COS1: an Arabidopsis coronatine insensitive1 suppressor essential for regulation of jasmonate-mediated plant defense and senescence

ERF114/115/109 are essential for jasmonate-repressed non-canonical JAZ8 activity in JA signaling

Jasmonate (JA), a key plant hormone, regulates various aspects of plant development and stress responses, primarily through the degradation of canonical jasmonate-ZIM domain (JAZ) proteins by the SCFCOI1 complex. While JAZ8, a non-canonical JAZ protein lacking the degron signal, has been shown to repress JA responses, the mechanism by which JA inhibits JAZ8 activity remains unclear. Here, we demonstrate that Arabidopsis ethylene response factor 114 (ERF114), ERF115, and ERF109 regulate JA signaling through interacting with JAZ8. This interaction disrupts the formation of the MYC2/3/4-JAZ8 and root hair defective 6 (RHD6)-JAZ8 complexes. We show that ERF114 positively regulates JA-induced transcriptional responses and that JA-promoted root hair growth is highly alleviated in erf114 mutants. Furthermore, the transcription of ERF114/115/109 is induced by JA in an MYC2-dependent manner, thus forming a positive feedback loop in JA signaling. Collectively, this study reveals a regulatory pathway in which ERF114/115/109 regulate JA signaling by targeting non-canonical JAZ proteins.

Identification of a drought stress response module in tomato plants commonly induced by fungal endophytes that confer increased drought tolerance

Global climate change exacerbates abiotic stresses, as drought, heat, and salt stresses are anticipated to increase significantly in the coming years. Plants coexist with a diverse range of microorganisms. Multiple inter-organismic relationships are known to confer benefits to plants, including growth promotion and enhanced tolerance to abiotic stresses. In this study, we investigated the mutualistic interactions between three fungal endophytes originally isolated from distinct arid environments and an agronomically relevant crop, Solanum lycopersicum. We demonstrated a significant increase in shoot biomass under drought conditions in co-cultivation with Penicillium chrysogenum isolated from Antarctica, Penicillium minioluteum isolated from the Atacama Desert, Chile, and Serendipita indica isolated from the Thar Desert, India. To elucidate plant gene modules commonly induced by the different endophytes that could explain the observed drought tolerance effect in tomato, a comprehensive transcriptomics analysis was conducted. This analysis led to the identification of a shared gene module in the fungus-infected tomato plants. Within this module, gene network analysis enabled us to identify genes related to abscisic acid (ABA) signaling, ABA transport, auxin signaling, ion homeostasis, proline biosynthesis, and jasmonic acid signaling, providing insights into the molecular basis of drought tolerance commonly mediated by fungal endophytes. Our findings highlight a conserved response in the mutualistic interactions between endophytic fungi isolated from unrelated environments and tomato roots, resulting in improved shoot biomass production under drought stress.

Cullin-Conciliated Regulation of Plant Immune Responses: Implications for Sustainable Crop Protection

Cullins are crucial components of the ubiquitin-proteasome system, playing pivotal roles in the regulation of protein metabolism. This review provides insight into the wide-ranging functions of cullins, particularly focusing on their impact on plant growth, development, and environmental stress responses. By modulating cullin-mediated protein mechanisms, researchers can fine-tune hormone-signaling networks to improve various agronomic traits, including plant architecture, flowering time, fruit development, and nutrient uptake. Furthermore, the targeted manipulation of cullins that are involved in hormone-signaling pathways, e.g., cytokinin, auxin, gibberellin, abscisic acids, and ethylene, can boost crop growth and development while increasing yield and enhancing stress tolerance. Furthermore, cullins also play important roles in plant defense mechanisms through regulating the defense-associated protein metabolism, thus boosting resistance to pathogens and pests. Additionally, this review highlights the potential of integrating cullin-based strategies with advanced biological tools, such as CRISPR/Cas9-mediated genome editing, genetic engineering, marker-associated selections, gene overexpression, and gene knockout, to achieve precise modifications for crop improvement and sustainable agriculture, with the promise of creating resilient, high-yielding, and environmentally friendly crop varieties.

AtDREB2G is involved in the regulation of riboflavin biosynthesis in response to low-temperature stress and abscisic acid treatment in Arabidopsis thaliana

Riboflavin (RF) serves as a precursor to flavin mononucleotide and flavin adenine dinucleotide, which are crucial cofactors in various metabolic processes. Strict regulation of cellular flavin homeostasis is imperative, yet information regarding the factors governing this regulation remains largely elusive. In this study, we first examined the impact of external flavin treatment on the Arabidopsis transcriptome to identify novel regulators of cellular flavin levels. Our analysis revealed alterations in the expression of 49 putative transcription factors. Subsequent reverse genetic screening highlighted a member of the dehydration-responsive element binding (DREB) family, AtDREB2G, as a potential regulator of cellular flavin levels. Knockout mutants of AtDREB2G (dreb2g) exhibited reduced flavin levels and decreased expression of RF biosynthetic genes compared to wild-type plants. Conversely, conditional overexpression of AtDREB2G led to an increase in the expression of RF biosynthetic genes and elevated flavin levels. In wild-type plants, exposure to low temperatures and abscisic acid treatment stimulated enhanced flavin levels and upregulated the expression of RF biosynthetic genes, concomitant with the induction of AtDREB2G. Notably, these responses were significantly attenuated in dreb2g mutants. Our findings establish AtDREB2G is involved in the positive regulation of flavin biosynthesis in Arabidopsis, particularly under conditions of low temperature and abscisic acid treatment.

The cap-binding complex modulates ABA-responsive transcript splicing during germination in barley (Hordeum vulgare)

To decipher the molecular bases governing seed germination, this study presents the pivotal role of the cap-binding complex (CBC), comprising CBP20 and CBP80, in modulating the inhibitory effects of abscisic acid (ABA) in barley. Using both single and double barley mutants in genes encoding the CBC, we revealed that the double mutant hvcbp20.ab/hvcbp80.b displays ABA insensitivity, in stark contrast to the hypersensitivity observed in single mutants during germination. Our comprehensive transcriptome and metabolome analysis not only identified significant alterations in gene expression and splicing patterns but also underscored the regulatory nexus among CBC, ABA, and brassinosteroid (BR) signaling pathways.

Role of F-box E3-ubiquitin ligases in plant development and stress responses

KEY MESSAGE

F-box E3-ubiquitin ligases regulate critical biological processes in plant development and stress responses. Future research could elucidate why and how plants have acquired a large number of F-box genes. The ubiquitin-proteasome system (UPS) is a predominant regulatory mechanism employed by plants to maintain the protein turnover in the cells and involves the interplay of three classes of enzymes, E1 (ubiquitin-activating), E2 (ubiquitin-conjugating), and E3 ligases. The diverse and most prominent protein family among eukaryotes, F-box proteins, are a vital component of the multi-subunit SCF (Skp1-Cullin 1-F-box) complex among E3 ligases. Several F-box proteins with multifarious functions in different plant systems have evolved rapidly over time within closely related species, but only a small part has been characterized. We need to advance our understanding of substrate-recognition regulation and the involvement of F-box proteins in biological processes and environmental adaptation. This review presents a background of E3 ligases with particular emphasis on the F-box proteins, their structural assembly, and their mechanism of action during substrate recognition. We discuss how the F-box proteins regulate and participate in the signaling mechanisms of plant development and environmental responses. We highlight an urgent need for research on the molecular basis of the F-box E3-ubiquitin ligases in plant physiology, systems biology, and biotechnology. Further, the developments and outlooks of the potential technologies targeting the E3-ubiquitin ligases for developing crop improvement strategies have been discussed.

