OsHK3 is a crucial regulator of abscisic acid signaling involved in antioxidant defense in rice

In this study, the role of the rice(Oryza sativa L.)histidine kinase Os HK3 in abscisic acid(ABA)-induced antioxidant defense was investigated. Treatments with ABA, H2O2,and polyethylene glycol(PEG) induced the expression of Os HK3 in rice leaves, and H2O2 is required for ABA-induced increase in the expression of Os HK3 under water stress. Subcellular localization analysis showed that Os HK3 is located in the cytoplasm and the plasma membrane. The transient expression analysis and the transient RNA interference test in rice protoplasts showed that Os HK3 is required for ABA-induced upregulation in the expression of antioxidant enzymes genes and the activities of antioxidant enzymes. Further analysis showed that Os HK3 functions upstream of the calcium/calmodulin-dependent protein kinase Os DMI3 and the mitogen-activated protein kinase Os MPK1 to regulate the activities of antioxidant enzymes in ABA signaling. Moreover, Os HK3was also shown to regulate the expression of nicotinamide adenine dinucleotide phosphate oxidase genes, Osrboh B and Osrboh E, and the production of H2O2 in ABA signaling. Our data indicate that Os HK3 play an important role in the regulation of ABA-induced antioxidant defense and in the feedback regulation of H2O2 production in ABA signaling.     

The C2H2 -type Zinc Finger Protein ZFP182 is Involved in Abscisic Acid-Induced Antioxidant Defense in Rice

C2H2 -type zinc finger proteins (ZFPs) are thought to play important roles in modulating the responses of plants to drought, salinity and oxidative stress. However, direct evidence is lacking for the involvement of these ZFPs in abscisic acid (ABA)-induced antioxidant defense in plants. In this study, the role of the rice (Oryza sativa L. sub. japonica cv. Nipponbare) C 2 H 2 -type ZFP ZFP182 in ABA-induced antioxidant defense and the relationship between ZFP182 and two rice MAPKs, OsMPK1 and OsMPK5 in ABA signaling were investigated. ABA treatment induced the increases in the expression of ZFP182, OsMPK1 and OsMPK5, and the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) in rice leaves. The transient gene expression analysis and the transient RNA interference (RNAi) analysis in protoplasts showed that ZFP182, OsMPK1 and OsMPK5 are involved in ABA-induced up-regulation in the activities of SOD and APX. Besides, OsMPK1 and OsMPK5 were shown to be required for the up-regulation in the expression of ZFP182 in ABA signaling, but ZFP182 did not mediate the ABA-induced up-regulation in the expression of OsMPK1 and OsMPK5. These results indicate that ZFP182 is required for ABA-induced antioxidant defense and the expression of ZFP182 is regulated by rice MAPKs in ABA signaling.     

OsSUV3 dual helicase functions in salinity stress tolerance by maintaining photosynthesis and antioxidant machinery in rice (Oryza sativa L. cv. IR64)

To overcome the salinity‐induced loss of crop yield, a salinity‐tolerant trait is required. The SUV3 helicase is involved in the regulation of RNA surveillance and turnover in mitochondria, but the helicase activity of plant SUV3 and its role in abiotic stress tolerance have not been reported so far. Here we report that the Oryza sativa (rice) SUV3 protein exhibits DNA and RNA helicase, and ATPase activities. Furthermore, we report that SUV3 is induced in rice seedlings in response to high levels of salt. Its expression, driven by a constitutive cauliflower mosaic virus 35S promoter in IR64 transgenic rice plants, confers salinity tolerance. The T₁ and T₂ sense transgenic lines showed tolerance to high salinity and fully matured without any loss in yields. The T₂ transgenic lines also showed tolerance to drought stress. These results suggest that the introduced trait is functional and stable in transgenic rice plants. The rice SUV3 sense transgenic lines showed lesser lipid peroxidation, electrolyte leakage and H₂O₂ production, along with higher activities of antioxidant enzymes under salinity stress, as compared with wild type, vector control and antisense transgenic lines. These results suggest the existence of an efficient antioxidant defence system to cope with salinity‐induced oxidative damage. Overall, this study reports that plant SUV3 exhibits DNA and RNA helicase and ATPase activities, and provides direct evidence of its function in imparting salinity stress tolerance without yield loss. The possible mechanism could be that OsSUV3 helicase functions in salinity stress tolerance by improving photosynthesis and antioxidant machinery in transgenic rice.     

