The complex regulation of WRKY53 during leaf senescence of Arabidopsis thaliana

Overexpression of AcWRKY31 Increases Sensitivity to Salt and Drought and Improves Tolerance to Mealybugs in Pineapple

Pineapple is a globally significant tropical fruit, but its cultivation faces numerous challenges due to abiotic and biotic stresses, affecting its quality and quantity. WRKY transcription factors are known regulators of stress responses, however, their specific functions in pineapple are not fully understood. This study investigates the role of AcWRKY31 by overexpressing it in pineapple and Arabidopsis. Transgenic pineapple lines were obtained using Agrobacterium-mediated transformation methods and abiotic and biotic stress treatments. Transgenic AcWRKY31-OE pineapple plants showed an increased sensitivity to salt and drought stress and an increased resistance to biotic stress from pineapple mealybugs compared to that of WT plants. Similar experiments in AcWRKY31-OE, AtWRKY53-OE, and the Arabidopsis Atwrky53 mutant were performed and consistently confirmed these findings. A comparative transcriptomic analysis revealed 5357 upregulated genes in AcWRKY31-OE pineapple, with 30 genes related to disease and pathogen response. Notably, 18 of these genes contained a W-box sequence in their promoter region. A KEGG analysis of RNA-Seq data showed that upregulated DEG genes are mostly involved in translation, protein kinases, peptidases and inhibitors, membrane trafficking, folding, sorting, and degradation, while the downregulated genes are involved in metabolism, protein families, signaling, and cellular processes. RT-qPCR assays of selected genes confirmed the transcriptomic results. In summary, the AcWRKY31 gene is promising for the improvement of stress responses in pineapple, and it could be a valuable tool for plant breeders to develop stress-tolerant crops in the future.

Pepper immunity against Ralstonia solanacearum is positively regulated by CaWRKY3 through modulation of different WRKY transcription factors

BACKGROUND

Several WRKY transcription factors (TFs), including CaWRKY6, CaWRKY22, CaWRKY27, and CaWRKY40 are known to govern the resistance of pepper (Capsicum annuum L.) plants to Ralstonia solanacearum infestation (RSI) and other abiotic stresses. However, the molecular mechanisms underlying these processes remain elusive.

METHODS

This study functionally described CaWRKY3 for its role in pepper immunity against RSI. The roles of phytohormones in mediating the expression levels of CaWRKY3 were investigated by subjecting pepper plants to 1 mM salicylic acid (SA), 100 µM methyl jasmonate (MeJA), and 100 µM ethylene (ETH) at 4-leaf stage. A virus-induced gene silencing (VIGS) approach based on the Tobacco Rattle Virus (TRV) was used to silence CaWRKY3 in pepper, and transiently over-expressed to infer its role against RSI.

RESULTS

Phytohormones and RSI increased CaWRKY3 transcription. The transcriptions of defense-associated marker genes, including CaNPR1, CaPR1, CaDEF1, and CaHIR1 were decreased in VIGS experiment, which made pepper less resistant to RSI. Significant hypersensitive (HR)-like cell death, H2O2 buildup, and transcriptional up-regulation of immunological marker genes were noticed in pepper when CaWRKY3 was transiently overexpressed. Transcriptional activity of CaWRKY3 was increased with overexpression of CaWRKY6, CaWRKY22, CaWRKY27, and CaWRKY40, and vice versa. In contrast, Pseudomonas syringae pv tomato DC3000 (Pst DC3000) was easily repelled by the innate immune system of transgenic Arabidopsis thaliana that overexpressed CaWRKY3. The transcriptions of defense-related marker genes like AtPR1, AtPR2, and AtNPR1 were increased in CaWRKY3-overexpressing transgenic A. thaliana plants.

CONCLUSION

It is concluded that CaWRKY3 favorably regulates phytohormone-mediated synergistic signaling, which controls cell death in plant and immunity of pepper plant against bacterial infections.

Plant U-Box 4 regulates chloroplast stress signaling and programmed cell death via Salicylic acid modulation

In response to environmental stress, chloroplasts generate reactive oxygen species, including singlet oxygen (1O2), which regulates nuclear gene expression (retrograde signaling), chloroplast turnover, and programmed cell death (PCD). Yet, the central signaling mechanisms and downstream responses remain poorly understood. The Arabidopsis thaliana plastid ferrochelatase two (fc2) mutant conditionally accumulates 1O2 and involves Plant U-Box 4 (PUB4), a cytoplasmic E3 ubiquitin ligase, in propagating these signals. To gain insights into 1O2 signaling pathways, we compared transcriptomes of fc2 and fc2 pub4 mutants. The accumulation of 1O2 in fc2 plants broadly repressed genes involved in chloroplast function and photosynthesis, while 1O2 induced genes and transcription factors involved in abiotic and biotic stress, the biosynthesis of jasmonic acid (JA), and Salicylic acid (SA). Elevated JA and SA levels were observed in stressed fc2 plants, but were not responsible for PCD. pub4 reversed the majority of 1O2-induced gene expression in fc2 and reduced the JA content, but maintained elevated levels of SA even in the absence of 1O2 stress. Reducing SA levels in fc2 pub4 restored 1O2 signaling and light sensitivity. Together, this work demonstrates that SA plays a protective role during photo-oxidative stress and that PUB4 mediates 1O2 signaling by modulating its levels.

Tulip transcription factor TgWRKY75 activates salicylic acid and abscisic acid biosynthesis to synergistically promote petal senescence

WRKY transcription factors play a central role in controlling plant organ senescence; however, it is unclear whether and how they regulate petal senescence in the widely grown ornamental plant tulip (Tulipa gesneriana). In this study, we report that TgWRKY75 promotes petal senescence by enhancing the synthesis of both abscisic acid (ABA) and salicylic acid (SA) in tulip and in transgenic Arabidopsis. The expression level of TgWRKY75 was up-regulated in senescent petals, and exogenous ABA or SA treatment induced its expression. The endogenous contents of ABA and SA significantly increased during petal senescence and in response to TgWRKY75 overexpression. Two SA synthesis-related genes, TgICS1 and TgPAL1, were identified as direct targets of TgWRKY75, which binds to their promoters. In parallel, TgWRKY75 activated the expression of the ABA biosynthesis-related gene TgNCED3 via directly binding to its promoter region. Site mutation of the W-box core motif located in the promoters of TgICS1, TgPAL1, and TgNCED3 eliminated their interactions with TgWRKY75. In summary, our study demonstrates a dual regulation of ABA and SA biosynthesis by TgWRKY75, revealing a synergistic process of tulip petal senescence through feedback regulation between TgWRKY75 and the accumulation of ABA and SA.

SmRAV1, an AP2 and B3 Transcription Factor, Positively Regulates Eggplant's Response to Salt Stress

Salt stress is a lethal abiotic stress threatening global food security on a consistent basis. In this study, we identified an AP2 and B3 domain-containing transcription factor (TF) named SmRAV1, and its expression levels were significantly up-regulated by NaCl, abscisic acid (ABA), and hydrogen peroxide (H2O2) treatment. High expression of SmRAV1 was observed in the roots and sepal of mature plants. The transient expression assay in Nicotiana benthamiana leaves revealed that SmRAV1 was localized in the nucleus. Silencing of SmRAV1 via virus-induced gene silencing (VIGS) decreased the tolerance of eggplant to salt stress. Significant down-regulation of salt stress marker genes, including SmGSTU10 and SmNCED1, was observed. Additionally, increased H2O2 content and decreased catalase (CAT) enzyme activity were recorded in the SmRAV1-silenced plants compared to the TRV:00 plants. Our findings elucidate the functions of SmRAV1 and provide opportunities for generating salt-tolerant lines of eggplant.

A truncated ETHYLENE INSENSITIVE3-like protein, GhLYI, regulates senescence in cotton

Numerous endogenous and environmental signals regulate the intricate and highly orchestrated process of plant senescence. Ethylene (ET), which accumulates as senescence progresses, is a major promoter of leaf senescence. The master transcription activator ETHYLENE INSENSITIVE3 (EIN3) activates the expression of a wide range of downstream genes during leaf senescence. Here, we found that a unique EIN3-LIKE 1 (EIL1) gene, cotton LINT YIELD INCREASING (GhLYI), encodes a truncated EIN3 protein in upland cotton (Gossypium hirsutum L.) that functions as an ET signal response factor and a positive regulator of senescence. Ectopic expression or overexpression of GhLYI accelerated leaf senescence in both Arabidopsis (Arabidopsis thaliana) and cotton. Cleavage under targets and tagmentation (CUT&Tag) analyses revealed that SENESCENCE-ASSOCIATED GENE 20 (SAG20) was a target of GhLYI. Electrophoretic mobility shift assay (EMSA), yeast 1-hybrid (Y1H), and dual-luciferase transient expression assay confirmed that GhLYI directly bound the promoter of SAG20 to activate its expression. Transcriptome analysis revealed that transcript levels of a series of senescence-related genes, SAG12, NAC-LIKE, ACTIVATED by APETALA 3/PISTILLATA (NAP/ANAC029), and WRKY53, are substantially induced in GhLYI overexpression plants compared with wild-type (WT) plants. Virus-induced gene silencing (VIGS) preliminarily confirmed that knockdown of GhSAG20 delayed leaf senescence. Collectively, our findings provide a regulatory module involving GhLYI-GhSAG20 in controlling senescence in cotton.

