Overexpression of a Plasma Membrane Protein Gene, SaPMP3, from Spartina alterniflora L. Enhances Salinity Tolerance in Rice (Oryza sativa L.)

Abstract

Salinity continues to be a major abiotic stress limiting crop productivity. As rice is staple food for nearly half of the world population, improvement in its salt tolerance will have a major impact on global food security. Compared to rice and other field crops, halophytes have evolved special physiological mechanisms to withstand high salinity. The overall goal of this study was to characterize plasma membrane protein 3 genes, SaPMP3-2 and SaPMP3-1, from a halophyte, Spartina alterniflora L., and evaluate their potential in single gene as well as pyramided transgenic plants in combination with the vacuolar ATPase subunit c1 (SaVHAc1) gene in improving salt tolerance in cv. Cocodrie background. Both genes, SaPMP3-2 and SaPMP3-1, enhanced the ability of E. coli to survive at 600 mM NaCl. Genetic complementation of the mutant yeast strain and enhanced salt tolerance in wild type yeast strain by SaPMP3-2 indicated its conserved functional role in salt tolerance. Subsequently, enhanced salt tolerance in transgenic rice plants was demonstrated through overexpression of SaPMP3-2 and SaPMP3-1 independently as well as the combination of SaPMP3-1 and SaVHAc1. Chlorophyll retention and relative water content were higher in transgenic plants compared to Cocodrie under salt stress during the vegetative stage. The transgenic plants survived wilting and drying symptoms with enhanced growth and higher K+/Na+ ratio at 100 mM NaCl stress during early seedling stage in hydroponic conditions. Salt stress screening during reproductive stage revealed that the single gene and the pyramided transgenic plants had better grain filling whereas only the pyramided plants showed significantly higher grain yield per plant and higher test weight compared to Cocodrie. The improvement in salt tolerance in transgenic rice plants could be due to the role played by SaPMP3-2 and SaPMP3-1 through maintenance of ion homoestasis by restricting uptake of salts. The impact of SaPMP3 gene was further amplified when combined with SaVHAc1 in pyramided transgenic plants, which showed better growth, vigor, and enhanced salt tolerance at all stages of crop growth compared with Cocodrie. Our study provided evidence that S. alterniflora could be a potential source for mining genes to enhance salt tolerance in rice and other cereal crops.