AtVQ25 promotes salicylic acid-related leaf senescence by fine-tuning the self-repression of AtWRKY53.

Most mechanistic details of chronologically ordered regulation of leaf senescence are unknown. Regulatory networks centered on AtWRKY53 are crucial for orchestrating and integrating various senescence-related signals. Notably, AtWRKY53 binds to its own promoter and represses transcription of AtWRKY53, but the biological significance and mechanism underlying this self-repression remain unclear. In this study, we identified the VQ motif-containing protein AtVQ25 as a cooperator of AtWRKY53. The expression level of AtVQ25 peaked at mature stage and was specifically repressed after the onset of leaf senescence. AtVQ25-overexpressing plants and atvq25 mutants displayed precocious and delayed leaf senescence, respectively. Importantly, we identified AtWRKY53 as an interacting partner of AtVQ25. We determined that interaction between AtVQ25 and AtWRKY53 prevented AtWRKY53 from binding to W-box elements on the AtWRKY53 promoter and thus counteracted the self-repression of AtWRKY53. In addition, our RNA-sequencing data revealed that the AtVQ25-AtWRKY53 module is related to the salicylic acid (SA) pathway. Precocious leaf senescence and SA-induced leaf senescence in AtVQ25-overexpressing lines were inhibited by an SA pathway mutant, atsid2, and NahG transgenic plants; AtVQ25-overexpressing/atwrky53 plants were also insensitive to SA-induced leaf senescence. Collectively, we demonstrated that AtVQ25 directly attenuates the self-repression of AtWRKY53 during the onset of leaf senescence, which is substantially helpful for understanding the timing of leaf senescence onset modulated by AtWRKY53.

Toxicological effects and mechanisms of lithium on growth, photosynthesis and antioxidant system in the freshwater microalga Chromochloris zofingiensis.

The growing prevalence of lithium (Li) batteries has drawn public attention to Li as an emerging pollutant. The present study investigates the toxicity of Li+ on Chromochloris zofingiensis, examining physiological, biochemical and omics aspects. Results reveal hormesis effects of Li+ on C. zofingiensis growth. At Li+ concentrations below 5 mg L-1, Li+ can enhance chlorophyll content, mitochondrial activity, and antioxidant capacity, leading to increased dry cell weight and cell number. Conversely, when it exceeded 10 mg L-1, Li+ can reduce chlorophyll content, induce oxidative stress, and disrupt chloroplast and mitochondria structure and function, ultimately impeding cell growth. In addition, under 50 mg L-1 Li+ stress, microalgae optimize absorbed light energy use (increasing Fv/Fm and E TR ) and respond to stress by up-regulating genes in starch and lipid biosynthesis pathways, promoting the accumulation of storage components. Weighted gene co-expression network analysis indicates that peptidylprolyl cis/trans isomerase, GTPase and L-ascorbate oxidase might be the key regulators in response to Li+ stress. This research marks the toxic effects and molecular mechanisms of Li+ on freshwater microalga, which would improve our understanding of Li's toxicology and contributing to the establishment of Li pollution standards.

Rice OsANN9 Enhances Drought Tolerance through Modulating ROS Scavenging Systems.

Drought is a critical abiotic stress which leads to crop yield and a decrease in quality. Annexins belong to a multi-gene family of calcium- and lipid-binding proteins and play diverse roles in plant growth and development. Herein, we report a rice annexin protein, OsANN9, which in addition to regular annexin repeats and type-II Ca2+ binding sites, also consists of a C2H2-type zinc-finger domain. We found that the expression of OsANN9 was upregulated by polyethylene glycol (PEG) or water-deficient treatment. Moreover, plants that overexpressed OsANN9 had increased survival rates under drought stress, while both OsANN9-RNAi and osann9 mutants showed sensitivity to drought. In addition, the overexpression of OsANN9 increased superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities, which regulate reactive oxygen species homeostasis. Collectively, these findings indicate that OsANN9 may function as a positive regulator in response to drought stress by modulating antioxidant accumulation. Interestingly, the setting rates of osann9 mutant rice plants significantly decreased in comparison to wild-type plants, suggesting that OsANN9 might be involved in other molecular mechanisms in the rice seed development stage.