Senescence-related receptor kinase 1 functions downstream of WRKY53 in regulating leaf senescence in Arabidopsis

Receptor-like kinases (RLKs) are the most important class of cell surface receptors, and play crucial roles in plant development and stress responses. However, few studies have been reported about the biofunctions of RLKs in leaf senescence. Here, we characterized a novel Arabidopsis RLK-encoding gene, SENESCENCE-RELATED RECEPTOR KINASE 1 (SENRK1), which was significantly down-regulated during leaf senescence. Notably, the loss-of-function senrk1 mutants displayed an early leaf senescence phenotype, while overexpression of SENRK1 significantly delayed leaf senescence, indicating that SENRK1 negatively regulates age-dependent leaf senescence in Arabidopsis. Furthermore, the senescence-promoting transcription factor WRKY53 repressed the expression of SENRK1. While the wrky53 mutant showed a delayed senescence phenotype as previously reported, the wrky53 senrk1-1 double mutant exhibited precocious leaf senescence, suggesting that SENRK1 functions downstream of WRKY53 in regulating age-dependent leaf senescence in Arabidopsis.

Multifaceted roles of LhWRKY44 in promoting anthocyanin accumulation in Asiatic hybrid lilies (Lilium spp.)

The Asiatic hybrid lily (Lilium spp.) is a horticultural crop with high commercial value and diverse anthocyanin pigmentation patterns. However, the regulatory mechanism underlying lily flower color has been largely unexplored. Here, we identified a WRKY transcription factor from lily tepals, LhWRKY44, whose expression was closely associated with anthocyanin accumulation. Functional verification indicated that LhWRKY44 positively regulated anthocyanin accumulation. LhWRKY44 physically interacted with LhMYBSPLATTER and directly bound to the LhMYBSPLATTER promoter, which enhanced the effect of the LhMYBSPLATTER-LhbHLH2 MBW complex activator on anthocyanin accumulation. Moreover, EMSA and dual-luciferase assays revealed that LhWRKY44 activated and bound to the promoters of gene LhF3H and the intracellular anthocyanin-related glutathione S-transferase gene LhGST. Interestingly, our further results showed that LhWRKY44 participated in light and drought-induced anthocyanin accumulation, and improved the drought tolerance in lily via activating stress-related genes. These results generated a multifaceted regulatory mechanism for the LhWRKY44-meditaed enhancement by the environmental signal pathway of anthocyanin accumulation and expanded our understanding of the WRKY-mediated transcriptional regulatory hierarchy modulating anthocyanin accumulation in Asiatic hybrid lilies.

Advances in the Research on Plant WRKY Transcription Factors Responsive to External Stresses

The WRKY transcription factors are a class of transcriptional regulators that are ubiquitous in plants, wherein they play key roles in various physiological activities, including responses to stress. Specifically, WRKY transcription factors mediate plant responses to biotic and abiotic stresses through the binding of their conserved domain to the W-box element of the target gene promoter and the subsequent activation or inhibition of transcription (self-regulation or cross-regulation). In this review, the progress in the research on the regulatory effects of WRKY transcription factors on plant responses to external stresses is summarized, with a particular focus on the structural characteristics, classifications, biological functions, effects on plant secondary metabolism, regulatory networks, and other aspects of WRKY transcription factors. Future research and prospects in this field are also proposed.

The CRK5 and WRKY53 Are Conditional Regulators of Senescence and Stomatal Conductance in Arabidopsis

In Arabidopsis thaliana, cysteine-rich receptor-like kinases (CRKs) constitute a large group of membrane-localized proteins which perceive external stimuli and transduce the signal into the cell. Previous reports based on their loss-of-function phenotypes and expression profile support their role in many developmental and stress-responsive pathways. Our study revealed that one member of this family, CRK5, acts as a negative regulator of leaf aging. Enrichment of the CRK5 promoter region in W-box cis-elements demonstrated that WRKY transcription factors control it. We observed significantly enhanced WRKY53 expression in crk5 and reversion of its early-senescence phenotype in the crk5 wrky53 line, suggesting a negative feedback loop between these proteins antagonistically regulating chlorophyll a and b contents. Yeast-two hybrid assay showed further that CRK5 interacts with several proteins involved in response to water deprivation or calcium signaling, while gas exchange analysis revealed a positive effect of CRK5 on water use efficiency. Consistent with that, the crk5 plants showed disturbed foliar temperature, stomatal conductance, transpiration, and increased susceptibility to osmotic stress. These traits were fully or partially reverted to wild-type phenotype in crk5 wrky53 double mutant. Obtained results suggest that WRKY53 and CRK5 are antagonistic regulators of chlorophyll synthesis/degradation, senescence, and stomatal conductance.

A cross-species co-functional gene network underlying leaf senescence

The complex leaf senescence process is governed by various levels of transcriptional and translational regulation. Several features of the leaf senescence process are similar across species, yet the extent to which the molecular mechanisms underlying the process of leaf senescence are conserved remains unclear. Currently used experimental approaches permit the identification of individual pathways that regulate various physiological and biochemical processes; however, the large-scale regulatory network underpinning intricate processes like leaf senescence cannot be built using these methods. Here, we discovered a series of conserved genes involved in leaf senescence in a common horticultural crop (Solanum lycopersicum), a monocot plant (Oryza sativa), and a eudicot plant (Arabidopsis thaliana) through analyses of the evolutionary relationships and expression patterns among genes. Our analyses revealed that the genetic basis of leaf senescence is largely conserved across species. We also created a multi-omics workflow using data from more than 10 000 samples from 85 projects and constructed a leaf senescence-associated co-functional gene network with 2769 conserved, high-confidence functions. Furthermore, we found that the mitochondrial unfolded protein response (UPR^mt) is the central biological process underlying leaf senescence. Specifically, UPR^mt responds to leaf senescence by maintaining mitostasis through a few cross-species conserved transcription factors (e.g. NAC13) and metabolites (e.g. ornithine). The co-functional network built in our study indicates that UPR^mt figures prominently in cross-species conserved mechanisms. Generally, the results of our study provide new insights that will aid future studies of leaf senescence.

WRKY transcription factor family in lettuce plant (Lactuca sativa): Genome-wide characterization, chromosome location, phylogeny structures, and expression patterns

WRKY transcription factors (TF) have been identified in many plant species and play critical roles in multiple stages of growth and development and under various stress conditions. As one of the most popular vegetable crops, asparagus lettuce has important medicinal and nutritional value. However, study of WRKY TFs family in asparagus lettuce is limited. With the lettuce (Lactuca sativa L.) genome publication, we identified 76 WRKY TFs and analyzed structural characteristics, phylogenetic relationships, chromosomal distribution, interaction network, and expression profiles. The 76 LsWRKY TFs were phylogenetically classified as Groups I, II (IIa-IIe), and III. Cis element analysis revealed complex regulatory relationships of LsWRKY genes in response to different biological progresses. Interaction network analysis indicated that LsWRKY TFs could interact with other proteins, such as SIB (sigma factor binding protein), WRKY TFs, and MPK. The WRKYIII subfamily genes showed different expression patterns during the progress of asparagus lettuce stem enlargement. According to qRT-PCR analysis, abiotic stresses (drought, salt, low temperature, and high temperature) and phytohormone treatment could induce specific LsWRKYIII gene expression. These results will provide systematic and comprehensive information on LsWRKY TFs and lay the foundation for further clarification of the regulatory mechanism of LsWRKY, especially LsWRKYIII TFs, involved in stress response and the progress of plant growth and development.