Shade-Induced Leaf Senescence in Plants

Leaf senescence is a vital developmental process that involves the orderly breakdown of macromolecules to transfer nutrients from mature leaves to emerging and reproductive organs. This process is essential for a plant's overall fitness. Multiple internal and external factors, such as leaf age, plant hormones, stresses, and light environment, regulate the onset and progression of leaf senescence. When plants grow close to each other or are shaded, it results in significant alterations in light quantity and quality, such as a decrease in photosynthetically active radiation (PAR), a drop in red/far-red light ratios, and a reduction in blue light fluence rate, which triggers premature leaf senescence. Recently, studies have identified various components involved in light, phytohormone, and other signaling pathways that regulate the leaf senescence process in response to shade. This review summarizes the current knowledge on the molecular mechanisms that control leaf senescence induced by shade.

Transcriptome analysis reveals candidate genes involved in nitrogen deficiency stress in apples

Nitrogen is one of the macroelements required for plant growth and development and the identification of candidate genes involved in nitrogen deficiency stress is of great importance to the sustainable development of agriculture. Here, we found that the color of apple leaves changed from dark green to yellow-green, the malondialdehyde (MDA) content, soluble protein content, and proline content significantly increased, the chlorophyll content significantly decreased in response to nitrate deficiency stress. According to the physiological and biochemical changes of apple leaves during nitrate deficiency stress, nitrogen deficiency stress was divided into two stages: early nitrogen deficiency stage (ES) and late nitrogen deficiency stage (LS). Transcriptome sequencing was performed in these two stress stages. 5773 differential expression genes (DEGs) were identified in the early nitrogen deficiency stress stage and 6130 DEGs were identified in the late nitrogen deficiency stress stage. Functional analysis of these DEGs revealed that a large number of DEGs were enriched in 'porphyrin and chlorophyll metabolic' pathways, the 'photosynthesis' pathway, the 'photosynthesis-antenna protein' pathway, and the 'ABA', 'ETH', and 'JA' signal transduction pathways, and the metabolic networks of these pathways were constructed. In addition, overexpression of MdNAC4 weakened the tolerance of apple calli to nitrogen deficiency stress. Taken together, our results reveal possible pathways for apple adaptation to nitrogen deficiency stress and identify the function of MdNAC4, a key transcription factor regulating nitrogen deficiency stress, which enriches the molecular mechanism of apple adapting to a nitrogen deficiency environment.

SQUINT Positively Regulates Resistance to the Pathogen Botrytis cinerea via miR156-SPL9 Module in Arabidopsis

SQUINT (SQN) regulates plant maturation by promoting the activity of miR156, which functions primarily in the miR156-SQUAMOSA PROMOTER BINDING PROTEIN-LIKE9 (SPL9) module regulating plant growth and development. Here, we show that SQN acts in the jasmonate (JA) pathway, a major signaling pathway regulating plant responses to insect herbivory and pathogen infection. Arabidopsis thaliana sqn mutants showed elevated sensitivity to the necrotrophic fungus Botrytis cinerea compared with wild type. However, SQN is not involved in the early pattern-triggered immunity response often triggered by fungal attack. Rather, SQN positively regulates the JA pathway, as sqn loss-of-function mutants treated with B. cinerea showed reduced JA accumulation, JA response and sensitivity to JA. Furthermore, the miR156-SPL9 module regulates plant resistance to B. cinerea: mir156 mutant, and SPL9 overexpression plants displayed elevated sensitivity to B. cinerea. Moreover, constitutively expressing miR156a or reducing SPL9 expression in the sqn-1 mutant restored the sensitivity of Arabidopsis to B. cinerea and JA responses. These results suggest that SQN positively modulates plant resistance to B. cinerea through the JA pathway, and the miR156-SPL9 module functions as a bridge between SQN and JA to mediate plant resistance to this pathogen.

The wound-activated ERF15 transcription factor drives Marchantia polymorpha regeneration by activating an oxylipin biosynthesis feedback loop

The regenerative potential in response to wounding varies widely among species. Within the plant lineage, the liverwort Marchantia polymorpha displays an extraordinary regeneration capacity. However, its molecular pathways controlling the initial regeneration response are unknown. Here, we demonstrate that the MpERF15 transcription factor gene is instantly activated after wounding and is essential for gemmaling regeneration following tissue incision. MpERF15 operates both upstream and downstream of the MpCOI1 oxylipin receptor by controlling the expression of oxylipin biosynthesis genes. The resulting rise in the oxylipin dinor-12-oxo-phytodienoic acid (dn-OPDA) levels results in an increase in gemma cell number and apical notch organogenesis, generating highly disorganized and compact thalli. Our data pinpoint MpERF15 as a key factor activating an oxylipin biosynthesis amplification loop after wounding, which eventually results in reactivation of cell division and regeneration. We suggest that the genetic networks controlling oxylipin biosynthesis in response to wounding might have been reshuffled over evolution.

Arabidopsis alkaline ceramidase ACER functions in defense against insect herbivory

Plant sphingolipids are important membrane components and bioactive molecules in development and defense responses. However, the function of sphingolipids in plant defense, especially against herbivores, is not fully understood. Here, we report that Spodoptera exigua feeding affects sphingolipid metabolism in Arabidopsis, resulting in increased levels of sphingoid long-chain bases, ceramides, and hydroxyceramides. Insect-induced ceramide and hydroxyceramide accumulation is dependent on the jasmonate signaling pathway. Loss of the Arabidopsis alkaline ceramidase ACER increases ceramides and decreases long-chain base levels in plants; in this work, we found that loss of ACER enhances plant resistance to S. exigua and improves response to mechanical wounding. Moreover, acer-1 mutants exhibited more severe root-growth inhibition and higher anthocyanin accumulation than wild-type plants in response to methyl jasmonate treatment, indicating that loss of ACER increases sensitivity to jasmonate and that ACER functions in jasmonate-mediated root growth and secondary metabolism. Transcript levels of ACER were also negatively regulated by jasmonates, and this process involves the transcription factor MYC2. Thus, our findings reveal that ACER is involved in mediating jasmonate-related plant growth and defense and that jasmonates function in regulating the expression of ACER.

Insights into the Genomic Regions and Candidate Genes of Senescence-Related Traits in Upland Cotton via GWAS

Senescence is the last stage of plant development and is controlled by both internal and external factors. Premature senescence significantly affects the yield and quality of cotton. However, the genetic architecture underlying cotton senescence remains unclear. In this study, genome-wide association studies (GWAS) were performed based on 3,015,002 high-quality SNP markers from the resequencing data of 355 upland cotton accessions to detect genomic regions for cotton senescence. A total of 977 candidate genes within 55 senescence-related genomic regions (SGRs), SGR1-SGR55, were predicted. Gene ontology (GO) analysis of candidate genes revealed that a set of biological processes was enriched, such as salt stress, ethylene processes, and leaf senescence. Furthermore, in the leaf senescence GO term, one candidate gene was focused on: Gohir.A12G270900 (GhMKK9), located in SGR36, which encodes a protein of the MAP kinase kinase family. Quantitative real-time PCR (qRT-PCR) analysis showed that GhMKK9 was up-regulated in old cotton leaves. Overexpression of GhMKK9 in Arabidopsis accelerated natural leaf senescence. Virus-induced gene silencing (VIGS) of GhMKK9 in cotton increased drought tolerance. These results suggest that GhMKK9 is a positive regulator and might be involved in drought-induced senescence in cotton. The results provide new insights into the genetic basis of cotton senescence and will be useful for improving cotton breeding in the future.