Salicylic acid inhibits rice endocytic protein trafficking mediated by OsPIN3t and clathrin to affect root growth.

Salicylic acid (SA) plays important roles in different aspects of plant development, including root growth, where auxin is also a major player by means of its asymmetric distribution. However, the mechanism underlying the effect of SA on the development of rice roots remains poorly understood. Here, we show that SA inhibits rice root growth by interfering with auxin transport associated with the OsPIN3t- and clathrin-mediated gene regulatory network (GRN). SA inhibits root growth as well as Brefeldin A-sensitive trafficking through a non-canonical SA signaling mechanism. Transcriptome analysis of rice seedlings treated with SA revealed that the OsPIN3t auxin transporter is at the center of a GRN involving the coat protein clathrin. The root growth and endocytic trafficking in both the pin3t and clathrin heavy chain mutants were SA insensitivity. SA inhibitory effect on the endocytosis of OsPIN3t was dependent on clathrin; however, the root growth and endocytic trafficking mediated by tyrphostin A23 (TyrA23) were independent of the pin3t mutant under SA treatment. These data reveal that SA affects rice root growth through the convergence of transcriptional and non-SA signaling mechanisms involving OsPIN3t-mediated auxin transport and clathrin-mediated trafficking as key components.

Grapevine bZIP transcription factor bZIP45 regulates VvANN1 and confers drought tolerance in Arabidopsis.

Drought is a severe environmental condition that restricts the vegetative growth and reduces the yield of grapevine (Vitis vinifera L.). However, the mechanisms underlying grapevine response and adaptation to drought stress remain unclear. In the present study, we characterized an ANNEXIN gene, VvANN1, which plays a positive role in the drought stress response. The results indicated that VvANN1 was significantly induced by osmotic stress. Expression of VvANN1 in Arabidopsis thaliana enhanced osmotic and drought tolerance through modulating the level of MDA, H2O2, and O2 ·- at the seedling stage, implying that VvANN1 might be involved in the process of ROS homeostasis under drought or osmotic stress conditions. Moreover, we used yeast one-hybridization and chromatin immunoprecipitation assays to show that VvbZIP45 could regulate VvANN1 expression by directly binding to the promoter region of VvANN1 in response to drought stress. We also generated transgenic Arabidopsis that constitutively expressed the VvbZIP45 gene (35S::VvbZIP45) and further produced VvANN1Pro::GUS/35S::VvbZIP45 Arabidopsis plants via crossing. The genetic analysis results subsequently indicated that VvbZIP45 could enhance GUS expression in vivo under drought stress. Our findings suggest that VvbZIP45 may modulate VvANN1 expression in response to drought stress and reduce the impact of drought on fruit quality and yield.

H3K36 demethylase JMJ710 negatively regulates drought tolerance by suppressing MYB48-1 expression in rice.

The homeostasis of histone methylation is maintained by histone methyltransferases and demethylases, which are important for the regulation of gene expression. Here, we report a histone demethylase from rice (Oryza sativa), Jumonji C domain-containing protein (JMJ710), which belongs to the JMJD6 group and plays an important role in the response to drought stress. Overexpression of JMJ710 causes a drought-sensitive phenotype, while RNAi and clustered regularly interspaced short palindromic repeats (CRISPR)-knockout mutant lines show drought tolerance. In vitro and in vivo assays showed that JMJ710 is a histone demethylase. It targets to MYB TRANSCRIPTION FACTOR 48 (MYB48-1) chromatin, demethylates H3K36me2, and negatively regulates the expression of MYB48-1, a positive regulator of drought tolerance. Under drought stress, JMJ710 is downregulated and the expression of MYB48-1 increases, and the subsequent activation of its downstream drought-responsive genes leads to drought tolerance. This research reports a negative regulator of drought stress-responsive genes, JMJ710, that ensures that the drought tolerance mechanism is not mis-activated under normal conditions but allows quick activation upon drought stress.

The bHLH transcription factor regulated gene OsWIH2 is a positive regulator of drought tolerance in rice.

