Physiological and transcriptional responses to low-temperature stress in rice genotypes at the reproductive stage.

Rice growth and productivity is adversely affected by low-temperature stress. From a previous screen of diverse rice genotypes for cold tolerance parameters at the vegetative stage, we selected the tolerant Nipponbare and M202 genotypes and sensitive Cypress and Secano do Brazil genotypes for further analysis at the reproductive stage for physiological and yield parameters. Cold stress severely affected grain yield as estimated by the number of grain per panicle, panicle length, and 100 seed weight. Analysis of gene expression of 21 genes involved in physiological responses to low temperature tested, in the flag leaf and inflorescence tissue of these genotypes, showed an increased expression of the Lipid Transfer Protein genes LTP7 and LTP10 in flag leaf tissue of the tolerant Nipponbare and M202, along with a significant increase in the relative expression of stress-responsive transcription factors (TFs) and cold-inducible genes. In flag leaf tissue OsNAC9, OsNAC10 and OsNAP genes showed high correlation with photosynthesis, stomatal conductance, transpiration and Quantum Efficiency of PSII. In consequence of the foregoing results, we conclude that Nipponbare and M202 are cold tolerant genotypes and that LTP7, LTP10, OsNAC9, OsNAC10 and OsNAP genes can be used as markers in screening for cold tolerance at the reproductive stage. Furthermore based on the results we propose a model of low-temperature tolerance mechanism of how stress is perceived, and how the signal cascade acts to promote tolerance at below-ideal temperatures.

Comparative analysis of gene expression in response to cold stress in diverse rice genotypes.

Cold stress is a major factor affecting rice (Oryza sativa) growth and productivity, limiting its distribution worldwide. Rice production is affected primarily due to its vulnerability to cold stress at seedling stage, as well as reproductive stage leading to spikelet sterility. We report here the analysis of 21 diverse rice genotypes from the USDA mini-core collection for cold tolerance and categorized their tolerance levels on the basis of reduction in growth measured by root and shoot length. The screening identified 12 cold tolerant genotypes from which six tolerant genotypes were characterized at the vegetative stage for cold tolerance and gas-exchange parameters. Two tolerant and two sensitive genotypes were used further for gene expression analysis. Lipid Transfer Protein (LTP) genes showed a clear difference in expression between cold tolerant and sensitive genotypes suggesting that they are good candidates for engineering cold tolerance in rice. Nipponbare was identified as a cold tolerant genotype with stress tolerance mechanism potentially operating via both ABA dependent and independent pathways.