Moderate DNA methylation changes associated with nitrogen remobilization and leaf senescence in Arabidopsis

The lifespan of plants is restricted by environmental and genetic components. Following the transition to reproductive growth, leaf senescence ends cellular life in monocarpic plants to remobilize nutrients to storage organs. In Arabidopsis, we initially observed altered leaf to seed ratios, faster senescence progression, altered leaf nitrogen recovery after transient nitrogen removal, and ultimately enhanced nitrogen remobilization from the leaves in two methylation mutants (ros1 and the triple dmr1/2 cmt3 knockout). Analysis of the DNA methylome in wild type Col-0 leaves identified an initial moderate decline of cytosine methylation with progressing leaf senescence, predominantly in the CG context. Late senescence was associated with moderate de novo methylation of cytosines, primarily in the CHH context. Relatively few differentially methylated regions, including one in the ROS1 promoter linked to down-regulation of ROS1, were present, but these were unrelated to known senescence-associated genes. Differential methylation patterns were identified in transcription factor binding sites, such as the W-boxes that are targeted by WRKYs. Methylation in artificial binding sites impaired transcription factor binding in vitro. However, it remains unclear how moderate methylome changes during leaf senescence are linked with up-regulated genes during senescence.

Transcriptome analysis of gene expression in the tomato leaf premature senescence mutant

Premature senescence of leaves can critically influence tomato yield and quality. In this study, the leaf premature senescence mutant MT318 was a spontaneous mutant and was controlled by a single recessive nuclear gene. The maximum photochemical efficiency (Fv/Fm), superoxide dismutase (SOD), and chlorophyll content in the leaves of mutant MT318 gradually decreased, while malondialdehyde (MDA) content significantly increased. Under the level 2 category, Gene Ontology (GO) enrichment analysis indicated that 45 terms were enriched, comprising 22 in biological process, 12 in cellular component, and 11 in molecular function. Genes are mainly involved in the metabolic processes (696 differentially expressed genes, DEGs), cellular processes (573 DEGs), single-organism processes (503 DEGs), and catalytic activity (675 DEGs). Kyoto Encyclopedia of Genes and Genomes pathway analysis demonstrated that the 4 pathways with the largest number of genes were biosynthesis of secondary metabolites, plant-pathogen interaction, plant hormone signal transduction, and MAPK signaling pathway-plant. The 'plant hormone signal transduction' pathway was the most significantly enriched at the T2 stage. Pearson correlation analysis showed that the auxin regulatory pathway and SA signal transduction pathway may play important roles. These results not only lay the foundation for the further cloning and functional analysis of the MT318 premature senescence gene but also provide a reference for the study of tomato leaf senescence.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01223-2.

GhWRKY33 Interacts with GhTIFY10A to Synergistically Modulate Both Ageing and JA-Mediated Leaf Senescence in Arabidopsis

WRKY transcription factors play critical roles in the modulation of transcriptional changes during leaf senescence, but the underlying mechanisms controlled by them in this progress still remain enigmatic. In this study, Gossypium hirsutum WRKY DNA-binding protein 33 (GhWRKY33) was characterized as a negative regulator of both ageing and JA-mediated leaf senescence. The overexpression of GhWRKY33 in Arabidopsis greatly delayed leaf senescence, as determined by elevated chlorophyll content, lower H₂O₂ content, and reduced expression of several senescence-associated genes (SAGs). An electrophoretic mobility shift assay (EMSA) and transient dual–luciferase reporter assay revealed that GhWRKY33 could bind to the promoters of both AtSAG12 and Ghcysp and suppress their expression. Yeast two-hybrid (Y2H) and firefly luciferase complementation imaging (LUC) assays showed that GhWRKY33 could interact with GhTIFY10A. Similarly, the overexpression of GhTIFY10A in Arabidopsis also dramatically delayed leaf senescence. Furthermore, both GhWRKY33 and GhTIFY10A negatively regulate JA-mediated leaf senescence. In addition, a transientdual-luciferase reporter assay indicated that GhWRKY33 and GhTIFY10A could function synergistically to inhibit the expression of both AtSAG12 and Ghcysp. Thus, our work suggested that GhWRKY33 may function as a negative regulator to modulate both ageing and JA-mediated leaf senescence and also contributes to a basis for further functional studies on cotton leaf senescence.

WHIRLY1 recruits the histone deacetylase HDA15 repressing leaf senescence and flowering in Arabidopsis

Leaf senescence is controlled by a complex regulatory network in which robustness is ensured by the activity of transcription factors and epigenetic regulators. However, how these coordinate the process of leaf senescence remains poorly understood. We found that WHIRLY1 interacts with Histone Deacetylase (HDA)15, a Reduced Potassium Dependence3 (RPD3)/HDA1-type HDA, by using green fluorescent protein-nanotrap-mass spectrum assays. The development-dependent interaction between WHIRLY1 and HDA15 was further confirmed by bimolecular fluorescence complementation assays and co-immunoprecipitation assays in Arabidopsis. Multi-omics genome-wide transcriptome and H3K9 acetylome enrichment analysis showed that HDA15 delays leaf senescence and flowering by repressing the expression of the positive regulators of leaf senescence and flowering, such as LOX2 and LARP1C, and reducing H3K9ac levels at these loci; WHIRLY1 and HDA15 co-target to the region near the transcription start site of a subset of nutrient recycling-related genes (e.g., Glutathione S-transferases 10, non-coding RNA, and photosystem II protein D1 synthesizer attenuator PDIL1-2), as well as WRKY53 and ELF4, and co-repress their expression by removing H3K9 acetylation. Our study revealed a key transcription regulatory node of nutrient recycling and senescence-associated genes involved in leaf senescence and flowering via the recruitment of HDA15 by the single-stranded DNA/RNA-binding protein WHIRLY1.

The Kandelia obovata transcription factor KoWRKY40 enhances cold tolerance in transgenic Arabidopsis

BACKGROUND

WRKY transcription factors play key roles in plant development processes and stress response. Kandelia obovata is the most cold-resistant species of mangrove plants, which are the important contributors to coastal marine environment. However, there is little known about the WRKY genes in K. obovata.

RESULTS

In this study, a WRKY transcription factor gene, named KoWRKY40, was identified from mangrove plant K. obovata. The full-length cDNA of KoWRKY40 gene was 1420 nucleotide bases, which encoded 318 amino acids. The KoWRKY40 protein contained a typical WRKY domain and a C2H2 zinc-finger motif, which were common signatures to group II of WRKY family. The three-dimensional (3D) model of KoWRKY40 was formed by one α-helix and five β-strands. Evolutionary analysis revealed that KoWRKY40 has the closest homology with a WRKY protein from another mangrove plant Bruguiera gymnorhiza. The KoWRKY40 protein was verified to be exclusively located in nucleus of tobacco epidermis cells. Gene expression analysis demonstrated that KoWRKY40 was induced highly in the roots and leaves, but lowly in stems in K. obovata under cold stress. Overexpression of KoWRKY40 in Arabidopsis significantly enhanced the fresh weight, root length, and lateral root number of the transgenic lines under cold stress. KoWRKY40 transgenic Arabidopsis exhibited higher proline content, SOD, POD, and CAT activities, and lower MDA content, and H₂O₂ content than wild-type Arabidopsis under cold stress condition. Cold stress affected the expression of genes related to proline biosynthesis, antioxidant system, and the ICE-CBF-COR signaling pathway, including AtP5CS1, AtPRODH1, AtMnSOD, AtPOD, AtCAT1, AtCBF1, AtCBF2, AtICE1, AtCOR47 in KoWRKY40 transgenic Arabidopsis plants.

CONCLUSION

These results demonstrated that KoWRKY40 conferred cold tolerance in transgenic Arabidopsis by regulating plant growth, osmotic balance, the antioxidant system, and ICE-CBF-COR signaling pathway. The study indicates that KoWRKY40 is an important regulator involved in the cold stress response in plants.

The genetic interaction of REVOLUTA and WRKY53 links plant development, senescence, and immune responses

In annual plants, tight coordination of successive developmental events is of primary importance to optimize performance under fluctuating environmental conditions. The recent finding of the genetic interaction of WRKY53, a key senescence-related gene with REVOLUTA, a master regulator of early leaf patterning, raises the question of how early and late developmental events are connected. Here, we investigated the developmental and metabolic consequences of an alteration of the REVOLUTA and WRKY53 gene expression, from seedling to fruiting. Our results show that REVOLUTA critically controls late developmental phases and reproduction while inversely WRKY53 determines vegetative growth at early developmental stages. We further show that these regulators of distinct developmental phases frequently, but not continuously, interact throughout ontogeny and demonstrated that their genetic interaction is mediated by the salicylic acid (SA). Moreover, we showed that REVOLUTA and WRKY53 are keys regulatory nodes of development and plant immunity thought their role in SA metabolic pathways, which also highlights the role of REV in pathogen defence. Together, our findings demonstrate how late and early developmental events are tightly intertwined by molecular hubs. These hubs interact with each other throughout ontogeny, and participate in the interplay between plant development and immunity.

