis, which is a suitable model plant showing monocarpic senescence. The regulation of senescence involves many regulatory elements composed of positive and negative elements to fine-tune the initiation and progression of senescence. This review gives an overview on chloroplast protein degradation during leaf senescence and abiotic stresses and also highlights the role of ROS management in both processes.

To close or not to close: plasmodesmata in defense

Cell death is a biological process that occurs during differentiation and maturation of certain cell types, during senescence, or as part of a defense mechanism against microbial pathogens. Intercellular coordination is thought to be necessary to restrict the spread of death signals, although little is known about how cell death is controlled at the tissue level. The recent characterization of a plasmodesmal protein, PDLP5, has revealed an important role for plasmodesmal control during salicylic acid-mediated cell death responses. Here, we discuss molecular factors that are potentially involved in PDLP5 expression, and explore possible signaling networks that PDLP5 interacts with during basal defense responses.

Role of ARABIDOPSIS A-FIFTEEN in regulating leaf senescence involves response to reactive oxygen species and is dependent on ETHYLENE INSENSITIVE2

Leaf senescence is a highly regulated developmental process that is coordinated by several factors. Many senescence-associated genes (SAGs) have been identified, but their roles during senescence remain unclear. A sweet potato (Ipomoea batatas) SAG, named SPA15, whose function was unknown, was identified previously. To understand the role of SPA15 in leaf senescence further, the orthologue of SPA15 in Arabidopsis thaliana was identified and characterized, and it was named ARABIDOPSIS A-FIFTEEN (AAF). AAF was expressed in early senescent leaves and in tissues with highly proliferative activities. AAF was localized to the chloroplasts by transient expression in Arabidopsis mesophyll protoplasts. Overexpression of AAF (AAF-OX) in Arabidopsis promoted, but the T-DNA insertion mutant (aaf-KO), delayed age-dependent leaf senescence. Furthermore, stress-induced leaf senescence caused by continuous darkness was enhanced in AAF-OX but suppressed in aaf-KO. Transcriptome analysis of expression profiles revealed up-regulated genes related to pathogen defence, senescence, and oxidative stress in 3-week-old AAF-OX plants. Indeed, elevated levels of reactive oxygen species (ROS) and enhanced sensitivity to oxidative and dark stress were apparent in AAF-OX but reduced in aaf-KO. ETHYLENE INSENSITIVE2 (EIN2) was required for the dark- and ROS-induced senescence phenotypes in AAF-OX and the induction of AAF expression by treatment with the immediate precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid. The results indicate the functional role of AAF is an involvement in redox homeostasis to regulate leaf senescence mediated by age and stress factors during Arabidopsis development.

GeBP/GPL transcription factors regulate a subset of CPR5-dependent processes

The CONSTITUTIVE EXPRESSOR OF PATHOGENESIS-RELATED GENES5 (CPR5) gene of Arabidopsis (Arabidopsis thaliana) encodes a putative membrane protein of unknown biochemical function and displays highly pleiotropic functions, particularly in pathogen responses, cell proliferation, cell expansion, and cell death. Here, we demonstrate a link between CPR5 and the GLABRA1 ENHANCER BINDING PROTEIN (GeBP) family of transcription factors. We investigated the primary role of the GeBP/GeBP-like (GPL) genes using transcriptomic analysis of the quadruple gebp gpl1,2,3 mutant and one overexpressing line that displays several cpr5-like phenotypes including dwarfism, spontaneous necrotic lesions, and increased pathogen resistance. We found that GeBP/GPLs regulate a set of genes that represents a subset of the CPR5 pathway. This subset includes genes involved in response to stress as well as cell wall metabolism. Analysis of the quintuple gebp gpl1,2,3 cpr5 mutant indicates that GeBP/GPLs are involved in the control of cell expansion in a CPR5-dependent manner but not in the control of cell proliferation. In addition, to our knowledge, we provide the first evidence that the CPR5 protein is localized in the nucleus of plant cells and that a truncated version of the protein with no transmembrane domain can trigger cpr5-like processes when fused to the VP16 constitutive transcriptional activation domain. Our results provide clues on how CPR5 and GeBP/GPLs play opposite roles in the control of cell expansion and suggest that the CPR5 protein is involved in transcription.