Drought is a major abiotic stress limiting crop growth and yield. In this study, we characterized a novel drought tolerance induced WIH gene in rice, OsWIH2. Overexpression of OsWIH2 in rice resulted in significantly higher drought tolerance, probably due to the decreased water loss rate and reactive oxygen species (ROS) accumulation under drought stress. We identified a long-chain fatty acid HOTHEAD (HTH) that interacted with OsWIH2 using yeast two-hybrid screening. OsWIH2 is an enzyme which is involved in fatty acid synthesis. We further demonstrated that the drought-inducible bHLH transcription factor OsbHLH130 could activate the expression of OsWIH2. Overall, our results suggest that drought stress may induce OsbHLH130 accumulation, which in turn activates OsWIH2 expression, and the latter improves rice drought tolerance by participating in cuticular wax biosynthesis and reducing the water loss rate as well as ROS accumulation. This research provides new genes for crop improvement.

OsANN4 modulates ROS production and mediates Ca2+ influx in response to ABA.

BACKGROUND: Plant annexins are calcium- and lipid-binding proteins that have multiple functions, and a significant amount of research on plant annexins has been reported in recent years. However, the functions of annexins in diverse biological processes in rice are largely unclear. RESULTS: Herein, we report that OsANN4, a calcium-binding rice annexin protein, was induced by abscisic acid (ABA). Under ABA treatment, the plants in which OsANN4 was knocked down by RNA interference showed some visible phenotypic changes compared to the wild type, such as a lower rooting rate and shorter shoot and root lengths. Moreover, the superoxide dismutase (SOD) and catalase (CAT) activities of the RNAi lines were significantly lower and further resulted in higher accumulation of O2.- and H2O2 than those of the wild-type. A Non-invasive Micro-test Technology (NMT) assay showed that ABA-induced net Ca2+ influx was inhibited in OsANN4 knockdown plants. Interestingly, the phenotypic differences caused by ABA were eliminated in the presence of LaCl3 (Ca2+ channel inhibitor). Apart from this, we demonstrated that OsCDPK24 interacted with and phosphorylated OsANN4. When the phosphorylated serine residue of OsANN4 was substituted by alanine, the interaction between OsANN4 and OsCDPK24 was still observed, however, both the conformation of OsANN4 and its binding activity with Ca2+ might be changed. CONCLUSIONS: OsANN4 plays a crucial role in the ABA response, partially by modulating ROS production, mediating Ca2+ influx or interacting with OsCDPK24.

High temperature inhibits the accumulation of storage materials by inducing alternative splicing of OsbZIP58 during filling stage in rice.

High temperature (HT) has an adverse effect on rice grain filling by inhibiting the accumulation of storage materials. However, the regulatory mechanism of this inhibition remains unknown. Here, we report that Opaque2 like transcription factor OsbZIP58 is a key factor regulating storage material accumulation under HT. The OsbZIP58 gene promotes expression of many seed storage protein genes and starch synthesis genes while inhibits expression of some starch hydrolyzing α-amylase genes under HT. The loss of OsbZIP58 function leads to floury and shrunken endosperms and dramatically reduced storage materials in the seeds under HT. HT is found to affect alternative splicing of OsbZIP58, promoting the formation of the truncated OsbZIP58ß protein form over the full-length OsbZIP58α protein form. The OsbZIP58ß form has a lower transcriptional activity than the OsbZIP58α form, especially under HT condition. Interestingly, rice varieties with less heat sensitivity have reduced alternative splicing of OsbZIP58. Therefore, OsbZIP58 is a crucial gene in regulating storage material accumulation under HT and lower alternative splicing of OsbZIP58 may contribute to heat tolerance during grain filling.

A calcium-dependent lipid binding protein, OsANN10, is a negative regulator of osmotic stress tolerance in rice.

Annexin, a multi-gene family in plants, is essential for plant growth and stress responses. Recent studies demonstrated a positive effect of annexin in abiotic stress responses. Interestingly, we found OsANN10, a putative annexin gene in rice, negatively regulated plant responses to osmotic stress. Knocking down OsANN10 significantly decreased the content of H2O2 by increasing Peroxidase (POD) and Catalase (CAT) activities, further reducing oxidative damage in rice leaves, suggesting a negative regulation of OsANN10 in protecting cell membrane against oxidative damage via scavenging ROS under osmotic stress.