CRISPR/Cas9 disruption of UGT71L1 in poplar connects salicinoid and salicylic acid metabolism and alters growth and morphology

Salicinoids are salicyl alcohol-containing phenolic glycosides with strong antiherbivore effects found only in poplars and willows. Their biosynthesis is poorly understood, but recently a UDP-dependent glycosyltransferase, UGT71L1, was shown to be required for salicinoid biosynthesis in poplar tissue cultures. UGT71L1 specifically glycosylates salicyl benzoate, a proposed salicinoid intermediate. Here, we analyzed transgenic CRISPR/Cas9-generated UGT71L1 knockout plants. Metabolomic analyses revealed substantial reductions in the major salicinoids, confirming the central role of the enzyme in salicinoid biosynthesis. Correspondingly, UGT71L1 knockouts were preferred to wild-type by white-marked tussock moth (Orgyia leucostigma) larvae in bioassays. Greenhouse-grown knockout plants showed substantial growth alterations, with decreased internode length and smaller serrated leaves. Reinserting a functional UGT71L1 gene in a transgenic rescue experiment demonstrated that these effects were due only to the loss of UGT71L1. The knockouts contained elevated salicylate (SA) and jasmonate (JA) concentrations, and also had enhanced expression of SA- and JA-related genes. SA is predicted to be released by UGT71L1 disruption, if salicyl salicylate is a pathway intermediate and UGT71L1 substrate. This idea was supported by showing that salicyl salicylate can be glucosylated by recombinant UGT71L1, providing a potential link of salicinoid metabolism to SA and growth impacts. Connecting this pathway with growth could imply that salicinoids are under additional evolutionary constraints beyond selective pressure by herbivores.

New Advances in the Regulation of Leaf Senescence by Classical and Peptide Hormones

Leaf senescence is the last stage of leaf development, manifested by leaf yellowing due to the loss of chlorophyll, along with the degradation of macromolecules and facilitates nutrient translocation from the sink to the source tissues, which is essential for the plants' fitness. Leaf senescence is controlled by a sophisticated genetic network that has been revealed through the study of the molecular mechanisms of hundreds of senescence-associated genes (SAGs), which are involved in multiple layers of regulation. Leaf senescence is primarily regulated by plant age, but also influenced by a variety of factors, including phytohormones and environmental stimuli. Phytohormones, as important signaling molecules in plant, contribute to the onset and progression of leaf senescence. Recently, peptide hormones have been reported to be involved in the regulation of leaf senescence, enriching the significance of signaling molecules in controlling leaf senescence. This review summarizes recent advances in the regulation of leaf senescence by classical and peptide hormones, aiming to better understand the coordinated network of different pathways during leaf senescence.

ABI5 binding protein2 inhibits ABA responses during germination without ABA-INSENSITIVE5 degradation

Overexpression of ABA-INSENSITIVE5 binding proteins (AFPs) results in extreme ABA resistance of seeds and failure to acquire desiccation tolerance, at least in part through effects on chromatin modification. We tested the hypothesis that AFPs promote germination in Arabidopsis (Arabidopsis thaliana) by also functioning as adapters for E3 ligases that ubiquitinate ABI5, leading to its degradation. Interactions between AFPs and two well-characterized classes of E3 ligases targeting ABI5, DWD HYPERSENSITIVE TO ABA (DWA)s and KEEP ON GOING, were analyzed by yeast two-hybrid, bimolecular fluorescence complementation, and genetic assays. Although weak direct interactions were detected between AFPs and E3 ligases, loss of function for these E3 ligases did not impair ABA-resistance conferred by overexpression of the YFP-AFP2 fusion. Comparison of ABI5 and AFP2 levels in these lines showed that AFP2 accumulation increased during germination, but that ABI5 degradation followed germination, demonstrating that AFP2 overexpression reduces ABA sensitivity, thereby permitting germination prior to ABI5 degradation. Surprisingly, AFP2 overexpression in the dwa1 dwa2 mutant background produced the unusual combination of extreme ABA resistance and desiccation tolerance, creating an opportunity to separate the underlying biochemical characteristics of ABA sensitivity and desiccation tolerance. Our quantitative proteomics analysis identified at least three-fold more differentially accumulated seed proteins than previous studies. Comparison of dry seed proteomes of wild-type or dwa1 dwa2 mutants with or without AFP2 overexpression allowed us to separate and refine the changes in protein accumulation patterns associated with desiccation tolerance independently of ABA sensitivity, or vice versa, to a subset of cold-induced and defense stress-responsive proteins and signaling regulators.

SlMYC2 mediates jasmonate-induced tomato leaf senescence by promoting chlorophyll degradation and repressing carbon fixation

Leaf senescence occurs as the last developmental phase of leaf. The initiation and progression of leaf senescence is highly regulated by a plethora of internal developmental signals and environmental stimuli. Being an important class of phytohormones, jasmonates (JAs) are shown to induce premature leaf senescence in tomato (Solanum lycopersicum), nevertheless, the underlying mechanisms remain enigmatic. Here, we report that tomato MYC2, a key factor in the JA signal transduction, functions in JA-induced tomato leaf senescence by promoting chlorophyll degradation and inhibiting photosynthetic carbon fixation. We found that exogenous application of MeJA reduced chlorophyll content, decreased carbon assimilation rates and disrupted membrane integrity. We further demonstrated using SlMYC2-RNAi tomato plants that SlMYC2 enhanced the expression of SlPAO, which encodes a chlorophyll degradation enzyme, but suppressed the expression of SlRCA and SlSBPASE, both of which are required for photosynthesis and growth in plants. Dual-luciferase assay confirmed that SlMYC2 activated the transcription of SlPAO, but inhibited the transcription of SlRCA and SlSBPASE. Furthermore, repression of SlRCA led to typical features associated with leaf senescence in tomato. Taken together, these results favor that tomato MYC2 acts positively in the regulation of JA-dependent tomato leaf senescence. The results extend our mechanistic understanding of JA-induced senescence in an important horticultural crop.

A regulatory GhBPE-GhPRGL module maintains ray petal length in Gerbera hybrida

The molecular mechanism regulating petal length in flowers is not well understood. Here we used transient transformation assays to confirm that GhPRGL (proline-rich and GASA-like)-a GASA (gibberellic acid [GA] stimulated in Arabidopsis) family gene-promotes the elongation of ray petals in gerbera (Gerbera hybrida). Yeast one-hybrid screening assay identified a bHLH transcription factor of the jasmonic acid (JA) signaling pathway, here named GhBPE (BIGPETAL), which binds to the GhPRGL promoter and represses its expression, resulting in a phenotype of shortened ray petal length when GhBPE is overexpressed. Further, the joint response to JA and GA of GhBPE and GhPRGL, together with their complementary expression profiles in the early stage of petal growth, suggests a novel GhBPE-GhPRGL module that controls the size of ray petals. GhPRGL promotes ray petal elongation in its early stage especially, while GhBPE inhibits ray petal elongation particularly in the late stage by inhibiting the expression of GhPRGL. JA and GA operate in concert to regulate the expression of GhBPE and GhPRGL genes, providing a regulatory mechanism by which ray petals could grow to a fixed length in gerbera species.

WHITE AND LESION-MIMIC LEAF1, encoding a lumazine synthase, affects reactive oxygen species balance and chloroplast development in rice

The riboflavin derivatives flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are essential cofactors for enzymes in multiple cellular processes. Characterizing mutants with impaired riboflavin metabolism can help clarify the role of riboflavin in plant development. Here, we characterized a rice (Oryza sativa) white and lesion-mimic (wll1) mutant, which displays a lesion-mimic phenotype with white leaves, chlorophyll loss, chloroplast defects, excess reactive oxygen species (ROS) accumulation, decreased photosystem protein levels, changes in expression of chloroplast development and photosynthesis genes, and cell death. Map-based cloning and complementation test revealed that WLL1 encodes lumazine synthase, which participates in riboflavin biosynthesis. Indeed, the wll1 mutant showed riboflavin deficiency, and application of FAD rescued the wll1 phenotype. In addition, transcriptome analysis showed that cytokinin metabolism was significantly affected in wll1 mutant, which had increased cytokinin and δ-aminolevulinic acid contents. Furthermore, WLL1 and riboflavin synthase (RS) formed a complex, and the rs mutant had a similar phenotype to the wll1 mutant. Taken together, our findings revealed that WLL1 and RS play pivotal roles in riboflavin biosynthesis, which is necessary for ROS balance and chloroplast development in rice.

