Enhanced gene expression rather than natural polymorphism in coding sequence of the OsbZIP23 determines drought tolerance and

Abstract

Drought is one of the major limiting factors for productivity of crops including rice (Oryza sativa L.). Understanding the role of allelic variations of key regulatory genes involved in stresstolerance is essential for developing an effective strategy to combat drought. The bZIP transcription factors play a crucial role in abioticstress adaptation in plants via abscisic acid (ABA) signaling pathway. The present study aimed to search for allelic polymorphism in the OsbZIP23 gene across selected droughttolerant and droughtsensitive rice genotypes, and to characterize the new allele through overexpression (OE) and genesilencing (RNAi). Analyses of the coding DNA sequence (CDS) of the cloned OsbZIP23 gene revealed single nucleotide polymorphism at four places and a 15nucleotide deletion at one place. The singlecopy OsbZIP23 gene is expressed at relatively higher level in leaf tissues of droughttolerant genotypes, and its abundance is more in reproductive stage. Cloning and sequence analyses of the OsbZIP23 promoter from droughttolerant O. rufipogon and droughtsensitive IR20 cultivar showed variation in the number of stressresponsive ciselements and a 35nucleotide deletion at 5'UTR in IR20. Analysis of the GFP reporter gene function revealed that the promoter activity of O. rufipogon is comparatively higher than that of IR20. The overexpression of any of the two polymorphic forms (1083 bp and 1068 bp CDS) of OsbZIP23 improved drought tolerance and yieldrelated traits significantly by retaining higher content of cellular water, soluble sugar and proline; and exhibited decrease in membrane lipid peroxidation in comparison to RNAi lines and nontransgenic plants. The OE lines showed higher expression of target genesOsRab16B, OsRab21 and OsLEA31 and increased ABA sensitivity; indicating that OsbZIP23 is a positive transcriptionalregulator of the ABAsignaling pathway. Taken together, the present study concludes that the enhanced gene expression rather than natural polymorphism in coding sequence of OsbZIP23 is accountable for improved drought tolerance and yield performance in rice genotypes.