Shade light interaction with salicylic acid in regulating growth of sun (alpine) and shade (prairie) ecotypes of Stellaria longipes

Magnesium (Mg) deficiency in plants is a widespread problem, affecting productivity and quality in agriculture. The mechanism of Mg deficiency inducing antioxidant enzyme activities has not been elucidated in rice. We examined the relationship among abscisic acid (ABA), H₂O₂, and antioxidant enzymes in the leaves of rice seedlings grown under conditions of Mg deficiency. The expression of OsRab16A, an ABA responsive gene, was used to determine the content of ABA. Mg deficiency resulted in increased ABA content in leaves of rice seedlings. The production of H₂O₂ was examined by 3,3-diaminobenzidine staining and a colorimetric method. Mg deficiency also induced H₂O₂ production in leaves, which was blocked by dipehnyleneiodonium chloride (DPI), an NADPH oxidase inhibitor. Tungstate (Tu), an ABA biosynthesis inhibitor, was effective in reducing Mg deficiency-increased ABA content, as well as Mg deficiency-induced H₂O₂ production. Both Tu and DPI were effective in reducing Mg deficiency-induced activities of superoxide dismutase, ascorbate peroxidase, glutathione reductase, and catalase in the leaves. Mg deficiency-induced ABA accumulation may trigger increased production of H₂O₂, which may involve plasma-membrane NADPH oxidase, and, in turn, up-regulates the activities of antioxidant enzymes in leaves of rice seedlings.     

Abscisic acid deficiency increases defence responses against Myzus persicae in Arabidopsis

Comparison of Arabidopsis thaliana (Arabidopsis) gene expression induced by Myzus persicae (green peach aphid) feeding, aphid saliva infiltration and abscisic acid (ABA) treatment showed a significant positive correlation. In particular, ABA‐regulated genes are over‐represented among genes that are induced by M. persicae saliva infiltration into Arabidopsis leaves. This suggests that the induction of ABA‐related gene expression could be an important component of the Arabidopsis–aphid interaction. Consistent with this hypothesis, M. persicae populations induced ABA production in wild‐type plants. Furthermore, aphid populations were smaller on Arabidopsis aba1‐1 mutants, which cannot synthesize ABA, and showed a significant preference for wild‐type plants compared with the mutant. Total free amino acids, which play an important role in aphid nutrition, were not altered in the aba1‐1 mutant line, but the levels of isoleucine (Ile) and tryptophan (Trp) were differentially affected by aphids in wild‐type and mutant plants. Recently, indole glucosinolates have been shown to promote aphid resistance in Arabidopsis. In this study, 4‐methoxyindol‐3‐ylmethylglucosinolate was more abundant in the aba1‐1 mutant than in wild‐type Arabidopsis, suggesting that the induction of ABA signals that decrease the accumulation of defence compounds may be beneficial for aphids.     