Jasmonates modulate sphingolipid metabolism and accelerate cell death in the ceramide kinase mutant acd5

Sphingolipids are structural components of the lipid bilayer that acts as signaling molecules in many cellular processes, including cell death. Ceramides, key intermediates in sphingolipid metabolism, are phosphorylated by the ceramide kinase ACCELERATED CELL DEATH5 (ACD5). The loss of ACD5 function leads to ceramide accumulation and spontaneous cell death. Here, we report that the jasmonate (JA) pathway is activated in the Arabidopsis (Arabidopsis thaliana) acd5 mutant and that methyl JA treatment accelerates ceramide accumulation and cell death in acd5. Moreover, the double mutants of acd5 with jasmonate resistant1-1 and coronatine insensitive1-2 exhibited delayed cell death, suggesting that the JA pathway is involved in acd5-mediated cell death. Quantitative sphingolipid profiling of plants treated with methyl JA indicated that JAs influence sphingolipid metabolism by increasing the levels of ceramides and hydroxyceramides, but this pathway is dramatically attenuated by mutations affecting JA pathway proteins. Furthermore, we showed that JAs regulate the expression of genes encoding enzymes in ceramide metabolism. Together, our findings show that JAs accelerate cell death in acd5 mutants, possibly by modulating sphingolipid metabolism and increasing ceramide levels.

Photoperiodic Signaling and Senescence, an Ancient Solution to a Modern Problem?

The length of the day (photoperiod) is a robust seasonal signal originated by earth orbital and translational movements, a resilient external cue to the global climate change, and a predictable hint to initiate or complete different developmental programs. In eukaryotic algae, the gene expression network that controls the cellular response to photoperiod also regulates other basic physiological functions such as starch synthesis or redox homeostasis. Land plants, evolving in a novel and demanding environment, imbued these external signals within the regulatory networks controlling organogenesis and developmental programs. Unlike algae that largely have to deal with cellular physical cues, within the course of evolution land plants had to transfer this external information from the receiving organs to the target tissues, and mobile signals such as hormones were recruited and incorporated in the regulomes. Control of senescence by photoperiod, as suggested in this perspective, would be an accurate way to feed seasonal information into a newly developed function (senescence) using an ancient route (photoperiodic signaling). This way, the plant would assure that two coordinated aspects of development such as flowering and organ senescence were sequentially controlled. As in the case of senescence, there is growing evidence to support the idea that harnessing the reliability of photoperiod regulation over other, more labile signaling pathways could be used as a robust breeding tool to enhance plants against the harmful effects of climate change.

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.

iTRAQ protein profile analysis of sugar beet under salt stress: different coping mechanisms in leaves and roots

BACKGROUND

Salinity is one of the most serious threats to world agriculture. An important sugar-yielding crop sugar beet, which shows some tolerance to salt via a mechanism that is poorly understood. Proteomics data can provide important clues that can contribute to finally understand this mechanism.

RESULTS

Differentially abundant proteins (DAPs) in sugar beet under salt stress treatment were identified in leaves (70 DAPs) and roots (76 DAPs). Functions of these DAPs were predicted, and included metabolism and cellular, environmental information and genetic information processing. We hypothesize that these processes work in concert to maintain cellular homeostasis. Some DAPs are closely related to salt resistance, such as choline monooxygenase, betaine aldehyde dehydrogenase, glutathione S-transferase (GST) and F-type H+-transporting ATPase. The expression pattern of ten DAPs encoding genes was consistent with the iTRAQ data.

CONCLUSIONS

During sugar beet adaptation to salt stress, leaves and roots cope using distinct mechanisms of molecular metabolism regulation. This study provides significant insights into the molecular mechanism underlying the response of higher plants to salt stress, and identified some candidate proteins involved in salt stress countermeasures.

OsJAZ13 Negatively Regulates Jasmonate Signaling and Activates Hypersensitive Cell Death Response in Rice

Jasmonate ZIM-domain (JAZ) proteins belong to the subgroup of TIFY family and act as key regulators of jasmonate (JA) responses in plants. To date, only a few JAZ proteins have been characterized in rice. Here, we report the identification and function of rice OsJAZ13 gene. The gene encodes three different splice variants: OsJAZ13a, OsJAZ13b, and OsJAZ13c. The expression of OsJAZ13 was mainly activated in vegetative tissues and transiently responded to JA and ethylene. Subcellular localization analysis indicated OsJAZ13a is a nuclear protein. Yeast two-hybrid assays revealed OsJAZ13a directly interacts with OsMYC2, and also with OsCOI1, in a COR-dependent manner. Furthermore, OsJAZ13a recruited a general co-repressor OsTPL via an adaptor protein OsNINJA. Remarkably, overexpression of OsJAZ13a resulted in the attenuation of root by methyl JA. Furthermore, OsJAZ13a-overexpressing plants developed lesion mimics in the sheath after approximately 30–45 days of growth. Tillers with necrosis died a few days later. Gene-expression analysis suggested the role of OsJAZ13 in modulating the expression of JA/ethylene response-related genes to regulate growth and activate hypersensitive cell death. Taken together, these observations describe a novel regulatory mechanism in rice and provide the basis for elucidating the function of OsJAZ13 in signal transduction and cell death in plants.

The expression characteristics of methyl jasmonate biosynthesis-related genes in Cymbidium faberi and influence of heterologous expression of CfJMT in Petunia hybrida

Cymbidium faberi Rolfe (Orchidaceae) is an herbaceous plant native to China, where it has a long history of cultivation owing to its beautiful flower pattern and floral fragrance. Previously, we conducted a transcriptome analysis of the flower and vegetative buds to elucidate the mechanisms of flower development in C. faberi. In the present study, we found nine secondary metabolic pathways through the KEGG pathway database that were related to the biosynthesis of methyl jasmonate (MeJA) and other volatile organic compounds. qRT-PCR was performed to analyze the expression levels of four key genes in the MeJA pathway. Among these, CfJMT (jasmonic acid carboxyl methyltransferase) had higher transcript levels in sepals, petals and labella than in other tissues. CfJMT was cloned from the petals of full-bloom flowers of C. faberi. The predicted CfJMT protein sequence contains conserved jasmonic acid methyl transferase-7 domains, indicating that it belongs to the SABATH protein family. The CfJMT coding sequence driven by the CaMV35S promoter was successfully transformed into Petunia hybrida through an Agrobacterium-mediated method. Although MeJA could not be detected in either wild-type or transgenic petunia plants, the leaves of the transgenic plants were smaller than those of wild-type plants and pollen development was abnormal. These results indicate that heterologous expression of CfJMT may change the levels of endogenous jasmonic acid and other hormones, but that the content of MeJA is not increased significantly by transformation with CfJMT alone. Thus, other related genes and regulation factors may play important roles in this process.

Secondary Mutation-Induced Alternative Splicing Suppresses RNA Splicing Defect of the jhs1 Mutant

jing he sheng1 (jhs1) is a mutant of the DNA2 homolog in Arabidopsis (Arabidopsis thaliana), which was previously identified as being involved in DNA damage repair, cell cycle regulation, and meristem maintenance. A mutation at the 3' intron splice site of the 11th intron causes alternative splicing of this intron at two other sites, which results in frame shifts and premature stop codons. Here, we screened suppressors of jhs1 to further study the function and regulatory networks of JHS1 Three suppressors with wild-type-like phenotypes were obtained. Sequencing analysis results showed that each of the suppressors has a second mutation in jhs1 that causes further alternative splicing of the intron and corrects the shifted reading frame with small insertions. Precursor mRNA sequence analysis and intron splice site evaluation results suggested that intron splicing was disturbed in the suppressors, and this switched the splice site, resulting in small insertions in the coding regions of JHS1. Structural analysis of JHS1 suggested that the insertions are in a disordered loop region of the DNA2 domain and do not seem to have much deleterious effect on the function of the protein. This work not only has implications for the evolution of protein sequences at exon junctions but also provides a strategy to study the mechanism of precursor mRNA splicing.