Loss-of-function of Constitutive Expresser of Pathogenesis Related Genes5 affects potassium homeostasis in Arabidopsis thaliana

Here, we demonstrate that the reduction in leaf K(+) observed in a mutant previously identified in an ionomic screen of fast neutron mutagenized Arabidopsis thaliana is caused by a loss-of-function allele of CPR5, which we name cpr5-3. This observation establishes low leaf K(+) as a new phenotype for loss-of-function alleles of CPR5. We investigate the factors affecting this low leaf K(+) in cpr5 using double mutants defective in salicylic acid (SA) and jasmonic acid (JA) signalling, and by gene expression analysis of various channels and transporters. Reciprocal grafting between cpr5 and Col-0 was used to determine the relative importance of the shoot and root in causing the low leaf K(+) phenotype of cpr5. Our data show that loss-of-function of CPR5 in shoots primarily determines the low leaf K(+) phenotype of cpr5, though the roots also contribute to a lesser degree. The low leaf K(+) phenotype of cpr5 is independent of the elevated SA and JA known to occur in cpr5. In cpr5 expression of genes encoding various Cyclic Nucleotide Gated Channels (CNGCs) are uniquely elevated in leaves. Further, expression of HAK5, encoding the high affinity K(+) uptake transporter, is reduced in roots of cpr5 grown with high or low K(+) supply. We suggest a model in which low leaf K(+) in cpr5 is driven primarily by enhanced shoot-to-root K(+) export caused by a constitutive activation of the expression of various CNGCs. This activation may enhance K(+) efflux, either indirectly via enhanced cytosolic Ca(2+) and/or directly by increased K(+) transport activity. Enhanced shoot-to-root K(+) export may also cause the reduced expression of HAK5 observed in roots of cpr5, leading to a reduction in uptake of K(+). All ionomic data presented is publically available at www.ionomicshub.org.

Regulatory subunit B'gamma of protein phosphatase 2A prevents unnecessary defense reactions under low light in Arabidopsis

Light is an important environmental factor that modulates acclimation strategies and defense responses in plants. We explored the functional role of the regulatory subunit B'γ (B'γ) of protein phosphatase 2A (PP2A) in light-dependent stress responses of Arabidopsis (Arabidopsis thaliana). The predominant form of PP2A consists of catalytic subunit C, scaffold subunit A, and highly variable regulatory subunit B, which determines the substrate specificity of PP2A holoenzymes. Mutant leaves of knockdown pp2a-b'γ plants show disintegration of chloroplasts and premature yellowing conditionally under moderate light intensity. The cell-death phenotype is accompanied by the accumulation of hydrogen peroxide through a pathway that requires CONSTITUTIVE EXPRESSION OF PR GENES5 (CPR5). Moreover, the pp2a-b'γ cpr5 double mutant additionally displays growth suppression and malformed trichomes. Similar to cpr5, the pp2a-b'γ mutant shows constitutive activation of both salicylic acid- and jasmonic acid-dependent defense pathways. In contrast to cpr5, however, pp2a-b'γ leaves do not contain increased levels of salicylic acid or jasmonic acid. Rather, the constitutive defense response associates with hypomethylation of DNA and increased levels of methionine-salvage pathway components in pp2a-b'γ leaves. We suggest that the specific B'γ subunit of PP2A is functionally connected to CPR5 and operates in the basal repression of defense responses under low irradiance.

Nitric oxide regulates DELLA content and PIF expression to promote photomorphogenesis in Arabidopsis

The transition from etiolated to green seedlings involves a shift from hypocotyl growth-promoting conditions to growth restraint. These changes occur through a complex light-driven process involving multiple and tightly coordinated hormonal signaling pathways. Nitric oxide (NO) has been lately characterized as a regulator of plant development interacting with hormone signaling. Here, we show that Arabidopsis (Arabidopsis thaliana) NO-deficient mutant hypocotyls are longer than those from wild-type seedlings under red light but not under blue or far-red light. Accordingly, exogenous treatment with the NO donor sodium nitroprusside and mutant plants with increased endogenous NO levels resulted in reduced hypocotyl length. In addition to increased hypocotyl elongation, NO deficiency led to increased anthocyanin levels and reduced PHYB content under red light, all processes governed by phytochrome-interacting factors (PIFs). NO-deficient plants accordingly showed an enhanced expression of PIF3, PIF1, and PIF4. Moreover, exogenous NO increased the levels of the gibberellin (GA)-regulated DELLA proteins and shortened hypocotyls, likely through the negative regulation of the GA Insensitive Dwarf1 (GID1)-Sleepy1 (SLY1) module. Consequently, NO-deficient seedlings displayed up-regulation of SLY1, defective DELLA accumulation, and altered GA sensitivity, thus resulting in defective deetiolation under red light. Accumulation of NO in wild-type seedlings undergoing red light-triggered deetiolation and elevated levels of NO in the GA-deficient ga1-3 mutant in darkness suggest a mutual NO-GA antagonism in controlling photomorphogenesis. PHYB-dependent NO production promotes photomorphogenesis by a GID1-GA-SLY1-mediated mechanism based on the coordinated repression of growth-promoting PIF genes and the increase in the content of DELLA proteins.