The Saltol QTL-localized transcription factor OsGATA8 plays an important role in stress tolerance and seed development in Arabidopsis and rice.

GATA represents a highly conserved family of transcription factors reported in organisms ranging from fungi to angiosperms. A member of this family, OsGATA8, localized within the Saltol QTL in rice, has been reported to be induced by salinity, drought, and ABA. However, its precise role in stress tolerance has not yet been elucidated. Using genetic, molecular, and physiological analyses, in this study we show that OsGATA8 increases seed size and tolerance to abiotic stresses in both Arabidopsis and rice. Transgenic lines of rice were generated with 3-fold overexpression of OsGATA8 compared to the wild-type together with knockdown lines with 2-fold lower expression. The overexpressing lines showed higher biomass accumulation and higher photosynthetic efficiency in seedlings compared to the wild-type and knockdown lines under both normal and salinity-stress conditions. OsGATA8 appeared to be an integrator of diverse cellular processes, including K+/Na+ content, photosynthetic efficiency, relative water content, Fv/Fm ratio, and the stability to sub-cellular organelles. It also contributed to maintaining yield under stress, which was ~46% higher in overexpression plants compared with the wild-type. OsGATA8 produced these effects by regulating the expression of critical genes involved in stress tolerance, scavenging of reactive oxygen species, and chlorophyll biosynthesis.

Abiotic Stresses Cause Differential Regulation of Alternative Splice Forms of GATA Transcription Factor in Rice.

The GATA gene family is one of the most conserved families of transcription factors, playing a significant role in different aspects of cellular processes, in organisms ranging from fungi to angiosperms. GATA transcription factors are DNA-binding proteins, having a class IV zinc-finger motif CX2CX17-20CX2C followed by a highly basic region and are known to bind a consensus sequence WGATAR. In plants, GATAs are known to be involved in light-dependent gene regulation and nitrate assimilation. However, a comprehensive analysis of these GATA gene members has not yet been highlighted in rice when subjected to environmental stresses. In this study, we present an overview of the GATA gene family in rice (OsGATA) in terms of, their chromosomal distribution, domain architecture, and phylogeny. Our study has revealed the presence of 28 genes, encoding 35 putative GATA transcription factors belonging to seven subfamilies in the rice genome. Transcript abundance analysis in contrasting genotypes of rice-IR64 (salt sensitive) and Pokkali (salt tolerant), for individual GATA members indicated their differential expression in response to various abiotic stresses such as salinity, drought, and exogenous ABA. One of the members of subfamily VII-OsGATA23a, emerged as a multi-stress responsive transcription factor giving elevated expression levels in response to salinity and drought. ABA also induces expression of OsGATA23a by 35 and 55-folds in IR64 and Pokkali respectively. However, OsGATA23b, an alternative splice variant of OsGATA23 did not respond to above-mentioned stresses. Developmental regulation of the OsGATA genes based on a publicly available microarray database showed distinct expression patterns for most of the GATA members throughout different stages of rice development. Altogether, our results suggest inherent roles of diverse OsGATA factors in abiotic stress signaling and also throw some light on the tight regulation of the spliced variants of OsGATA genes in response to different environmental conditions.

A nuclear-localized rice glyoxalase I enzyme, OsGLYI-8, functions in the detoxification of methylglyoxal in the nucleus.

The cellular levels of methylglyoxal (MG), a toxic byproduct of glycolysis, rise under various abiotic stresses in plants. Detoxification of MG is primarily through the glyoxalase pathway. The first enzyme of the pathway, glyoxalase I (GLYI), is a cytosolic metalloenzyme requiring either Ni2+ or Zn2+ for its activity. Plants possess multiple GLYI genes, of which only some have been partially characterized; hence, the precise molecular mechanism, subcellular localization and physiological relevance of these diverse isoforms remain enigmatic. Here, we report the biochemical properties and physiological role of a putative chloroplast-localized GLYI enzyme, OsGLYI-8, from rice, which is strikingly different from all hitherto studied GLYI enzymes in terms of its intracellular localization, metal dependency and kinetics. In contrast to its predicted localization, OsGLYI-8 was found to localize in the nucleus along with its substrate, MG. Further, OsGLYI-8 does not show a strict requirement for metal ions for its activity, is functional as a dimer and exhibits unusual biphasic steady-state kinetics with a low-affinity and a high-affinity substrate-binding component. Loss of AtGLYI-2, the closest Arabidopsis ortholog of OsGLYI-8, results in severe germination defects in the presence of MG and growth retardation under salinity stress conditions. These defects were rescued upon complementation with AtGLYI-2 or OsGLYI-8. Our findings thus provide evidence for the presence of a GLYI enzyme and MG detoxification in the nucleus.