AtMOB1 Genes Regulate Jasmonate Accumulation and Plant Development

The MOB1 proteins are highly conserved in yeasts, animals, and plants. Previously, we showed that the Arabidopsis (Arabidopsis thaliana) MOB1A gene (AtMOB1A/NCP1) plays critical roles in auxin-mediated plant development. Here, we report that AtMOB1A and AtMOB1B redundantly and negatively regulate jasmonate (JA) accumulation and function in Arabidopsis development. The two MOB1 genes exhibited similar expression patterns, and the MOB1 proteins displayed similar subcellular localizations and physically interacted in vivo. Furthermore, the atmob1a atmob1b (mob1a/1b) double mutant displayed severe developmental defects, which were much stronger than those of either single mutant. Interestingly, many JA-related genes were up-regulated in mob1a/1b, suggesting that AtMOB1A and AtMOB1B negatively regulate the JA pathways. mob1a/1b plants accumulated more JA and were hypersensitive to exogenous JA treatments. Disruption of MYC2, a key gene in JA signaling, in the mob1a/1b background partially alleviated the root defects and JA hypersensitivity observed in mob1a/1b. Moreover, the expression levels of the MYC2-repressed genes PLT1 and PLT2 were significantly decreased in the mob1a/1b double mutant. Our results showed that MOB1A/1B genetically interact with SIK1 and antagonistically modulate JA-related gene expression. Taken together, our findings indicate that AtMOB1A and AtMOB1B play important roles in regulating JA accumulation and Arabidopsis development.

Brassinosteroids Antagonize Jasmonate-Activated Plant Defense Responses through BRI1-EMS-SUPPRESSOR1 (BES1)

Brassinosteroids (BRs) and jasmonates (JAs) regulate plant growth, development, and defense responses, but how these phytohormones mediate the growth-defense tradeoff is unclear. Here, we identified the Arabidopsis (Arabidopsis thaliana) dwarf at early stages1 (dwe1) mutant, which exhibits enhanced expression of defensin genes PLANT DEFENSIN1.2a (PDF1.2a) and PDF1.2b The dwe1 mutant showed increased resistance to herbivory by beet armyworms (Spodoptera exigua) and infection by botrytis (Botrytis cinerea). DWE1 encodes ROTUNDIFOLIA3, a cytochrome P450 protein essential for BR biosynthesis. The JA-inducible transcription of PDF1.2a and PDF1.2b was significantly reduced in the BRASSINOSTEROID INSENSITIVE1-ETHYL METHANESULFONATE-SUPPRESSOR1 (BES1) gain-of-function mutant bes1- D, which was highly susceptible to S. exigua and B. cinerea BES1 directly targeted the terminator regions of PDF1.2a/PDF1.2b and suppressed their expression. PDF1.2a overexpression diminished the enhanced susceptibility of bes1- D to B. cinerea but did not improve resistance of bes1- D to S. exigua In response to S. exigua herbivory, BES1 inhibited biosynthesis of the JA-induced insect defense-related metabolite indolic glucosinolate by interacting with transcription factors MYB DOMAIN PROTEIN34 (MYB34), MYB51, and MYB122 and suppressing expression of genes encoding CYTOCHROME P450 FAMILY79 SUBFAMILY B POLYPEPTIDE3 (CYP79B3) and UDP-GLUCOSYL TRANSFERASE 74B1 (UGT74B1). Thus, BR contributes to the growth-defense tradeoff by suppressing expression of defensin and glucosinolate biosynthesis genes.

iTRAQ-Based Quantitative Proteomic Analysis of the Arabidopsis Mutant opr3-1 in Response to Exogenous MeJA

Jasmonates (JAs) regulate the defense of biotic and abiotic stresses, growth, development, and many other important biological processes in plants. The comprehensive proteomic profiling of plants under JAs treatment provides insights into the regulation mechanism of JAs. Isobaric tags for relative and absolute quantification (iTRAQ)-based quantitative proteomic analysis was performed on the Arabidopsis wild type (Ws) and JA synthesis deficiency mutant opr3-1. The effects of exogenous MeJA treatment on the proteome of opr3-1, which lacks endogenous JAs, were investigated. A total of 3683 proteins were identified and 126 proteins were differentially regulated between different genotypes and treatment groups. The functional classification of these differentially regulated proteins showed that they were involved in metabolic processes, responses to abiotic stress or biotic stress, the defense against pathogens and wounds, photosynthesis, protein synthesis, and developmental processes. Exogenous MeJA treatment induced the up-regulation of a large number of defense-related proteins and photosynthesis-related proteins, it also induced the down-regulation of many ribosomal proteins in opr3-1. These results were further verified by a quantitative real-time PCR (qRT-PCR) analysis of 15 selected genes. Our research provides the basis for further understanding the molecular mechanism of JAs’ regulation of plant defense, photosynthesis, protein synthesis, and development.

Proteome and Ubiquitome Changes during Rose Petal Senescence

Petal senescence involves numerous programmed changes in biological and biochemical processes. Ubiquitination plays a critical role in protein degradation, a hallmark of organ senescence. Therefore, we investigated changes in the proteome and ubiquitome of senescing rose (Rosa hybrida) petals to better understand their involvement in petal senescence. Of 3859 proteins quantified in senescing petals, 1198 were upregulated, and 726 were downregulated during senescence. We identified 2208 ubiquitinated sites, including 384 with increased ubiquitination in 298 proteins and 1035 with decreased ubiquitination in 674 proteins. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that proteins related to peptidases in proteolysis and autophagy pathways were enriched in the proteome, suggesting that protein degradation and autophagy play important roles in petal senescence. In addition, many transporter proteins accumulated in senescing petals, and several transport processes were enriched in the ubiquitome, indicating that transport of substances is associated with petal senescence and regulated by ubiquitination. Moreover, several components of the brassinosteroid (BR) biosynthesis and signaling pathways were significantly altered at the protein and ubiquitination levels, implying that BR plays an important role in petal senescence. Our data provide a comprehensive view of rose petal senescence at the posttranslational level.

Maize Dek33 encodes a pyrimidine reductase in riboflavin biosynthesis that is essential for oil-body formation and ABA biosynthesis during seed development

The maize (Zea mays) defective kernel 33 (dek33) mutant produces defective and occasionally viviparous kernel phenotypes. In this study, we cloned Dek33 by positional cloning and found that it encodes a pyrimidine reductase in riboflavin biosynthesis. In dek33, a single-base mutation (G to A) in the C-terminal COG3236 domain caused a premature stop codon (TGA), producing a weak mutant allele with only a truncated form of the DEK33 protein that occurred at much lower levels that the completed WT form, and with a reduced riboflavin content. The dek33 mutation significantly affected oil-body formation and suppressed endoreduplication. It also disrupted ABA biosynthesis, resulting in lower ABA content that might be responsible for the viviparous embryo. In addition, our results indicated that the COG3236 domain is important for the protein stability of DEK33. Yeast two-hybrid experiments identified several proteins that interacted with DEK33, including RGLG2 and SnRK1, suggesting possible post-translational regulation of DEK33 stability. The interaction between DEK33 and these proteins was further confirmed by luciferase complementation image assays. This study provides a weak mutant allele that can be utilized to explore cellular responses to impaired riboflavin biosynthesis during seed development. Our findings indicate that the COG3236 domain might be an essential regulatory structure for DEK33 stability in maize.

