Physiological and molecular responses to abiotic stress in rice (Oryza sativa) and characterization of an up-regulated gene family

Plants respond to abiotic stresses by complex molecular and physiological changes. Analyses of germination, survival rate and electrolyte leakage of five rice genotypes enabled us to identify two genotypes contrasting in their cold-tolerance. OsLti6a and OsLti6b were cloned from a cold subtracted cDNA library from the cold-tolerant genotype. The OsLti6 genes are related to evolutionarily conserved abiotic stress-related genes encoded for small hydrophobic proteins. Expression analyses revealed that OsLti6 genes are induced by cold, water-deficit, and salinity stresses. Each of the abiotic stresses induced OsLti6 genes more rapidly in the cold-tolerant genotype than in the cold-susceptible genotype. The OsLti6 gene family is regulated via a CBF transcription factor and through ABA-dependent and -independent pathways. Subcellular localization of OsLti confirmed its membrane nature by western blot. Heterologous expression of OsLti6a in yeast showed higher survival, growth and regrowth rates and lower electrolyte leakage and cell mortality under different stresses compared to the same strain transformed with a null plasmid. Concurrence of tolerance to abiotic stresses in rice and yeast implied a role for the proteins in increased membrane stability during stress. Moreover, no interaction between OsLti6a and rice other proteins was detected by the yeast two-hybrid assay. We hypothesized that OsLti may increase membrane stability by changing the lipid/protein ratio upon stress.; Physiological analyses showed that the cold-sensitive genotype is more salt-tolerant compared to the cold-tolerant genotype. Accumulation of stress-related compounds indicated several possible mechanisms in both genotypes that might be related to the tolerant phenotype. The cold-tolerant genotype may depend on maintaining membrane integrity by reduction of lipid peroxidation via increased levels of antioxidant enzymes during cold stress. Moreover, accumulation of some osmoprotectant, such as raffinose may increase membrane stability. On the other hand, the tolerant mechanism in the salt-stress tolerant genotype might be associated with accumulation of trehalose and mannitol. Comparison of gene expression profiles between the two genotypes showed expression of the same set of genes during stress but possibly under different regulation.