Chloroplast Protein Tic55 Involved in Dark-Induced Senescence through AtbHLH/AtWRKY-ANAC003 Controlling Pathway of Arabidopsis thaliana

The chloroplast comprises the outer and inner membranes that are composed of the translocon protein complexes Toc and Tic (translocon at the outer/inner envelope membrane of chloroplasts), respectively. Tic55, a chloroplast Tic protein member, was shown to be not vital for functional protein import in Arabidopsis from previous studies. Instead, Tic55 was revealed to be a dark-induced senescence-related protein in our earlier study. To explore whether Tic55 elicits other biological functions, a tic55-II knockout mutant (SALK_086048) was characterized under different stress treatments. Abiotic stress conditions, such as cold, heat, and high osmotic pressure, did not cause visible effects on tic55-II mutant plant, when compared to the wild type (WT). In contrast, senescence was induced in the individually darkened leaves (IDLs), resulting in the differential expression of the senescence-related genes PEROXISOME DEFECTIVE 1 (PED1), BLUE COPPER-BINDING PROTEIN (BCB), SENESCENCE 1 (SEN1), and RUBISCO SMALL SUBUNIT GENE 2B (RBCS2B). The absence of Tic55 in tic55-II knockout mutant inhibited expression of the senescence-related genes PED1, BCB, and SEN1 at different stages of dark adaptation, while causing stimulation of RBCS2B gene expression at an early stage of dark response. Finally, yeast one-hybrid assays located the ANAC003 promoter region with cis-acting elements are responsible for binding to the different AtbHLH proteins, thereby causing the transactivation of an HIS3 reporter gene. ANAC003 was shown previously as a senescence-related protein and its activation would lead to expression of senescence-associated genes (SAGs), resulting in plant senescence. Thus, we propose a hypothetical model in which three signaling pathways may be involved in controlling the expression of ANAC003, followed by expression of SAGs that in turn leads to leaf senescence in Arabidopsis by this study and previous data.

WRKY Transcription Factor Response to High-Temperature Stress

Plant growth and development are closely related to the environment, and high-temperature stress is an important environmental factor that affects these processes. WRKY transcription factors (TFs) play important roles in plant responses to high-temperature stress. WRKY TFs can bind to the W-box cis-acting elements of target gene promoters, thereby regulating the expression of multiple types of target genes and participating in multiple signaling pathways in plants. A number of studies have shown the important biological functions and working mechanisms of WRKY TFs in plant responses to high temperature. However, there are few reviews that summarize the research progress on this topic. To fully understand the role of WRKY TFs in the response to high temperature, this paper reviews the structure and regulatory mechanism of WRKY TFs, as well as the related signaling pathways that regulate plant growth under high-temperature stress, which have been described in recent years, and this paper provides references for the further exploration of the molecular mechanisms underlying plant tolerance to high temperature.

Genome-wide identification and analysis of WRKY gene family in maize provide insights into regulatory network in response to abiotic stresses

BACKGROUND

The WRKY transcription factor family plays significant roles in biotic and abiotic stress responses, which has been associated with various biological processes in higher plants. However, very little is known regarding the structure and function of WRKY genes in maize.

RESULTS

In this study, a total of 140 ZmWRKY proteins encoded by 125 ZmWRKY genes were eventually identified in maize. On the basis of features of molecular structure and a comparison of phylogenetic relationships of WRKY transcription factor families from Arabidopsis, rice and maize, all 140 ZmWRKY proteins in maize were divided into three main groups (Groups I, II and III) and the Group II was further classified into five subgroups. The characteristics of exon-intron structure of these putative ZmWRKY genes and conserved protein motifs of their encoded ZmWRKY proteins were also presented respectively, which was in accordance with the group classification results. Promoter analysis suggested that ZmWRKY genes shared many abiotic stress-related elements and hormone-related elements. Gene duplication analysis revealed that the segmental duplication and purifying selection might play a significant role during the evolution of the WRKY gene family in maize. Using RNA-seq data, transcriptome analysis indicated that most of ZmWRKY genes displayed differential expression patterns at different developmental stages of maize. Further, by quantitative real-time PCR analysis, twenty-one ZmWRKY genes were confirmed to respond to two different abiotic stress treatments, suggesting their potential roles in various abiotic stress responses. In addition, RNA-seq dataset was used to conduct weighted gene co-expression network analysis (WGCNA) in order to recognize gene subsets possessing similar expression patterns and highly correlated with each other within different metabolic networks. Further, subcellular localization prediction, functional annotation and interaction analysis of ZmWRKY proteins were also performed to predict their interactions and associations involved in potential regulatory network.

CONCLUSIONS

Taken together, the present study will serve to present an important theoretical basis for further exploring function and regulatory mechanism of ZmWRKY genes in the growth, development, and adaptation to abiotic stresses in maize.

Arabidopsis WRKY71 regulates ethylene-mediated leaf senescence by directly activating EIN2, ORE1 and ACS2 genes

Leaf senescence is a pivotal step in the last stage of the plant life cycle and is influenced by various external and endogenous cues. A series of reports have indicated the involvement of the WRKY transcription factors in regulating leaf senescence, but the molecular mechanisms and signaling pathways remain largely unclear. Here we provide evidence demonstrating that WRKY71 acts as a positive regulator of leaf senescence in Arabidopsis. WRKY71-1D, an overexpressor of WRKY71, exhibited early leaf senescence, while wrky71-1, the WRKY71 loss-of-function mutant, displayed delayed leaf senescence. Accordingly, a set of senescence-associated genes (SAGs) were substantially elevated in WRKY71-1D but markedly decreased in wrky71-1. Chromatin immunoprecipitation assays indicated that WRKY71 can bind directly to the promoters of SAG13 and SAG201. Transcriptome analysis suggested that WRKY71 might mediate multiple cues to accelerate leaf senescence, such as abiotic stresses, dark and ethylene. WRKY71 was ethylene inducible, and treatment with the ethylene precursor 1-amino-cyclopropane-1-carboxylic acid enhanced leaf senescence in WRKY71-1D but caused only a marginal delay in leaf senescence in wrky71-1. In vitro and in vivo assays demonstrated that WRKY71 can directly regulate ETHYLENE INSENSITIVE2 (EIN2) and ORESARA1 (ORE1), genes of the ethylene signaling pathway. Consistently, leaf senescence of WRKY71-1D was obviously retarded in the ein2-5 and nac2-1 mutants. Moreover, WRKY71 was also proved to interact with ACS2 in vitro and in vivo. Treatment with AgNO₃ and aminoethoxyvinylglycine and acs2-1 could greatly arrest the leaf senescence of WRKY71-1D. In conclusion, our data revealed that WRKY71 mediates ethylene signaling and synthesis to hasten leaf senescence in Arabidopsis.

Leaf senescence: progression, regulation, and application

Leaf senescence, the last stage of leaf development, is a type of postmitotic senescence and is characterized by the functional transition from nutrient assimilation to nutrient remobilization which is essential for plants' fitness. The initiation and progression of leaf senescence are regulated by a variety of internal and external factors such as age, phytohormones, and environmental stresses. Significant breakthroughs in dissecting the molecular mechanisms underpinning leaf senescence have benefited from the identification of senescence-altered mutants through forward genetic screening and functional assessment of hundreds of senescence-associated genes (SAGs) via reverse genetic research in model plant Arabidopsis thaliana as well as in crop plants. Leaf senescence involves highly complex genetic programs that are tightly tuned by multiple layers of regulation, including chromatin and transcription regulation, post-transcriptional, translational and post-translational regulation. Due to the significant impact of leaf senescence on photosynthesis, nutrient remobilization, stress responses, and productivity, much effort has been made in devising strategies based on known senescence regulatory mechanisms to manipulate the initiation and progression of leaf senescence, aiming for higher yield, better quality, or improved horticultural performance in crop plants. This review aims to provide an overview of leaf senescence and discuss recent advances in multi-dimensional regulation of leaf senescence from genetic and molecular network perspectives. We also put forward the key issues that need to be addressed, including the nature of leaf age, functional stay-green trait, coordination between different regulatory pathways, source-sink relationship and nutrient remobilization, as well as translational researches on leaf senescence.