Signal Transduction in Leaf Senescence: Progress and Perspective

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

A comparison of inducible, ontogenetic, and interspecific sources of variation in the foliar metabolome in tropical trees

Plant interactions with other organisms are mediated by chemistry, yet chemistry varies among conspecific and within individual plants. The foliar metabolome—the suite of small-molecule metabolites found in the leaf—changes during leaf ontogeny and is influenced by the signaling molecule jasmonic acid. Species differences in secondary metabolites are thought to play an important ecological role by limiting the host ranges of herbivores and pathogens, and hence facilitating competitive coexistence among plant species in species-rich plant communities such as tropical forests. Yet it remains unclear how inducible and ontogenetic variation compare with interspecific variation, particularly in tropical trees. Here, we take advantage of novel methods to assemble mass spectra of all compounds in leaf extracts into molecular networks that quantify their chemical structural similarity in order to compare inducible and ontogenetic chemical variation to among-species variation in species-rich tropical tree genera. We ask (i) whether young and mature leaves differ chemically, (ii) whether jasmonic acid-inducible chemical variation differs between young and mature leaves, and (iii) whether interspecific exceeds intraspecific chemical variation for four species from four hyperdiverse tropical tree genera. We observed significant effects of the jasmonic acid treatment for three of eight combinations of species and ontogenetic stage evaluated. Three of the four species also exhibited large metabolomic differences with leaf ontogenetic stage. The profound effect of leaf ontogenetic stage on the foliar metabolome suggests a qualitative turnover in secondary chemistry with leaf ontogeny. We also quantified foliar metabolomes for 45 congeners of the four focal species. Chemical similarity was much greater within than between species for all four genera, even when within-species comparisons included leaves that differed in age and jasmonic acid treatment. Despite ontogenetic and inducible variation within species, chemical differences among congeneric species may be sufficient to partition niche space with respect to chemical defense.

Transcriptome divergence during leaf development in two contrasting switchgrass (Panicum virgatum L.) cultivars

The genetics and responses to biotic stressors of tetraploid switchgrass (Panicum virgatum L.) lowland cultivar 'Kanlow' and upland cultivar Summer are distinct and can be exploited for trait improvement. In general, there is a paucity of data on the basal differences in transcription across tissue developmental times for switchgrass cultivars. Here, the changes in basal and temporal expression of genes related to leaf functions were evaluated for greenhouse grown 'Kanlow', and 'Summer' plants. Three biological replicates of the 4th leaf pooled from 15 plants per replicate were harvested at regular intervals beginning from leaf emergence through senescence. Increases and decreases in leaf chlorophyll and N content were similar for both cultivars. Likewise, multidimensional scaling (MDS) analysis indicated both cultivar-independent and cultivar-specific gene expression. Cultivar-independent genes and gene-networks included those associated with leaf function, such as growth/senescence, carbon/nitrogen assimilation, photosynthesis, chlorophyll biosynthesis, and chlorophyll degradation. However, many genes encoding nucleotide-binding leucine rich repeat (NB-LRRs) proteins and wall-bound kinases associated with detecting and responding to environmental signals were differentially expressed. Several of these belonged to unique cultivar-specific gene co-expression networks. Analysis of genomic resequencing data provided several examples of NB-LRRs genes that were not expressed and/or apparently absent in the genomes of Summer plants. It is plausible that cultivar (ecotype)-specific genes and gene-networks could be one of the drivers for the documented differences in responses to leaf-borne pathogens between these two cultivars. Incorporating broad resistance to plant pathogens in elite switchgrass germplasm could improve sustainability of biomass production under low-input conditions.

Arabidopsis Flowers Unlocked the Mechanism of Jasmonate Signaling

The Arabidopsis male-sterile phenotype has been a wonderful model for jasmonate action in plants. It has allowed us to identify transcription factors that control gene expression during stamen and pollen maturation and provided for the discovery of the JAZ repressor proteins and the mechanism of jasmonate signaling. More recently, it has revealed intriguing details of the spatial localization of jasmonate synthesis and perception in stamen tissues. The extensive and thoughtful application of protein–protein interaction assays to identify JAZ-interacting partners has led to a much richer appreciation of the mechanisms by which jasmonate integrates with the actions of other hormones to regulate plant growth and physiological responses. This integration is strikingly evident in stamen and pollen development in Arabidopsis, which requires the actions of many hormones. Just as importantly, it is now evident that jasmonate has very different actions during flower development and reproduction in other plant species. This integration and diversity of action indicates that many exciting discoveries remain to be made in this area of jasmonate hormone signaling and response.

Heterologous Expression of the Grapevine JAZ7 Gene in Arabidopsis Confers Enhanced Resistance to Powdery Mildew but Not to Botrytis cinerea

Jasmonate ZIM-domain (JAZ) family proteins comprise a class of transcriptional repressors that silence jasmonate-inducible genes. Although a considerable amount of research has been carried out on this gene family, there is still very little information available on the role of specific JAZ gene members in multiple pathogen resistance, especially in non-model species. In this study, we investigated the potential resistance function of the VqJAZ7 gene from a disease-resistant wild grapevine, Vitis quinquangularis cv. “Shang-24”, through heterologous expression in Arabidopsis thaliana. VqJAZ7-expressing transgenic Arabidopsis were challenged with three pathogens: the biotrophic fungus Golovinomyces cichoracearum, necrotrophic fungus Botrytis cinerea, and semi-biotrophic bacteria Pseudomonas syringae pv. tomato DC3000. We found that plants expressing VqJAZ7 showed greatly reduced disease symptoms for G. cichoracearum, but not for B. cinerea or P. syringae. In response to G cichoracearum infection, VqJAZ7-expressing transgenic lines exhibited markedly higher levels of cell death, superoxide anions (O₂¯, and H₂O₂ accumulation, relative to nontransgenic control plants. Moreover, we also tested the relative expression of defense-related genes to comprehend the possible induced pathways. Taken together, our results suggest that VqJAZ7 in grapevine participates in molecular pathways of resistance to G. cichoracearum, but not to B. cinerea or P. syringae.

Arabidopsis phospholipase Dα1 and Dδ oppositely modulate EDS1- and SA-independent basal resistance against adapted powdery mildew

Plants use a tightly regulated immune system to fight off various pathogens. Phospholipase D (PLD) and its product, phosphatidic acid, have been shown to influence plant immunity; however, the underlying mechanisms remain unclear. Here, we show that the Arabidopsis mutants pldα1 and pldδ, respectively, exhibited enhanced resistance and enhanced susceptibility to both well-adapted and poorly adapted powdery mildew pathogens, and a virulent oomycete pathogen, indicating that PLDα1 negatively while PLDδ positively modulates post-penetration resistance. The pldα1δ double mutant showed a similar infection phenotype to pldα1, genetically placing PLDα1 downstream of PLDδ. Detailed genetic analyses of pldδ with mutations in genes for salicylic acid (SA) synthesis (SID2) and/or signaling (EDS1 and PAD4), measurement of SA and jasmonic acid (JA) levels, and expression of their respective reporter genes indicate that PLDδ contributes to basal resistance independent of EDS1/PAD4, SA, and JAsignaling. Interestingly, while PLDα1-enhanced green fluorescent protein (eGFP) was mainly found in the tonoplast before and after haustorium invasion, PLDδ-eGFP's focal accumulation to the plasma membrane around the fungal penetration site appeared to be suppressed by adapted powdery mildew. Together, our results demonstrate that PLDα1 and PLDδ oppositely modulate basal, post-penetration resistance against powdery mildew through a non-canonical mechanism that is independent of EDS1/PAD4, SA, and JA.

Plant organ senescence - regulation by manifold pathways

Senescence is the final stage of plant ontogeny before death. Senescence may occur naturally because of age or may be induced by various endogenous and exogenous factors. Despite its destructive character, senescence is a precisely controlled process that follows a well-defined order. It is often inseparable from programmed cell death (PCD), and a correlation between these processes has been confirmed during the senescence of leaves and petals. Despite suggestions that senescence and PCD are two separate processes, with PCD occurring after senescence, cell death responsible for senescence is accompanied by numerous changes at the cytological, physiological and molecular levels, similar to other types of PCD. Independent of the plant organ analysed, these changes are focused on initiating the processes of cellular structural degradation via fluctuations in phytohormone levels and the activation of specific genes. Cellular structural degradation is genetically programmed and dependent on autophagy. Phytohormones/plant regulators are heavily involved in regulating the senescence of plant organs and can either promote [ethylene, abscisic acid (ABA), jasmonic acid (JA), and polyamines (PAs)] or inhibit [cytokinins (CKs)] this process. Auxins and carbohydrates have been assigned a dual role in the regulation of senescence, and can both inhibit and stimulate the senescence process. In this review, we introduce the basic pathways that regulate senescence in plants and identify mechanisms involved in controlling senescence in ephemeral plant organs. Moreover, we demonstrate a universal nature of this process in different plant organs; despite this process occurring in organs that have completely different functions, it is very similar. Progress in this area is providing opportunities to revisit how, when and which way senescence is coordinated or decoupled by plant regulators in different organs and will provide a powerful tool for plant physiology research.