OsANN3, a calcium-dependent lipid binding annexin is a positive regulator of ABA-dependent stress tolerance in rice.

Annexin is a multigene family that plays critical roles in plant stress responses and various cellular processes. Here, we reported the cloning and functional characterization of a novel rice annexin protein, OsANN3. We found that expression of OsANN3 was induced by polyethylene glycol (PEG) and abscisic acid (ABA) treatments. Overexpression of OsANN3 in rice significantly increased survival rates under drought stress, while knocking down OsANN3 resulted in sensitivity to drought. Meanwhile, OsANN3 overexpression showed enhanced sensitivity to exogenous ABA. Together with its Ca2+ and phospholipid binding activity, we proposed that when plants were subjected to drought stress, OsANN3 might mediate Ca2+ influx by binding to phospholipid to activate ABA signaling pathways. In addition, overexpression OsANN3 showed better growth under drought stress comparing to wild type, such as longer root length and more stomata closure for reducing water loss by regulating ABA-dependent stress response pathways.

OsJMJ703, a rice histone demethylase gene, plays key roles in plant development and responds to drought stress.

JmjC-domain-containing (JmjC) protein, an important kind of histone demethylase in plants, plays key roles in multiple growth and development processes and in adversity resistance. In this study, we found that OsJMJ703, a known histone demethylase, is expressed in various tissues. Furthermore, over-expression of OsJMJ703 influenced the type of rice panicle, and knock-down of the expression of OsJMJ703 showed an earlier flowering time in rice. In addition, OsJMJ703 is involved in abiotic stress. Transgenic rice of over-expressing OsJMJ703 is sensitive to drought stress, whereas knocking down OsJMJ703 enhances the tolerance to drought stress. This study provides a theoretical basis of the biological function of JmjC protein and further promotes the study of drought resistance.

STERILE APETALA modulates the stability of a repressor protein complex to control organ size in Arabidopsis thaliana.

Organ size control is of particular importance for developmental biology and agriculture, but the mechanisms underlying organ size regulation remain elusive in plants. Meristemoids, which possess stem cell-like properties, have been recognized to play important roles in leaf growth. We have recently reported that the Arabidopsis F-box protein STERILE APETALA (SAP)/SUPPRESSOR OF DA1 (SOD3) promotes meristemoid proliferation and regulates organ size by influencing the stability of the transcriptional regulators PEAPODs (PPDs). Here we demonstrate that KIX8 and KIX9, which function as adaptors for the corepressor TOPLESS and PPD, are novel substrates of SAP. SAP interacts with KIX8/9 and modulates their protein stability. Further results show that SAP acts in a common pathway with KIX8/9 and PPD to control organ growth by regulating meristemoid cell proliferation. Thus, these findings reveal a molecular mechanism by which SAP targets the KIX-PPD repressor complex for degradation to regulate meristemoid cell proliferation and organ size.

FRUCTOKINASE-LIKE PROTEIN 1 interacts with TRXz to regulate chloroplast development in rice.

Chloroplast genes are transcribed by the plastid-encoded RNA polymerase (PEP) or nucleus-encoded RNA polymerase. FRUCTOKINASE-LIKE PROTEINS (FLNs) are phosphofructokinase-B (PfkB)-type carbohydrate kinases that act as part of the PEP complex; however, the molecular mechanisms underlying FLN activity in rice remain elusive. Previously, we identified and characterized a heat-stress sensitive albino (hsa1) mutant in rice. Map-based cloning revealed that HSA1 encodes a putative OsFLN2. Here, we further demonstrated that knockdown or knockout of the OsFLN1, a close homolog of HSA1/OsFLN2, considerably inhibits chloroplast biogenesis and the fln1 knockout mutants, created by clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associate protein 9, exhibit severe albino phenotype and seedling lethality. Moreover, OsFLN1 localizes to the chloroplast. Yeast two-hybrid, pull-down and bimolecular fluorescence complementation experiments revealed that OsFLN1 and HSA1/OsFLN2 interact with THIOREDOXINZ (OsTRXz) to regulate chloroplast development. In agreement with this, knockout of OsTRXz resulted in a similar albino and seedling lethality phenotype to that of the fln1 mutants. Quantitative reverse transcription polymerase chain reaction and immunoblot analysis revealed that the transcription and translation of PEP-dependent genes were strongly inhibited in fln1 and trxz mutants, indicating that loss of OsFLN1, HSA1/OsFLN2, or OsTRXz function perturbs the stability of the transcriptionally active chromosome complex and PEP activity. These results show that OsFLN1 and HSA1/OsFLN2 contribute to chloroplast biogenesis and plant growth.