PRAT Proteins Operate in Organellar Protein Import and Export in Arabidopsis thaliana

Chloroplasts need to import preproteins and amino acids from the cytosol during their light-induced differentiation. Similarly, chloroplasts have to export organic matter including proteins and amino acids during leaf senescence. Members of the PRAT (preprotein and amino acid transporter) family are candidate transporters for both processes. Here, we defined the role of two small PRAT gene families, At4g26670 and At5g55510 (HP20 subfamily) versus At3g49560 and At5g24650 (HP30 subfamily) during greening of etiolated plants and during leaf senescence. Using a combination of reverse genetics, protein biochemistry and physiological tools, evidence was obtained for a role of chloroplast HP20, HP30 and HP30-2 in protein, but not amino acid, import into chloroplasts. HP20, HP30 and HP30-2 form larger complexes involved in the uptake of transit sequence-less cytosolic precursors. In addition, we identified a fraction of HP30-2 in mitochondria where it served a similar function as found for chloroplasts and operated in the uptake of transit sequence-less cytosolic precursor proteins. By contrast, HP22 was found to act in the export of proteins from chloroplasts during leaf senescence, and thus its role is entirely different from that of its orthologue, HP20. HP22 is part of a unique protein complex in the envelope of senescing chloroplasts that comprises at least 11 proteins and contains with HP65b (At5g55220) a protein that is related to the bacterial trigger factor chaperone. An ortholog of HP65b exists in the cyanobacterium Synechocystis and has previously been implicated in protein secretion. Whereas plants depleted of either HP22 or HP65b or even both were increasingly delayed in leaf senescence and retained much longer stromal chloroplast constituents than wild-type plants, HP22 overexpressors showed premature leaf senescence that was associated with accelerated losses of stromal chloroplast proteins. Together, our results identify the PRAT protein family as a unique system for importing and exporting proteins from chloroplasts.

ASCORBATE PEROXIDASE6 delays the onset of age-dependent leaf senescence

Age-dependent changes in reactive oxygen species (ROS) levels are critical in leaf senescence. While H2O2-reducing enzymes such as catalases and cytosolic ASCORBATE PEROXIDASE1 (APX1) tightly control the oxidative load during senescence, their regulation and function are not specific to senescence. Previously, we identified the role of ASCORBATE PEROXIDASE6 (APX6) during seed maturation in Arabidopsis (Arabidopsis thaliana). Here, we show that APX6 is a bona fide senescence-associated gene. APX6 expression is specifically induced in aging leaves and in response to senescence-promoting stimuli such as abscisic acid (ABA), extended darkness, and osmotic stress. apx6 mutants showed early developmental senescence and increased sensitivity to dark stress. Reduced APX activity, increased H2O2 level, and altered redox state of the ascorbate pool in mature pre-senescing green leaves of the apx6 mutants correlated with the early onset of senescence. Using transient expression assays in Nicotiana benthamiana leaves, we unraveled the age-dependent post-transcriptional regulation of APX6. We then identified the coding sequence of APX6 as a potential target of miR398, which is a key regulator of copper redistribution. Furthermore, we showed that mutants of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE7 (SPL7), the master regulator of copper homeostasis and miR398 expression, have a higher APX6 level compared with the wild type, which further increased under copper deficiency. Our study suggests that APX6 is a modulator of ROS/redox homeostasis and signaling in aging leaves that plays an important role in developmental- and stress-induced senescence programs.

MdABI5 works with its interaction partners to regulate abscisic acid-mediated leaf senescence in apple

Abscisic acid (ABA) induces chlorophyll degradation and leaf senescence; however, the molecular mechanism remains poorly understood, especially in woody plants. In this study, we found that MdABI5 plays an essential role in the regulation of ABA-triggered leaf senescence in Malus domestica (apple). Through yeast screening, three transcription factors, MdBBX22, MdWRKY40 and MdbZIP44, were found to interact directly with MdABI5 in vitro and in vivo. Physiological and biochemical assays showed that MdBBX22 delayed leaf senescence in two pathways. First, MdBBX22 interacted with MdABI5 to inhibit the transcriptional activity of MdABI5 on the chlorophyll catabolic genes MdNYE1 and MdNYC1, thus negatively regulating chlorophyll degradation and leaf senescence. Second, MdBBX22 interacted with MdHY5 to interfere with the transcriptional activation of MdHY5 on MdABI5, thereby inhibiting the expression of MdABI5, which also contributed to the delay of leaf senescence. MdWRKY40 and MdbZIP44 were identified as positive regulators of leaf senescence. They accelerated MdABI5-promoted leaf senescence through the same regulatory pathways, i.e., interacting with MdABI5 to enhance the transcriptional activity of MdABI5 on MdNYE1 and MdNYC1. Taken together, our results suggest that MdABI5 works with its positive or negative interaction partners to regulate ABA-mediated leaf senescence in apple, in which it acts as a core regulator. The antagonistic regulation pathways ensure that plants respond to external stresses flexibly and efficiently. Our results provide a concept for further study on the regulation mechanisms of leaf senescence.

Genetic loci underlying quantitative resistance to necrotrophic pathogens Sclerotinia and Diaporthe (Phomopsis), and correlated resistance to both pathogens

We provide results rooted in quantitative genetics, which combined with knowledge of candidate gene function, helps us to better understand the resistance to two major necrotrophic pathogens of sunflower. Necrotrophic pathogens can avoid or even benefit from plant defenses used against biotrophic pathogens, and thus represent a distinct challenge to plant populations in natural and agricultural systems. Sclerotinia and Phomopsis/Diaporthe are detrimental pathogens for many dicotyledonous plants, including many economically important plants. With no well-established methods to prevent infection in susceptible plants, host-plant resistance is currently the most effective strategy. Despite knowledge of a moderate, positive correlation in resistance to the two diseases in sunflower, detailed analysis of the genetics, in the same populations, has not been conducted. We present results of genome-wide analysis of resistance to both pathogens in a diversity panel of 218 domesticated sunflower genotypes of worldwide origin. We identified 14 Sclerotinia head rot and 7 Phomopsis stem canker unique QTLs, plus 1 co-located QTL for both traits, and observed extensive patterns of linkage disequilibrium between sites for both traits. Most QTLs contained one credible candidate gene, and gene families were common for the two disease resistance traits. These results suggest there has been strong, simultaneous selection for resistance to these two diseases and that a generalized mechanism for defense against these necrotrophic pathogens exists.

Comparative Transcriptome Analysis of Salt Stress-Induced Leaf Senescence in Medicago truncatula

Leaves are the most critical portion of forage crops such as alfalfa (Medicago sativa). Leaf senescence caused by environmental stresses significantly impacts the biomass and quality of forages. To understand the molecular mechanisms and identify the key regulator of the salt stress-induced leaf senescence process, we conducted a simple and effective salt stress-induced leaf senescence assay in Medicago truncatula, which was followed by RNA-Seq analysis coupled with physiological and biochemical characterization. By comparing the observed expression data with that derived from dark-induced leaf senescence at different time points, we identified 3,001, 3,787, and 4,419 senescence-associated genes (SAGs) for salt stress-induced leaf senescence on day 2, 4, and 6, respectively. There were 1546 SAGs shared by dark and salt stress treatment across the three time points. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses showed that the 1546 SAGs were mainly related to protein and amino acids metabolism, photosynthesis, chlorophyll metabolism, and hormone signaling during leaf senescence. Strikingly, many different transcription factors (TFs) families out of the 1546 SAGs, including NAC, bHLH, MYB, and ERF, were associated with salt stress-induced leaf senescence processes. Using the transient expression system in Nicotiana benthamiana, we verified that three functional NAC TF genes from the 1546 SAGs were related to leaf senescence. These results clarify SAGs under salt stress in M. truncatula and provide new insights and additional genetic resources for further forage crop breeding.

Multiple Layers of Regulation on Leaf Senescence: New Advances and Perspectives

Leaf senescence is the last stage of leaf development and is an orderly biological process accompanied by degradation of macromolecules and nutrient recycling, which contributes to plant fitness. Forward genetic mutant screening and reverse genetic studies of senescence-associated genes (SAGs) have revealed that leaf senescence is a genetically regulated process, and the initiation and progression of leaf senescence are influenced by an array of internal and external factors. Recently, multi-omics techniques have revealed that leaf senescence is subjected to multiple layers of regulation, including chromatin, transcriptional and post-transcriptional, as well as translational and post-translational levels. Although impressive progress has been made in plant senescence research, especially the identification and functional analysis of a large number of SAGs in crop plants, we still have not unraveled the mystery of plant senescence, and there are some urgent scientific questions in this field, such as when plant senescence is initiated and how senescence signals are transmitted. This paper reviews recent advances in the multiple layers of regulation on leaf senescence, especially in post-transcriptional regulation such as alternative splicing.