Crystal Structure of Tetrameric Arabidopsis MYC2 Reveals the Mechanism of Enhanced Interaction with DNA

Jasmonates (JAs) are essential plant hormones that play important roles in the regulation of plant growth and the response to environmental stress. In the JA signaling pathway, the core transcription factors are a class of basic helix-loop-helix (bHLH) proteins, including MYC2, MYC3, and MYC4, that have different regulatory capacities. Here, we report the 2.7 Å crystal structure of the MYC2 bHLH domain complexed with G-box DNA, showing a cis-tetrameric structure. Biochemical assays confirmed that full-length MYC2 forms a stable homo-tetramer both in solution and in DNA-bound states, whereas MYC3 forms only a homodimer. Isothermal titration calorimetry (ITC) assays demonstrated that tetramerization enhanced DNA binding affinity, and fluorescence resonance energy transfer (FRET) assay indicated DNA looping potential of tetrameric MYC2. Luciferase assay further confirmed the importance of tetramerization in transcriptional regulation. Our studies provide a mechanistic explanation for the regulatory differences of MYC transcription factors.

Global transcriptome analysis of the maize (Zea mays L.) inbred line 08LF during leaf senescence initiated by pollination-prevention

In maize (Zea mays), leaf senescence acts as a nutrient recycling process involved in proteins, lipids, and nucleic acids degradation and transport to the developing sink. However, the molecular mechanisms of pre-maturation associated with pollination-prevention remain unclear in maize. To explore global gene expression changes during the onset and progression of senescence in maize, the inbred line 08LF, with severe early senescence caused by pollination prevention, was selected. Phenotypic observation showed that the onset of leaf senescence of 08LF plants occurred approximately 14 days after silking (DAS) by pollination prevention. Transcriptional profiling analysis of the leaf at six developmental stages during induced senescence revealed that a total of 5,432 differentially expressed genes (DEGs) were identified, including 2314 up-regulated genes and 1925 down-regulated genes. Functional annotation showed that the up-regulated genes were mainly enriched in multi-organism process and nitrogen compound transport, whereas down-regulated genes were involved in photosynthesis. Expression patterns and pathway enrichment analyses of early-senescence related genes indicated that these DEGs are involved in complex regulatory networks, especially in the jasmonic acid pathway. In addition, transcription factors from several families were detected, particularly the CO-like, NAC, ERF, GRAS, WRKY and ZF-HD families, suggesting that these transcription factors might play important roles in driving leaf senescence in maize as a result of pollination-prevention.

MYC5 is Involved in Jasmonate-Regulated Plant Growth, Leaf Senescence and Defense Responses

Jasmonates (JAs), lipid-derived phytohormones, regulate plant growth, development and defenses against biotic stresses. CORONATINE INSENSITIVE1 perceives bioactive JA and recruits JASMONATE ZIM-DOMAIN (JAZ) proteins for ubiquitination and subsequent degradation via the 26S proteasome, which de-represses JAZ-targeted transcription factors that regulate diverse JA responses. Recent studies showed that the Arabidopsis basic helix-loop-helix transcription factor MYC5 interacts with JAZs and regulates stamen development. However, whether MYC5 mediates other JA responses is unclear. Here, we show that MYC5 functions redundantly with MYC2, MYC3 and MYC4 to modulate JA-regulated root growth inhibition and plant defenses against insect attack and pathogen infection, and that it positively regulates JA-induced leaf senescence. Our findings define MYC5 as an important regulator that is essential for diverse JA responses.

Ameliorative effects of melatonin on dark-induced leaf senescence in gardenia (Gardenia jasminoides Ellis): leaf morphology, anatomy, physiology and transcriptome

Cut gardenia (Gardenia jasminoides Ellis) foliage is widely used as a vase material or flower bouquet indoors; however, insufficient indoor light accelerates its senescence, which shortens its viewing time. In this study, applying melatonin to delay gardenia leaf senescence when exposed to extremely low light condition (darkness), and the results showed that 1.0 mM was the effective concentration. At this concentration, chlorophyll contents and chlorophyll fluorescence parameters (Fv/Fm, Fv/F0 and Y(II)) increased, while the carotenoid and flavonoid contents decreased. Meanwhile, stress physiological indices decreased in response to exogenous melatonin application, whereas an increase in glutamine synthetase activity, water and soluble protein contents was observed. Moreover, exogenous melatonin application also reduced leaf programmed cell death under darkness, increased the endogenous melatonin level, expression levels of tryptophan decarboxylase gene, superoxide dismutase and catalase activities and the ascorbate-glutathione cycle, and maintained more intact anatomical structures. Furthermore, transcriptome sequencing revealed that various biological processes responded to exogenous melatonin application, including carbohydrate metabolism, amino acid metabolism, lipid metabolism, plant hormone signal transduction and pigment biosynthesis. Consequently, dark-induced leaf senescence in gardenia was significantly delayed. These results provided a better understanding for improving the ornamental value of cut gardenia foliage using melatonin.

MdSnRK1.1 interacts with MdJAZ18 to regulate sucrose-induced anthocyanin and proanthocyanidin accumulation in apple

Sugars induce anthocyanin biosynthesis in plants. As a conserved energy sensor, SnRK1 (SNF1-related kinase 1) is involved in sucrose-induced anthocyanin accumulation. However, the exact molecular mechanism by which SnRK1 regulates the biosynthesis of anthocyanins and proanthocyanidins (PAs) in response to sucrose in plants is not clear. In this study, it was found that MdSnRK1.1 interacted with MdJAZ18 protein which acts as a repressor in the jasmonate (JA) signaling pathway. MdSnRK1.1 then phosphorylated MdJAZ18 to facilitate its 26S proteasome-mediated degradation, which released MdbHLH3 thereby activating the expression of the regulatory and structural genes, thus finally promoting the biosynthesis of anthocyanins and PAs. Taken together, these results demonstrate the involvement of MdSnRK1.1 in sucrose-induced accumulation of anthocyanins and PAs. For the first time, our findings shed light on the molecular mechanism by which the crosstalk of sucrose and JA signaling regulates flavonoid biosynthesis.

Jasmonate Regulates Plant Responses to Postsubmergence Reoxygenation through Transcriptional Activation of Antioxidant Synthesis

Submergence induces hypoxia in plants; exposure to oxygen following submergence, termed reoxygenation, produces a burst of reactive oxygen species. The mechanisms of hypoxia sensing and signaling in plants have been well studied, but how plants respond to reoxygenation remains unclear. Here, we show that reoxygenation in Arabidopsis (Arabidopsis thaliana) involves rapid accumulation of jasmonates (JAs) and increased transcript levels of JA biosynthesis genes. Application of exogenous methyl jasmonate improved tolerance to reoxygenation in wild-type Arabidopsis; also, mutants deficient in JA biosynthesis and signaling were very sensitive to reoxygenation. Moreover, overexpression of the transcription factor gene MYC2 enhanced tolerance to posthypoxic stress, and myc2 knockout mutants showed increased sensitivity to reoxygenation, indicating that MYC2 functions as a key regulator in the JA-mediated reoxygenation response. MYC2 transcriptionally activates members of the VITAMIN C DEFECTIVE (VTC) and GLUTATHIONE SYNTHETASE (GSH) gene families, which encode rate-limiting enzymes in the ascorbate and glutathione synthesis pathways. Overexpression of VTC1 and GSH1 in the myc2-2 mutant suppressed the posthypoxic hypersensitive phenotype. The JA-inducible accumulation of antioxidants may alleviate oxidative damage caused by reoxygenation, improving plant survival after submergence. Taken together, our findings demonstrate that JA signaling interacts with the antioxidant pathway to regulate reoxygenation responses in Arabidopsis.