Physiological characteristics and related gene expression of after‐ripening on seed dormancy release in rice

After‐ripening is a common method used for dormancy release in rice. In this study, the rice variety Jiucaiqing (Oryza sativa L. subsp. japonica) was used to determine dormancy release following different after‐ripening times (1, 2 and 3 months). Germination speed, germination percentage and seedling emergence increased with after‐ripening; more than 95% germination and 85% seedling emergence were observed following 1 month of after‐ripening within 10 days of imbibition, compared with <45% germination and 20% seedling emergence in freshly harvested seed. Hence, 3 months of after‐ripening could be considered a suitable treatment period for rice dormancy release. Dormancy release by after‐ripening is mainly correlated with a rapid decline in ABA content and increase in IAA content during imbibition. Subsequently, GA₁/ABA, GA₇/ABA, GA₁₂/ABA, GA₂₀/ABA and IAA/ABA ratios significantly increased, while GA₃/ABA, GA₄/ABA and GAs/IAA ratio significantly decreased in imbibed seeds following 3 months of after‐ripening, thereby altering α‐amylase activity during seed germination. Peak α‐amylase activity occurred at an earlier germination stage in after‐ripened seeds than in freshly harvested seeds. Expression of ABA, GA and IAA metabolism genes and dormancy‐related genes was regulated by after‐ripening time upon imbibition. Expression of OsCYP707A5, OsGA2ox1, OsGA2ox2, OsGA2ox3, OsILR1, OsGH3‐2, qLTG3‐1 and OsVP1 increased, while expression of Sdr4 decreased in imbibed seeds following 3 months of after‐ripening. Dormancy release through after‐ripening might be involved in weakening tissues covering the embryo via qLTG3‐1 and decreased ABA signalling and sensitivity via Sdr4 and OsVP1.     

A rice fungal MAMP‐responsive MAPK cascade regulates metabolic flow to antimicrobial metabolite synthesis

Plants recognize potential microbial pathogens through microbial‐associated molecular patterns (MAMPs) and activate a series of defense responses, including cell death and the production of reactive oxygen species (ROS) and diverse anti‐microbial secondary metabolites. Mitogen‐activated protein kinase (MAPK) cascades are known to play a pivotal role in mediating MAMP signals; however, the signaling pathway from a MAPK cascade to the activation of defense responses is poorly understood. Here, we found in rice that the chitin elicitor, a fungal MAMP, activates two rice MAPKs (OsMPK3 and OsMPK6) and one MAPK kinase (OsMKK4). OsMPK6 was essential for the chitin elicitor‐induced biosynthesis of diterpenoid phytoalexins. Conditional expression of the active form of OsMKK4 (OsMKK4^DD) induced extensive alterations in gene expression, which implied dynamic changes of metabolic flow from glycolysis to secondary metabolite biosynthesis while suppressing basic cellular activities such as translation and cell division. OsMKK4^DD also induced various defense responses, such as cell death, biosynthesis of diterpenoid phytoalexins and lignin but not generation of extracellular ROS. OsMKK4^DD‐induced cell death and expression of diterpenoid phytoalexin pathway genes, but not that of phenylpropanoid pathway genes, were dependent on OsMPK6. Collectively, the OsMKK4–OsMPK6 cascade plays a crucial role in reprogramming plant metabolism during MAMP‐triggered defense responses.     

Arabidopsis acyl‐CoA‐binding protein ACBP1 participates in the regulation of seed germination and seedling development

A family of six genes encoding acyl‐CoA‐binding proteins (ACBPs), ACBP1–ACBP6, has been characterized in Arabidopsis thaliana. In this study, we demonstrate that ACBP1 promotes abscisic acid (ABA) signaling during germination and seedling development. ACBP1 was induced by ABA, and transgenic Arabidopsis ACBP1‐over‐expressors showed increased sensitivity to ABA during germination and seedling development, whereas the acbp1 mutant showed decreased ABA sensitivity during these processes. Subsequent RNA assays showed that ACBP1 over‐production in 12‐day‐old seedlings up‐regulated the expression of PHOSPHOLIPASE Dα1 (PLDα1) and three ABA/stress‐responsive genes: ABA‐RESPONSIVE ELEMENT BINDING PROTEIN1 (AREB1), RESPONSE TO DESICCATION29A (RD29A) and bHLH‐TRANSCRIPTION FACTOR MYC2 (MYC2). The expression of AREB1 and PLDα1 was suppressed in the acbp1 mutant in comparison with the wild type following ABA treatment. PLDα1 has been reported to promote ABA signal transduction by producing phosphatidic acid, an important lipid messenger in ABA signaling. Using lipid profiling, seeds and 12‐day‐old seedlings of ACBP1‐over‐expressing lines were shown to accumulate more phosphatidic acid after ABA treatment, in contrast to lower phosphatidic acid in the acbp1 mutant. Bimolecular fluorescence complementation assays indicated that ACBP1 interacts with PLDα1 at the plasma membrane. Their interaction was further confirmed by yeast two‐hybrid analysis. As recombinant ACBP1 binds phosphatidic acid and phosphatidylcholine, ACBP1 probably promotes PLDα1 action. Taken together, these results suggest that ACBP1 participates in ABA‐mediated seed germination and seedling development.     