Function and Mechanism of WRKY Transcription Factors in Abiotic Stress Responses of Plants

The WRKY gene family is a plant-specific transcription factor (TF) group, playing important roles in many different response pathways of diverse abiotic stresses (drought, saline, alkali, temperature, and ultraviolet radiation, and so forth). In recent years, many studies have explored the role and mechanism of WRKY family members from model plants to agricultural crops and other species. Abiotic stress adversely affects the growth and development of plants. Thus, a review of WRKY with stress responses is important to increase our understanding of abiotic stress responses in plants. Here, we summarize the structural characteristics and regulatory mechanism of WRKY transcription factors and their responses to abiotic stress. We also discuss current issues and future perspectives of WRKY transcription factor research.

Copper deficiency alters shoot architecture and reduces fertility of both gynoecium and androecium in Arabidopsis thaliana

Copper deficiency reduces plant growth, male fertility, and seed set. The contribution of copper to female fertility and the underlying molecular aspects of copper deficiency-caused phenotypes are not well known. We show that among copper deficiency-caused defects in Arabidopsis thaliana were also the increased shoot branching, delayed flowering and senescence, and entirely abolished gynoecium fertility. The increased shoot branching of copper-deficient plants was rescued by the exogenous application of auxin or copper. The delayed flowering was associated with the decreased expression of the floral activator, FT. Copper deficiency also decreased the expression of senescence-associated genes, WRKY53 and SAG13, but increased the expression of SAG12. The reduced fertility of copper-deficient plants stemmed from multiple factors including the abnormal stigma papillae development, the abolished gynoecium fertility, and the failure of anthers to dehisce. The latter defect was associated with reduced lignification, the upregulation of copper microRNAs and the downregulation of their targets, laccases, implicated in lignin synthesis. Copper-deficient plants accumulated ROS in pollen and had reduced cytochrome c oxidase activity in both leaves and floral buds. This study opens new avenues for the investigation into the relationship between copper homeostasis, hormone-mediated shoot architecture, gynoecium fertility, and copper deficiency-derived nutritional signals leading to the delay in flowering and senescence.

The hawthorn CpLRR-RLK1 gene targeted by ACLSV-derived vsiRNA positively regulate resistance to bacteria disease

Virus-derived small interfering RNAs (vsiRNAs) can target not only viruses but also plant genes. Apple chlorotic leaf spot virus (ACLSV) is an RNA virus that infects Rosaceae plants extensively, including apple, pear and hawthorn. Here, we report an ACLSV-derived vsiRNA [vsiR1360(-)] that targets and down-regulates the leucine-rich repeat receptor-like kinase 1 (LRR-RLK1) gene of hawthorn (Crataegus pinnatifida). The targeting and cleavage of the CpLRR-RLK1 gene by vsiR1360(-) were validated by RNA ligase-mediated 5' rapid amplification of cDNA ends and tobacco transient transformation assays. And the CpLRR-RLK1 protein fused to green fluorescent protein localized to the cell membrane. Conserved domain and phylogenetic tree analyses showed that CpLRR-RLK1 is closely related to the proteins of the LRRII-RLK subfamily. The biological function of CpLRR-RLK1 was explored by heterologous overexpression of CpLRR-RLK1 gene in Arabidopsis. The results of inoculation of Pst DC3000 in Arabidopsis leaves showed that the symptoms of CpLRR-RLK1 overexpression plants infected with Pst DC3000 were significantly reduced compared with the wild type. In addition, the detection of reactive oxygen species and callose deposition and the expression analysis of defense-related genes showed that the CpLRR-RLK1 gene can indeed enhance the resistance of Arabidopsis to bacteria disease.

A WRKY transcription factor, TaWRKY40-D, promotes leaf senescence associated with jasmonic acid and abscisic acid pathways in wheat

Leaf senescence is a complex and precise regulatory process that is correlated with numerous internal and environmental factors. Leaf senescence is tightly related to the redistribution of nutrients, which significantly affects productivity and quality, especially in crops. Evidence shows that the mediation of transcriptional regulation by WRKY transcription factors is vital for the fine-tuning of leaf senescence. However, the underlying mechanisms of the involvement of WRKY in leaf senescence are still unclear in wheat. Using RNA sequencing data, we isolated a novel WRKY transcription factor, TaWRKY40-D, which localizes in the nucleus and is basically induced by the progression of leaf senescence. TaWRKY40-D is a promoter of natural and dark-induced leaf senescence in transgenic Arabidopsis thaliana and wheat. We also demonstrated a positive response of TaWRKY40-D in wheat upon jasmonic acid (JA) and abscisic acid (ABA) treatment. Consistent with this, the detached leaves of TaWRKY40-D VIGS (virus-induced gene silencing) wheat plants showed a stay-green phenotype, while TaWRKY40-D overexpressing Arabidopsis plants showed premature leaf senescence after JA and ABA treatment. Moreover, our results revealed that TaWRKY40-D positively regulates leaf senescence, possibly by altering the biosynthesis and signalling of JA and ABA pathway genes. Together, our results suggest a new regulator of JA- and ABA-related leaf senescence, as well as a new candidate gene that can be used for molecular breeding in wheat.

Suppression of a hexokinase gene, SlHXK1, leads to accelerated leaf senescence and stunted plant growth in tomato

Sugars are the key regulatory molecules that impact diverse biological processes in plants. Hexokinase, the key rate-limiting enzyme in hexose metabolism, takes part in the first step of glycolytic pathway. Acting as a sensor that mediates sugar regulation, hexokinase has been proved to play significant roles in regulating plant growth and development. Here, we isolated a hexokinase gene SlHXK1 from tomato. Its transcript levels were higher in flowers and leaves than in other organs and decreased during leaf and petiole development. SlHXK1-RNAi lines displayed advanced leaf senescence and stunted plant growth. Physiological features including plant height, leaf length, thickness and size, the contents of chlorophyll, starch and MDA, and hexokinase activity were dramatically altered in SlHXK1-RNAi plants. Dark-induced leaf senescence were advanced and the transcripts of senescence-related genes after darkness treatment were markedly increased in SlHXK1-RNAi plants. RNA-seq and qRT-PCR analyses showed that the transcripts of genes related to plant hormones, photosynthesis, chloroplast development, chlorophyll synthesis and metabolism, cellular process, starch and sucrose metabolism, and senescence were significantly altered in SlHXK1-RNAi plants. Taken together, our data demonstrate that SlHXK1 is a significant gene involved in leaf senescence and plant growth and development in tomato through affecting starch turnover.

ATM suppresses leaf senescence triggered by DNA double-strand break through epigenetic control of senescence-associated genes in Arabidopsis

All living organisms are unavoidably exposed to various endogenous and environmental stresses that trigger potentially fatal DNA damage, including double-strand breaks (DSBs). Although a growing body of evidence indicates that DNA damage is one of the prime drivers of aging in animals, little is known regarding the importance of DNA damage and its repair on lifespan control in plants. We found that the level of DSBs increases but DNA repair efficiency decreases as Arabidopsis leaves age. Generation of DSBs by inducible expression of I-PpoI leads to premature senescence phenotypes. We examined the senescence phenotypes in the loss-of-function mutants for 13 key components of the DNA repair pathway and found that deficiency in ATAXIA TELANGIECTASIA MUTATED (ATM), the chief transducer of the DSB signal, results in premature senescence in Arabidopsis. ATM represses DSB-induced expression of senescence-associated genes, including the genes encoding the WRKY and NAC transcription factors, central components of the leaf senescence process, via modulation of histone lysine methylation. Our work highlights the significance of ATM in the control of leaf senescence and has significant implications for the conservation of aging mechanisms in animals and plants.

Expression of the tomato WRKY gene, SlWRKY23, alters root sensitivity to ethylene, auxin and JA and affects aerial architecture in transgenic Arabidopsis

WRKY transcription factors (TFs) are a large plant-specific family of TFs that govern development and biotic/abiotic stress responses in plants. We have identified SlWRKY23 as a gene primarily expressed in roots. SlWRKY23 encodes a protein of 320 amino acids that functions as a transcriptional activator. It is transcriptionally up-regulated by ethylene, BAP and salicylic acid treatment but suppressed by IAA. Expression of SlWRKY23 in transgenic Arabidopsis affects sensitivity of roots to ethylene, JA and auxin with transgenic plants showing hypersensitivity to ethylene, JA and auxin-mediated primary root growth inhibition. This hypersensitivity is correlated with higher expression of ERF1 and ARF5 that mediate responses to these hormones. SlWRKY23 expression also affects aerial growth with transgenic plants showing greater number of leaves but smaller rosettes. Flowering time is reduced in transgenic lines and these plants also show a greater number of inflorescence branches, siliques and seeds. The siliques are longer and compactly packed with seeds but seeds are smaller in size. Root biomass shows a 25% decrease in transgenic SlWRKY23 Arabidopsis plants at harvest compared with controls. The studies show that SlWRKY23 regulates plant growth possibly through modulation of genes controlling hormone responses.