Jasmonate regulates leaf senescence and tolerance to cold stress: crosstalk with other phytohormones

Plants are challenged with numerous abiotic stresses, such as drought, cold, heat, and salt stress. These environmental stresses are major causes of crop failure and reduced yields worldwide. Phytohormones play essential roles in regulating various plant physiological processes and alleviating stressful perturbations. Jasmonate (JA), a group of oxylipin compounds ubiquitous in the plant kingdom, acts as a crucial signal to modulate multiple plant processes. Recent studies have shown evidence supporting the involvement of JA in leaf senescence and tolerance to cold stress. Concentrations of JA are much higher in senescent leaves compared with those in non-senescent ones. Treatment with exogenous JA induces leaf senescence and expression of senescence-associated genes. In response to cold stress, exogenous application of JA enhances Arabidopsis freezing tolerance with or without cold acclimation. Consistently, biosynthesis of endogenous JA is activated in response to cold exposure. JA positively regulates the CBF (C-REPEAT BINDING FACTOR) transcriptional pathway to up-regulate downstream cold-responsive genes and ultimately improve cold tolerance. JA interacts with other hormone signaling pathways (such as auxin, ethylene, and gibberellin) to regulate leaf senescence and tolerance to cold stress. In this review, we summarize recent studies that have provided insights into JA-mediated leaf senescence and cold-stress tolerance.

Suppressor of rid1 (SID1) shares common targets with RID1 on florigen genes to initiate floral transition in rice

The transition from vegetative to reproductive growth is a critical process in the life cycle of higher plants. Previously, we cloned Rice Indeterminate 1 (RID1), which acts as the master switch for the transition from the vegetative to reproductive phase in rice. Although the photoperiod pathway of RID1 inducing expression of the florigen genes Hd3a and RFT1 via Ehd1 has been established, the alternative pathways for the essential flowering transition need to be further examined. Here, we identified a Suppressor of rid1 (SID1), which rescues the never-flowering phenotype of rid1. SID1 encodes an INDETERMINATE DOMAIN (IDD) transcription factor. Mutation in SID1 showed the delayed flowering phenotype. Gain-of-function of SID1, OsIDD1, or OsIDD6 could restore the rid1 to flowering. Further analyses showed SID1 and RID1 directly target the promoter regions of Hd3a and RFT1, two florigen genes in rice. Taken together, our results reveal an autonomous flowering pathway might be mediated by RID1, thereby controlling the phase transition from vegetative to reproductive development in rice.

Transition from vegetative to reproductive phase is a critical developmental switch in the life cycle of higher plants. In rice, our previous work suggested Rice Indeterminate 1 (RID1) acts as the master switch for the transition to flowering. Mutation in RID1 results in a never-flowering phenotype. In order to uncover the molecular network regulated by RID1, a Suppressor of rid1 (SID1) was identified in this study. Both SID1 and RID1 encode a plant-specific INDETERMINATE DOMAIN (IDD) transcription factor. Overexpression of SID1, OsIDD1, or OsIDD6 could rescue the never-flowering phenotype of rid1. Molecular data indicate both SID1 and RID1 physically bind the promoters of the florigen genes Hd3a and RFT1 in rice. Thus, we propose that the transition to flowering could be regulated by RID1 through the autonomous pathway, in addition to the photoperiod pathway.

GhJAZ2 negatively regulates cotton fiber initiation by interacting with the R2R3-MYB transcription factor GhMYB25-like

Jasmonic acid (JA) signaling has been well studied in Arabidopsis. Most reports focus on the role of JA in biological pathways, such as stress resistance, trichome initiation and anthocyanin accumulation. The JASMONATE ZIM-DOMAIN (JAZ) protein is one of the important repressors in the JA signaling pathway. Previous studies showed that JA functions in fiber initiation and elongation, but little is known about how JAZ genes function in fiber development. In this study, a cotton JAZ protein (GhJAZ2) containing a highly conserved TIFY motif and a C-terminal Jas domain was identified, and its function during cotton fiber development was analysed. Gene expression analysis showed that GhJAZ2 was preferentially expressed in the root, hypocotyl, flower and ovule 1 day before anthesis. Overexpression of GhJAZ2 inhibited both lint and fuzz fiber initiation, and reduced the fiber length. Yeast two-hybrid assays showed that GhJAZ2 interacted with the R2R3-MYB transcription factors GhMYB25-like and GhGL1, the bHLH transcription factor GhMYC2, the WD repeat protein GhWD40 and the unknown protein GhJI1. Among these transcription factors, previous studies showed that downregulation of GhMYB25-like leads to a fiberless phenotype in cotton seeds. Molecular and genetic evidence showed that the GhJAZ2 protein suppresses fiber initiation in the overexpressing lines by interacting with GhMYB25-like and suppressing GhMYB25-like activity. Our results suggested that GhJAZ2 functions as a primary transcription repressor during lint and fuzz fiber initiation by interacting with GhMYB25-like, GhGL1, GhMYC2, GhWD40 and GhJI1 to regulate the JA signaling pathway.

MYC2, MYC3, and MYC4 function redundantly in seed storage protein accumulation in Arabidopsis

Basic helix-loop-helix transcription factors (TFs), namely MYC2, MYC3, and MYC4, interact with Jasmonate Zim-domain proteins and are their direct targets. These TFs have been shown to function synergistically to control Arabidopsis growth and development. Our results showed similar MYC2, MYC3, and MYC4 expression patterns during Arabidopsis seed development, which remained relatively high during seed mid-maturation. MYC2, MYC3, and MYC4 acted redundantly in seed size, weight control, and in regulating seed storage protein accumulation. Triple mutants produced the largest seeds and single and double mutants' seeds were much larger than those of wild type. The weight of triple mutants' seeds was significantly higher than that of wild-type seeds, which was accompanied by an increase in seed storage protein contents. Triple mutants' seeds presented a marked decrease in 2S amounts relative to those in wild-type seeds. Liquid chromatography tandem mass spectra sequencing results indicated that both the relative abundance and the peptide number of CRA1 and CRU3 were greatly increased in triple mutants compared to wild type. The expression of 2S1-2S5 decreased and that of CRA1 and CRU3 increased in triple mutants relative to those in wild types during seed development, which might have contributed to the low 2S and high 12S contents in triple mutants. Our results contribute to understanding the function of MYC2, MYC3, and MYC4 on seed development, and provide promising targets for genetic manipulations of protein-producing crops to improve the quantity and quality of seed storage proteins.

Vitamins for enhancing plant resistance

This paper provides an overview on vitamins with inducing activities in plants, the molecular and cellular mechanisms implicated, and the hormonal signalling-network regulating this process. Moreover, it reports how vitamins might be part of the molecular events linked to induced resistance by the conventional elicitors. Induced resistance (IR), exploiting the plant innate-defense system is a sustainable strategy for plant disease control. In the last decade, vitamins have been proven to act as inducers of disease resistance, and these findings have received an important attention owing to their safety and cost effectiveness. Vitamins, including thiamine (TH, vitamin B1), riboflavin (RF, vitamin B2), menadione sodium bisulfite (MSB, vitamin K3), Para-aminobenzoic acid (PABA, vitamin Bx), and folic acid (FA, vitamin B9) provided an efficient protection against a wide range of pathogens through the modulation of specific host-defense facets. However, other vitamins, such as ascorbic acid (AA, vitamin C) and tocopherols (vitamin E), have been shown to be a part of the molecular mechanisms associated to IR. The present review is the first to summarize what vitamins are acting as inducers of disease resistance in plants and how could they be modulated by the conventional elicitors. Thus, this report provides an overview on the protective abilities of vitamins and the molecular and cellular mechanisms underlying their activities. Moreover, it describes the hormonal-signalling network regulating vitamin-signal transduction during IR. Finally, a biochemical model describing how vitamins are involved in the establishment of IR process is discussed.