A calcium-binding protein, rice annexin OsANN1, enhances heat stress tolerance by modulating the production of H2O2.

OsANN1 is a member of the annexin protein family in rice. The function of this protein and the mechanisms of its involvement in stress responses and stress tolerance are largely unknown. Here it is reported that OsANN1 confers abiotic stress tolerance by modulating antioxidant accumulation under abiotic stress. OsANN1-knockdown [RNA interference (RNAi)] plants were more sensitive to heat and drought stresses, whereas OsANN1-overexpression (OE) lines showed improved growth with higher expression of OsANN1 under abiotic stress. Overexpression of OsANN1 promoted SOD (superoxide dismutase) and CAT (catalase) activities, which regulate H2O2 content and redox homeostasis, suggesting the existence of a feedback mechanism between OsANN1 and H2O2 production under abiotic stress. Higher expression of OsANN1 can provide overall cellular protection against abiotic stress-induced damage, and a significant accumulation of OsANN1-green fluorescent protein (GFP) signals was found in the cytosol after heat shock treatment. OsANN1 also has calcium-binding and ATPase activities in vitro, indicating that OsANN1 has multiple functions in rice growth. Furthermore, yeast two-hybrid and bimolecular fluorescence complementation (BiFC) assays demonstrated that OsANN1 interacts with OsCDPK24. This cross-talk may provide additional layers of regulation in the abiotic stress response.

The ubiquitin receptors DA1, DAR1, and DAR2 redundantly regulate endoreduplication by modulating the stability of TCP14/15 in Arabidopsis.

Organ growth involves the coordination of cell proliferation and cell growth with differentiation. Endoreduplication is correlated with the onset of cell differentiation and with cell and organ size, but little is known about the molecular mechanisms linking cell and organ growth with endoreduplication. We have previously demonstrated that the ubiquitin receptor DA1 influences organ growth by restricting cell proliferation. Here, we show that DA1 and its close family members DAR1 and DAR2 are redundantly required for endoreduplication during leaf development. DA1, DAR1, and DAR2 physically interact with the transcription factors TCP14 and TCP15, which repress endoreduplication by directly regulating the expression of cell-cycle genes. We also show that DA1, DAR1, and DAR2 modulate the stability of TCP14 and TCP15 proteins in Arabidopsis thaliana. Genetic analyses demonstrate that DA1, DAR1, and DAR2 function in a common pathway with TCP14/15 to regulate endoreduplication. Thus, our findings define an important genetic and molecular mechanism involving the ubiquitin receptors DA1, DAR1, and DAR2 and the transcription factors TCP14 and TCP15 that links endoreduplication with cell and organ growth.

SKIP Confers Osmotic Tolerance during Salt Stress by Controlling Alternative Gene Splicing in Arabidopsis.

Deciphering the mechanisms underlying plant responses to abiotic stress is key for improving plant stress resistance. Much is known about the regulation of gene expression in response to salt stress at the transcriptional level; however, little is known about this process at the posttranscriptional level. Recently, we demonstrated that SKIP is a component of spliceosome that interacts with clock gene pre-mRNAs and is essential for regulating their alternative splicing and mRNA maturation. In this study, we found that skip-1 plants are hypersensitive to both salt and osmotic stresses, and that SKIP is required for the alternative splicing and mRNA maturation of several salt-tolerance genes, including NHX1, CBL1, P5CS1, RCI2A, and PAT10. A genome-wide analysis revealed that SKIP mediates the alternative splicing of many genes under salt-stress conditions, and that most of the alternative splicing events in skip-1 involve intron retention and can generate a premature termination codon in the transcribed mRNA. SKIP also controls alternative splicing by modulating the recognition or cleavage of 5' and 3' splice donor and acceptor sites under salt-stress conditions. Therefore, this study addresses the fundamental question of how the mRNA splicing machinery in plants contributes to salt-stress responses at the posttranscriptional level, and provides a link between alternative splicing and salt tolerance.