Multi-level analysis of the interactions between REVOLUTA and MORE AXILLARY BRANCHES 2 in controlling plant development reveals parallel, independent and antagonistic functions

Class III homeodomain leucine zipper (HD-ZIPIII) transcription factors play fundamental roles in controlling plant development. The known HD-ZIPIII target genes encode proteins involved in the production and dissipation of the auxin signal, HD-ZIPII transcription factors and components that feedback to regulate HD-ZIPIII expression or protein activity. Here, we have investigated the regulatory hierarchies of the control of MORE AXILLARY BRANCHES2 (MAX2) by the HD-ZIPIII protein REVOLUTA (REV). We found that REV can interact with the promoter of MAX2 In agreement, rev10D gain-of-function mutants had increased levels of MAX2 expression, while rev loss-of-function mutants showed lower levels of MAX2 in some tissues. Like REV, MAX2 plays known roles in the control of plant architecture, photobiology and senescence, which prompted us to initiate a multi-level analysis of growth phenotypes of hd-zipIII, max2 and respective higher order mutants thereof. Our data suggest a complex relationship of synergistic and antagonistic activities between REV and MAX2; these interactions appear to depend on the developmental context and do not all involve the direct regulation of MAX2 by REV.

Histone Deacetylase HDA9 and WRKY53 Transcription Factor Are Mutual Antagonists in Regulation of Plant Stress Response

Epigenetic regulation of gene expression is important for plant adaptation to environmental changes. Previous results showed that Arabidopsis RPD3-like histone deacetylase HDA9 is known to function in repressing plant response to stress in Arabidopsis. However, how HDA9 targets to specific chromatin loci and controls gene expression networks involved in plant response to stress remains largely unclear. Here, we show that HDA9 represses stress tolerance response by interacting with and regulating the DNA binding and transcriptional activity of WRKY53, which functions as a high-hierarchy positive regulator of stress response. We found that WRKY53 is post-translationally modified by lysine acetylation at multiple sites, some of which are removed by HDA9, resulting in inhibition of WRKY53 transcription activity. Conversely, WRKY53 negatively regulates HDA9 histone deacetylase activity. Collectively, our results indicate that HDA9 and WRK53 are reciprocal negative regulators of each other's activities, illustrating how the functional interplay between a chromatin regulator and a transcription factor regulates stress tolerance in plants.

PyWRKY26 and PybHLH3 cotargeted the PyMYB114 promoter to regulate anthocyanin biosynthesis and transport in red-skinned pears

Red pear is favored because of its bright appearance and abundant anthocyanins. Anthocyanin biosynthesis is controlled by transcription factors (TFs) forming regulatory complexes. In red-skinned pears, the WRKY TFs have a significant relationship with anthocyanin biosynthesis, but the molecular mechanism of the WRKY TFs involved in regulating color formation in red-skinned pear is unclear. In this study, the TFs PyWRKY31 and PyWRKY26 were screened as candidate genes for controlling anthocyanin biosynthesis by transcriptome data and bioinformatics analysis. The effect of anthocyanin accumulations after cotransformation of PyWRKY31 or PyWRKY26 with its partners PyMYB10, PyMYB114, and PybHLH3 was verified in tobacco leaves and strawberry receptacles by a transient expression system. RT-qPCR analysis and a dual-luciferase reporter system further confirmed that this cotransformation activated the expression of PyDFR, PyANS, and PyUFGT in anthocyanin biosynthesis and PyGST in anthocyanin transport instead of the PyABC transporter and PyAVP. Furthermore, the cotransformed PyWRKY26 and PybHLH3 could bind to the PyMYB114 promoter, and PyWRKY26 directly activated the transcription of PyMYB114. In addition, the TF PyWRKY26 could interact with PybHLH3, as confirmed by firefly luciferase complementation and yeast two-hybrid (Y2H) assays. These results showed that the interaction of PyWRKY26 and PybHLH3 could cotarget the PyMYB114 promoter, which resulted in anthocyanin accumulation in red-skinned pear. This study further strengthened the understanding of the regulatory mechanism of anthocyanin accumulation and contributed to improving the appearance of red-skinned pears.

Arabidopsis thaliana WRKY25 Transcription Factor Mediates Oxidative Stress Tolerance and Regulates Senescence in a Redox-Dependent Manner

Senescence is the last developmental step in plant life and is accompanied by a massive change in gene expression implying a strong participation of transcriptional regulators. In the past decade, the WRKY53 transcription factor was disclosed to be a central node of a complex regulatory network of leaf senescence and to underlie a tight multi-layer control of expression, activity and protein stability. Here, we identify WRKY25 as a redox-sensitive up-stream regulatory factor of WRKY53 expression. Under non-oxidizing conditions, WRKY25 binds to a specific W-box in the WRKY53 promoter and acts as a positive regulator of WRKY53 expression in a transient expression system using Arabidopsis protoplasts, whereas oxidizing conditions dampened the action of WRKY25. However, overexpression of WRKY25 did not accelerate senescence but increased lifespan of Arabidopsis plants, whereas the knock-out of the gene resulted in the opposite phenotype, indicating a more complex regulatory function of WRKY25 within the WRKY subnetwork of senescence regulation. In addition, overexpression of WRKY25 mediated higher tolerance to oxidative stress and the intracellular H2O2 level is lower in WRKY25 overexpressing plants and higher in wrky25 mutants compared to wildtype plants suggesting that WRKY25 is also involved in controlling intracellular redox conditions. Consistently, WRKY25 overexpressers had higher and wrky mutants lower H2O2 scavenging capacity. Like already shown for WRKY53, MEKK1 positively influenced the activation potential of WRKY25 on the WRKY53 promoter. Taken together, WRKY53, WRKY25, MEKK1 and H2O2 interplay with each other in a complex network. As H2O2 signaling molecule participates in many stress responses, WRKK25 acts most likely as integrators of environmental signals into senescence regulation.

Arabidopsis WRKY53, a Node of Multi-Layer Regulation in the Network of Senescence

Leaf senescence is an integral part of plant development aiming at the remobilization of nutrients and minerals out of the senescing tissue into developing parts of the plant. Sequential as well as monocarpic senescence maximize the usage of nitrogen, mineral, and carbon resources for plant growth and the sake of the next generation. However, stress-induced premature senescence functions as an exit strategy to guarantee offspring under long-lasting unfavorable conditions. In order to coordinate this complex developmental program with all kinds of environmental input signals, complex regulatory cues have to be in place. Major changes in the transcriptome imply important roles for transcription factors. Among all transcription factor families in plants, the NAC and WRKY factors appear to play central roles in senescence regulation. In this review, we summarize the current knowledge on the role of WRKY factors with a special focus on WRKY53. In contrast to a holistic multi-omics view we want to exemplify the complexity of the network structure by summarizing the multilayer regulation of WRKY53 of Arabidopsis.

H2O2 as a Feedback Signal on Dual-Located WHIRLY1 Associates with Leaf Senescence in Arabidopsis

Leaf senescence, either as a natural stage of development or as an induced process under stress conditions, incorporates multiple intricate signaling pathways. At the cellular level, retrograde signals have been considered as important players during the initiation and progression of senescence in both animals and plants. The plant-specific single-strand DNA-binding protein WHIRLY1 (WHY1), a repressor of leaf natural senescence, is dually located in both nucleus and plastids. Despite many years of studies, the myth about its dual location and the underlying functional implications remain elusive. Here, we provide further evidence in Arabidopsis showing that alteration in WHY1 allocation between the nucleus and chloroplast causes perturbation in H₂O₂ homeostasis, resulting in adverse plant senescence phenotypes. The knockout of WHY1 increased H₂O₂ content at 37 days post-germination, coincident with an early leaf senescence phenotype, which can be rescued by ectopic expression of the nuclear isoform (nWHY1), but not by the plastid isoform (pWHY1). Instead, accumulated pWHY1 greatly provoked H₂O₂ in cells. On the other hand, exogenous H₂O₂ treatment induced a substantial plastid accumulation of WHY1 proteins and at the same time reduced the nuclear isoforms. This H₂O₂-induced loss of nucleus WHY1 isoform was accompanied by enhanced enrichments of histone H3 lysine 9 acetylation (H3K9ac) and recruitment of RNA polymerase II (RNAP II) globally, and specifically at the promoter of the senescence-related transcription factor WRKY53, which in turn activated WRKY53 transcription and led to a senescence phenotype. Thus, the distribution of WHY1 organelle isoforms and the feedback of H₂O₂ intervene in a circularly integrated regulatory network during plant senescence in Arabidopsis.