Carbon dioxide enrichment alleviates heat stress by improving cellular redox homeostasis through an ABA‐independent process in tomato plants

Plant responses to elevated CO₂ and high temperature are critically regulated through a complex network of phytohormones and redox homeostasis. However, the involvement of abscisic acid (ABA) in plant adaptation to heat stress under elevated CO₂ conditions has not been thoroughly studied. This study investigated the interactive effects of elevated CO₂ (800 μmol·mol^?1) and heat stress (42 °C for 24 h) on the endogenous level of ABA and the cellular redox state of two genotypes of tomato with different ABA biosynthesis capacities. Heat stress significantly decreased maximum photochemical efficiency of PSII (Fv/Fm) and leaf water potential, but also increased levels of malondialdehyde (MDA) and electrolyte leakage (EL) in both genotypes. Heat‐induced damage was more severe in the ABA‐deficient mutant notabilis (not) than in its parental cultivar Ailsa Craig (Ailsa), suggesting that a certain level of endogenous ABA is required to minimise the heat‐induced oxidative damage to the photosynthetic apparatus. Irrespective of genotype, the enrichment of CO₂ remarkably stimulated Fv/Fm, MDA and EL in heat‐stressed plants towards enhanced tolerance. In addition, elevated CO₂ significantly strengthened the antioxidant capacity of heat‐stressed tomato seedlings towards a reduced cellular redox state for a prolonged period, thereby mitigating oxidative stress. However, elevated CO₂ and heat stress did not alter the endogenous level of ABA or the expression of its biosynthetic gene NCED2 in either genotype, indicating that ABA is not involved in elevated CO₂‐induced heat stress alleviation. The results of this study suggest that elevated CO₂ alleviated heat stress through efficient regulation of the cellular redox poise in an ABA‐independent manner in tomato plants.     

Abscisic acid alleviates iron deficiency by promoting root iron reutilization and transport from root to shoot in Arabidopsis

Abscisic acid (ABA) has been demonstrated to be involved in iron (Fe) homeostasis, but the underlying mechanism is largely unknown. Here, we found that Fe deficiency induced ABA accumulation rapidly (within 6 h) in the roots of Arabidopsis. Exogenous ABA at 0.5 μM decreased the amount of root apoplastic Fe bound to pectin and hemicellulose, and increased the shoot Fe content significantly, thus alleviating Fe deficiency‐induced chlorosis. Exogenous ABA promoted the secretion of phenolics to release apoplastic Fe and up‐regulated the expression of AtNRAMP3 to enhance reutilization of Fe stored in the vacuoles, leading to a higher level of soluble Fe and lower ferric–chelate reductase (FCR) activity in roots. Treatment with ABA also led to increased Fe concentrations in the xylem sap, partially because of the up‐regulation of AtFRD3, AtYSL2 and AtNAS1, genes related to long‐distance transport of Fe. Exogenous ABA could not alleviate the chlorosis of abi5 mutant resulting from the significantly low expression of AtYSL2 and low transport of Fe from root to shoot. Taken together, our data support the conclusion that ABA is involved in the reutilization and transport of Fe from root to shoot under Fe deficiency conditions in Arabidopsis.