A MITE variation-associated heat-inducible isoform of a heat-shock factor confers heat tolerance through regulation of JASMONATE ZIM-DOMAIN genes in rice.

High temperatures cause huge yield losses in rice. Heat-shock factors (Hsfs) are key transcription factors which regulate the expression of heat stress-responsive genes, but natural variation in and functional characterization of Hsfs have seldom been reported. A significant heat response locus was detected via a genome-wide association study (GWAS) using green leaf area as an indicative trait. A miniature inverted-repeat transposable element (MITE) in the promoter of a candidate gene, HTG3 (heat-tolerance gene on chromosome 3), was found to be significantly associated with heat-induced expression of HTG3 and heat tolerance (HT). The MITE-absent variant has been selected in heat-prone rice-growing regions. HTG3a is an alternatively spliced isoform encoding a functional Hsf, and experiments using overexpression and knockout rice lines showed that HTG3a positively regulates HT at both vegetative and reproductive stages. The HTG3-regulated genes were enriched for heat shock proteins and jasmonic acid signaling. Two heat-responsive JASMONATE ZIM-DOMAIN (JAZ) genes were confirmed to be directly upregulated by HTG3a, and one of them, OsJAZ9, positively regulates HT. We conclude that HTG3 plays an important role in HT through the regulation of JAZs and other heat-responsive genes. The MITE-absent allele may be valuable for HT breeding in rice.

Serine Hydroxymethyltransferase 1 Is Essential for Primary-Root Growth at Low-Sucrose Conditions.

Plant roots are essential organs for absorbing nutrients from the soil or medium. Sucrose functions as a vital carbon source in root development, and sucrose starvation interferes with the redox state of plant cells. However, the mechanism of root growth at sucrose starvation remains unclear. Here, we report that SHMT1 (serine hydroxymethyltransferase 1) plays a crucial role in primary-root growth. SHMT1 mutation caused decreased sugar levels, excessive H2O2 accumulation, and severe root-growth arrest at sucrose-free conditions, whereas plants with SHMT1 overexpression had increased sugar and decreased H2O2 levels, and longer primary roots. Sucrose supply fully restored root growth of shm1-2, but CO2 alone could not, and SHMT1 is much more stable in roots than shoots at sucrose conditions, suggesting that SHMT1 accumulation in roots is critical for sucrose accumulation and root growth. Further ROS scavenging by GSH application or ROS synthesis inhibition by apocynin application or RBOHD mutation reduced H2O2 levels and partially restored the root-growth arrest phenotype of shm1-2 at low-sucrose conditions, suggesting that SHMT1 modulates root growth via sucrose-mediated ROS accumulation. Our findings demonstrated the role of SHMT1 in primary-root growth by regulating sucrose accumulation and ROS homeostasis in roots.

Common and specific genetic basis of metabolite-mediated drought responses in rice.

Plants orchestrate drought responses at metabolic level but the genetic basis remains elusive in rice. In this study, 233 drought-responsive metabolites (DRMs) were quantified in a large rice population comprised of 510 diverse accessions at the reproductive stage. Large metabolic variations in drought responses were detected, and little correlation of metabolic levels between drought and normal conditions were observed. Interestingly, most of these DRMs could predict drought resistance in high accuracy. Genome-wide association study revealed 2522 significant association signals for 233 DRMs, and 98% (2471/2522) of the signals were co-localized with the association loci for drought-related phenotypic traits in the same population or the linkage-mapped QTLs for drought resistance in other populations. Totally, 10 candidate genes were efficiently identified for nine DRMs, seven of which harbored cis-eQTLs under drought condition. Based on comparative GWAS of common DRMs in rice and maize, representing irrigated and upland crops, we have identified three pairs of homologous genes associated with three DRMs between the two crops. Among the homologous genes, a transferase gene responsible for metabolic variation of N-feruloylputrescine was confirmed to confer enhanced drought resistance in rice. Our study provides not only genetic architecture of metabolic responses to drought stress in rice but also metabolic data resources to reveal the common and specific metabolite-mediated drought responses in different crops.

Control of rice ratooning ability by a nucleoredoxin that inhibits histidine kinase dimerization to attenuate cytokinin signaling in axillary buds.