Transcription dynamics of Saltol QTL localized genes encoding transcription factors, reveals their differential regulation in contrasting genotypes of rice.

Salinity is one of the major environmental factors affecting the growth and yield of rice crop. Salinity stress response is a multigenic trait and numerous approaches have been used to dissect out the key determinants of complex salt tolerance trait and their regulation in plant. In the current study, we have investigated expression dynamics of the genes encoding transcription factors (SalTFs) localized within a major salinity tolerance related QTL-'Saltol' in the contrasting cultivars of rice. SalTFs were found to be differentially regulated between the contrasting genotypes of rice, with higher constitutive expression in the salt tolerant landrace, Pokkali than the cultivar IR64. Moreover, SalTFs were found to exhibit inducibility in the salt sensitive cultivar at late duration (after 24 h) of salinity stress. Further, the transcript abundance analysis of these SalTFs at various developmental stages of rice revealed that low expressing genes may be involved in developmental responses, while high expressing genes can be linked with the salt stress response. Grouping of these genes was well supported by in silico protein-protein interaction studies and distribution of single-nucleotide polymorphisms (SNPs) and insertions/deletions (InDels) in the promoter and genic regions of these genes. Taken together, we propose that out of 14 SalTFs, eight members are strongly correlated with the salinity stress tolerance in rice and six are involved in plant growth and development.

Understanding salinity responses and adopting 'omics-based' approaches to generate salinity tolerant cultivars of rice.

Soil salinity is one of the main constraints affecting production of rice worldwide, by reducing growth, pollen viability as well as yield of the plant. Therefore, detailed understanding of the response of rice towards soil salinity at the physiological and molecular level is a prerequisite for its effective management. Various approaches have been adopted by molecular biologists or breeders to understand the mechanism for salinity tolerance in plants and to develop salt tolerant rice cultivars. Genome wide analysis using 'omics-based' tools followed by identification and functional validation of individual genes is becoming one of the popular approaches to tackle this task. On the other hand, mutation breeding and insertional mutagenesis has also been exploited to obtain salinity tolerant crop plants. This review looks into various responses at cellular and whole plant level generated in rice plants toward salinity stress thus, evaluating the suitability of intervention of functional genomics to raise stress tolerant plants. We have tried to highlight the usefulness of the contemporary 'omics-based' approaches such as genomics, proteomics, transcriptomics and phenomics towards dissecting out the salinity tolerance trait in rice. In addition, we have highlighted the importance of integration of various 'omics' approaches to develop an understanding of the machinery involved in salinity response in rice and to move forward to develop salt tolerant cultivars of rice.

A nuclear-localized histone-gene binding protein from rice (OsHBP1b) functions in salinity and drought stress tolerance by maintaining chlorophyll content and improving the antioxidant machinery.

Plants have evolved a number of molecular strategies and regulatory mechanisms to cope with abiotic stresses. Among the various key factors/regulators, transcription factors (TFs) play critical role(s) towards regulating the gene expression patterns in response to stress conditions. Altering the expression of the key TFs can greatly influence plant stress tolerance. OsHBP1b (accession no. KM096571) is one such TF belonging to bZIP family, localized within the Saltol QTL, whose expression is induced upon salinity treatment in the rice seedlings. qRT-PCR based expression studies for OsHBP1b in seedlings of contrasting genotypes of rice showed its differential regulation in response to salinity stress. A GFP based in vivo study showed that the OsHBP1b protein is nuclear localized and possesses the trans-activation activity. As compared to the WT tobacco plants, the transgenic plants ectopically expressing OsHBP1b showed better survival and favourable osmotic parameters (such as germination and survival rate, membrane stability, K(+)/Na(+) ratio, lipid peroxidation, electrolyte leakage and proline contents) under salinity and drought stress. Under salinity conditions, the transgenic plants accumulated lower levels of reactive oxygen species as compared to the WT. It was also accompanied by higher activities of antioxidant enzymes (such as ascorbate peroxidase and superoxide dismutase), thereby demonstrating that transgenic plants are physiologically better adapted towards the oxidative damage. Taken together, our findings suggest that OsHBP1b contributes to abiotic stress tolerance through multiple physiological pathways and thus, may serve as a useful 'candidate gene' for improving multiple stress tolerance in crop plants.