The putative auxin efflux carrier OsPIN3t is involved in the drought stress response and drought tolerance.

The phytohormone auxin plays a critical role in plant growth and development, and its spatial distribution largely depends on the polar localization of the PIN-FORMED (PIN) auxin efflux carrier family members. In this study, we identify a putative auxin efflux carrier gene in rice, OsPIN3t, which acts in auxin polar transport but is also involved in the drought stress response in rice. We show that OsPIN3t-GFP fusion proteins are localized in plasma membranes, and this subcellular localization changes under 1-N-naphthylphthalamic acid (NPA) treatment. The tissue-specific expression patterns of OsPIN3t were also investigated using a ß-glucuronidase (GUS) reporter, which showed that OsPIN3t was mainly expressed in vascular tissue. The GUS activity in OsPIN3tpro::GUS plants increased by NAA treatment and decreased by NPA treatment. Moreover, knockdown of OsPIN3t caused crown root abnormalities in the seedling stage that could be phenocopied by treatment of wild-type plants with NPA, which indicated that OsPIN3t is involved in the control of polar auxin transport. Overexpression of OsPIN3t led to improved drought tolerance, and GUS activity significantly increased when OsPIN3tpro::GUS plants were subjected to 20% polyethylene glycol stress. Taken together, these results suggest that OsPIN3t is involved in auxin transport and the drought stress response, which suggests that a polar auxin transport pathway is involved in the regulation of the response to water stress in plants.

The plant-specific G protein γ subunit AGG3 influences organ size and shape in Arabidopsis thaliana.

• Control of organ size and shape by cell proliferation and cell expansion is a fundamental developmental process, but the mechanisms that set the size and shape of determinate organs are largely unknown in plants. • Molecular, genetic, cytological and biochemical approaches were used to characterize the roles of the Arabidopsis thaliana G protein γ subunit (AGG3) gene in organ growth. • Here, we describe A. thaliana AGG3, which promotes petal growth by increasing the period of cell proliferation. Both the N-terminal region and the C-terminal domains of AGG3 are necessary for the function of AGG3. By contrast, analysis of a series of AGG3 derivatives with deletions in specific domains showed that the deletion of any of these domains cannot completely abolish the function of AGG3. The GFP-AGG3 fusion protein is localized to the plasma membrane. The predicted transmembrane domain plays an important role in the plasma membrane localization of AGG3. Genetic analyses revealed that AGG3 action requires a functional G protein α subunit (GPA1) and G protein ß subunit (AGB1). • Our findings demonstrate that AGG3, GPA1 and AGB1 act in the same genetic pathway to influence organ size and shape in A. thaliana.

Crinkly4 receptor-like kinase is required to maintain the interlocking of the palea and lemma, and fertility in rice, by promoting epidermal cell differentiation.

The palea and lemma are unique organs in grass plants that form a protective barrier around the floral organs and developing kernel. The interlocking of the palea and lemma is critical for maintaining fertility and seed yield in rice; however, the molecules that control the interlocking structure remain largely unknown. Here, we showed that when OsCR4 mRNA expression was knocked down in rice by RNA interference, the palea and lemma separated at later spikelet stages and gradually turned brown after heading, resulting in the severe interruption of pistil pollination and damage to the development of embryo and endosperm, with defects in aleurone. The irregular architecture of the palea and lemma was caused by tumour-like cell growth in the outer epidermis and wart-like cell masses in the inner epidermis. These abnormal cells showed discontinuous cuticles and uneven cell walls, leading to organ self-fusion that distorted the interlocking structures. Additionally, the faster leakage of chlorophyll, reduced silica content and elevated accumulation of anthocyanin in the palea and lemma indicated a lesion in the protective barrier, which also impaired seed quality. OsCR4 is an active receptor-like kinase associated with the membrane fraction. An analysis of promoter::GUS reporter plants showed that OsCR4 is specifically expressed in the epidermal cells of paleas and lemmas. Together, these results suggest that OsCR4 plays an essential role in maintaining the interlocking of the palea and lemma by promoting epidermal cell differentiation.