Salinity tolerance in Oryza sativa L. and its wild relatives

Abstract

Cultivated rice (Oryza sativa L.) is salt sensitive, and its growth, physiology and yield are negatively affected by moderate salinity. Rice employs three major physiological mechanisms to minimize salt-induced damage i.e., maintenance of Na+/K+ homeostasis, osmotic adjustment, and reactive oxygen species (ROS) detoxification. Oryza coarctata is the only halophyte member in the genus Oryza. It can survive and grow abundantly at high salinity level up to 40 dS/m in the coastal regions of many South Asian countries. The superior salt tolerance of O. coarctata is associated with several unique morphological and physiological features including salt-secreting hairs on the leaf surface, Kranz-like leaf anatomy and activity of C4-related enzymes, synthesis of pinitol as a unique organic osmoprotectant, compartmentalization of Na+ in roots, efficient restriction of Na+ xylem loading in roots and transport to shoots, and the ability to use Na+ as cheap osmoticum. Interestingly, O. coarctata possesses highly efficient vacuolar Na+ sequestration resulting in tissue tolerance which is an uncommon mechanism in cultivated rice. Some non-halophytic wild rice species that display higher level of salt tolerance than the salt-tolerant cultivated rice varieties also possess the tissue tolerance mechanism. This review provides an account on some physiological mechanisms of salt tolerance in cultivated rice, halophytic rice, and some wild rice relatives to explore new traits and candidate genes for future genetic improvement of rice.