Transcription Factor Networks in Leaves of Cichorium endivia: New Insights into the Relationship Between Photosynthesis and Leaf Development

Cichorium endivia is a leafy crop closely related to Lactuca sativa that comprises two major botanical varieties characterized by a high degree of intraspecific morphological variation: var. latifolium with broad leaves (escarole) and var. crispum with narrow crisp curly leaves (endive). To investigate the relationship between leaf morphology and photosynthetic activity, escaroles and endives were used as a crop model due to the striking morphological diversity of their leaves. We constructed a leaf database for transcription factors (TFs) and photosynthesis-related genes from a refined C. endivia transcriptome and used RNA-seq transcriptomic data from leaves of four commercial endive and escarole cultivars to explore transcription factor regulatory networks. Cluster and gene co-expression network (GCN) analyses identified two main anticorrelated modules that control photosynthesis. Analysis of the GCN network topological properties identified known and novel hub genes controlling photosynthesis, and candidate developmental genes at the boundaries between shape and function. Differential expression analysis between broad and curly leaves suggested three novel TFs putatively involved in leaf shape diversity. Physiological analysis of the photosynthesis properties and gene expression studies on broad and curly leaves provided new insights into the relationship between leaf shape and function.

Genome-Wide Characterization, Expression Profile Analysis of WRKY Family Genes in Santalum album and Functional Identification of Their Role in Abiotic Stress

WRKY proteins are a large superfamily of transcription factors that are involved in diverse biological processes including development, as well as biotic and abiotic stress responses in plants. WRKY family proteins have been extensively characterized and analyzed in many plant species, including Arabidopsis, rice, and poplar. However, knowledge on WRKY transcription factors in Santalum album is scarce. Based on S. album genome and transcriptome data, 64 SaWRKY genes were identified in this study. A phylogenetic analysis based on the structures of WRKY protein sequences divided these genes into three major groups (I, II, III) together with WRKY protein sequences from Arabidopsis. Tissue-specific expression patterns showed that 37 SaWRKY genes were expressed in at least one of five tissues (leaves, roots, heartwood, sapwood, or the transition zone), while the remaining four genes weakly expressed in all of these tissues. Analysis of the expression profiles of the 42 SaWRKY genes after callus was initiated by salicylic acid (SA) and methyl jasmonate (MeJA) revealed that 25 and 24 SaWRKY genes, respectively, were significantly induced. The function of SaWRKY1, which was significantly up-regulated by SA and MeJA, was analyzed. SaWRKY1 was localized in the nucleus and its overexpression improved salt tolerance in transgenic Arabidopsis. Our study provides important information to further identify the functions of SaWRKY genes and to understand the roles of SaWRKY family genes involved in the development and in SA- and MeJA-mediated stress responses.

Exploring gene networks in two sunflower lines with contrasting leaf senescence phenotype using a system biology approach

BACKGROUND

Leaf senescence is a complex process, controlled by multiple genetic and environmental variables. In sunflower, leaf senescence is triggered abruptly following anthesis thereby limiting the capacity of plants to keep their green leaf area during grain filling, which subsequently has a strong impact on crop yield. Recently, we performed a selection of contrasting sunflower inbred lines for the progress of leaf senescence through a physiological, cytological and molecular approach. Here we present a large scale transcriptomic analysis using RNA-seq and its integration with metabolic profiles for two contrasting sunflower inbred lines, R453 and B481-6 (early and delayed senescence respectively), with the aim of identifying metabolic pathways associated to leaf senescence.

RESULTS

Gene expression profiles revealed a higher number of differentially expressed genes, as well as, higher expression levels in R453, providing evidence for early activation of the senescence program in this line. Metabolic pathways associated with sugars and nutrient recycling were differentially regulated between the lines. Additionally, we identified transcription factors acting as hubs in the co-expression networks; some previously reported as senescence-associated genes in model species but many are novel candidate genes.

CONCLUSIONS

Understanding the onset and the progress of the senescence process in crops and the identification of these new candidate genes will likely prove highly useful for different management strategies to mitigate the impact of senescence on crop yield. Functional characterization of candidate genes will help to develop molecular tools for biotechnological applications in breeding crop yield.

Comparative Genomic Analysis Reveals Extensive Genetic Variations of WRKYs in Solanaceae and Functional Variations of CaWRKYs in Pepper

As a conserved protein family, WRKY has been shown to be involved in multiple biological processes in plants. However, the mechanism of functional diversity for WRKYs in pepper has not been well elucidated. Here, a total of 223 WRKY members from solanaceae crops including pepper, tomato and potato, were analyzed using comparative genomics. A tremendous genetic variation among WRKY members of different solanaceous plants or groups was demonstrated by the comparison of some WRKY features, including number/size, group constitution, gene structure, and domain composition. The phylogenetic analysis showed that except for the known WRKY groups (I, IIa/b/c/d/e and III), two extra WRKY subgroups specifically existed in solanaceous plants, which were named group IIf and group IIg in this study, and their genetic variations were also revealed by the characteristics of some group IIf and IIg WRKYs. Except for the extensive genetic variations, certain degrees of conservatism for solanaceae WRKYs were also revealed. Moreover, the variant zinc-finger structure (CX4,7CX22-24HXC) in group III of solanaceae WRKYs was identified. Expression profiles of CaWRKY genes suggested their potential roles in pepper development and stress responses, and demonstrated a functional division pattern for pepper CaWRKYs. Furthermore, functional analysis using virus induced gene silencing (VIGS) revealed critical roles of two CaWRKYs (CaWRKY45 and CaWRKY58) in plant responses to disease and drought, respectively. This study provides a solid foundation for further dissection of the evolutionary and functional diversity of solanaceae WRKYs in crop plants.

The WRKY transcription factor GhWRKY27 coordinates the senescence regulatory pathway in upland cotton (Gossypium hirsutum L.)

BACKGROUND

Premature senescence can reduce the yield and quality of crops. WRKY transcription factors (TFs) play important roles during leaf senescence, but little is known about their ageing mechanisms in cotton.

RESULTS

In this study, a group III WRKY TF, GhWRKY27, was isolated and characterized. The expression of GhWRKY27 was induced by leaf senescence and was higher in an early-ageing cotton variety than in a non-early-ageing cotton variety. Overexpression of GhWRKY27 in Arabidopsis promoted leaf senescence, as determined by reduced chlorophyll content and elevated expression of senescence-associated genes (SAGs). Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays showed that GhWRKY27 interacted with an MYB TF, GhTT2. Putative target genes of GhWRKY27 were identified via chromatin immunoprecipitation followed by sequencing (ChIP-seq). Yeast one-hybrid (Y1H) assay and electrophoretic mobility shift assay (EMSA) revealed that GhWRKY27 binds directly to the promoters of cytochrome P450 94C1 (GhCYP94C1) and ripening-related protein 2 (GhRipen2-2). In addition, the expression patterns of GhTT2, GhCYP94C1 and GhRipen2-2 were identified during leaf senescence. Transient dual-luciferase reporter assay indicated that GhWRKY27 could activate the expression of GhCYP94C1 and GhRipen2-2.

CONCLUSIONS

Our work lays the foundation for further study of the functional roles of WRKY genes during leaf senescence in cotton. In addition, our data provide new insights into the senescence-associated mechanisms of WRKY genes in cotton.

Complementation studies of the Arabidopsis fc1 mutant substantiate essential functions of ferrochelatase 1 during embryogenesis and salt stress

Ferrochelatase (FC) is the final enzyme for haem formation in the tetrapyrrole biosynthesis pathway and encoded by two genes in higher plants. FC2 exists predominantly in green tissue, whereas FC1 is constitutively expressed. We intended to substantiate the specific roles of FC1. The embryo-lethal fc1-2 mutant was used to express the two genomic FC-encoding sequences under the FC1 and FC2 promoter and explore the complementation of the FC1 deficiency. Apart from the successful complementation with FC1, expression of FC2 under control of the FC1 promoter (pFC1::FC2) compensates for missing FC1 but not by FC2 promoter expression. The complementing lines pFC1FC2(fc1/fc1) succeeded under standard growth condition but failed under salt stress. The pFC1FC2(fc1/fc1) line exhibited symptoms of leaf senescence, including accelerated loss of haem and chlorophyll and elevated gene expression for chlorophyll catabolism. In contrast, ectopic FC1 expression (p35S::FC1) resulted in increased chlorophyll accumulation. The limited ability of FC2 to complement fc1 is explained by a faster turnover of FC2 mRNA during stress. It is suggested that FC1-produced haem is essential for embryogenesis and stress response. The pFC1::FC2 expression readily complements the fc1-2 embryo lethality, whereas higher FC1 transcript content contributes essentially to stress tolerance.