Rice ratooning, the fast outgrowth of dormant buds on stubble, is an important cropping practice in rice production. However, the low ratooning ability (RA) of most rice varieties restricts the application of this cost-efficient system, and the genetic basis of RA remains unknown. In this study, we dissected the genetic architecture of RA by a genome-wide association study in a natural rice population. Rice ratooning ability 3 (RRA3), encoding a hitherto not characterized nucleoredoxin involved in reduction of disulfide bonds, was identified as the causal gene of a major locus controlling RA. Overexpression of RRA3 in rice significantly accelerated leaf senescence and reduced RA, whereas knockout of RRA3 significantly delayed leaf senescence and increased RA and ratoon yield. We demonstrated that RRA3 interacts with Oryza sativa histidine kinase 4 (OHK4), a cytokinin receptor, and inhibits the dimerization of OHK4 through disulfide bond reduction. This inhibition ultimately led to decreased cytokinin signaling and reduced RA. In addition, variations in the RRA3 promoter were identified to be associated with RA. Introgression of a superior haplotype with weak expression of RRA3 into the elite rice variety Guichao 2 significantly increased RA and ratoon yield by 23.8%. Collectively, this study not only uncovers an undocumented regulatory mechanism of cytokinin signaling through de-dimerization of a histidine kinase receptor-but also provides an eximious gene with promising value for ratoon rice breeding.

Reversible Histone H2B Monoubiquitination Fine-Tunes Abscisic Acid Signaling and Drought Response in Rice.

Histone H2B monoubiquitination (H2Bub1) plays important roles in several physiological and developmental processes, but its roles in the regulation of plant stress responses remain elusive. Here, we report that H2Bub1 is crucially involved in abscisic acid (ABA) signaling and drought response in rice. We found that rice HISTONE MONOUBIQUITINATION2 (OsHUB2), an E3 ligase for H2Bub1, interacted with OsbZIP46, a key transcription factor regulating ABA signaling and drought response in rice. Genetic analyses suggest that OsHUB2, upregulated by drought and ABA, positively modulates ABA sensitivity and drought resistance. The H2Bub1 levels were increased in the target genes of OsbZIP46 under the drought stress and ABA treatments, which were positively correlated with their increased expression levels. Interestingly, MODD, a reported suppressor of ABA signaling and drought resistance by mediating OsbZIP46 deactivation and degradation, could reduce the H2Bub1 levels in the target genes of OsbZIP46 by recruiting a putative deubiquitinase OsOTLD1. Suppression of OsOTLD1 in vivo resulted in increased H2Bub1 levels and expression of OsbZIP46 target genes. Collectively, these findings established an elaborate mechanism of histone monoubiquitination in the fine-turning of ABA signaling and drought response by balancing H2Bub1 deposition and removal.

OsJAZ1 Attenuates Drought Resistance by Regulating JA and ABA Signaling in Rice.

Jasmonates (JAs) and abscisic acid (ABA) are phytohormones known play important roles in plant response and adaptation to various abiotic stresses including salinity, drought, wounding, and cold. JAZ (JASMONATE ZIM-domain) proteins have been reported to play negative roles in JA signaling. However, direct evidence is still lacking that JAZ proteins regulate drought resistance. In this study, OsJAZ1 was investigated for its role in drought resistance in rice. Expression of OsJAZ1 was strongly responsive to JA treatment, and it was slightly responsive to ABA, salicylic acid, and abiotic stresses including drought, salinity, and cold. The OsJAZ1-overexpression rice plants were more sensitive to drought stress treatment than the wild-type (WT) rice Zhonghua 11 (ZH11) at both the seedling and reproductive stages, while the jaz1 T-DNA insertion mutant plants showed increased drought tolerance compared to the WT plants. The OsJAZ1-overexpression plants were hyposensitive to MeJA and ABA, whereas the jaz1 mutant plants were hypersensitive to MeJA and ABA. In addition, there were significant differences in shoot and root length between the OsJAZ1 transgenic and WT plants under the MeJA and ABA treatments. A subcellular localization assay indicated that OsJAZ1 was localized in both the nucleus and cytoplasm. Transcriptome profiling analysis by RNA-seq revealed that the expression levels of many genes in the ABA and JA signaling pathways exhibited significant differences between the OsJAZ1-overexpression plants and WT ZH11 under drought stress treatment. Quantitative real-time PCR confirmed the expression profiles of some of the differentially expressed genes, including OsNCED4, OsLEA3, RAB21, OsbHLH006, OsbHLH148, OsDREB1A, OsDREB1B, SNAC1, and OsCCD1. These results together suggest that OsJAZ1 plays a role in regulating the drought resistance of rice partially via the ABA and JA pathways.