Recent Updates on Salinity Stress in Rice: From Physiological to Molecular Responses

One-fifth of irrigated agriculture is negatively affected by high soil salinity. The expected population growth, over 9 billion by 2050, enhances the pressure for agricultural production in marginal saline lands. Rice (Oryza sativa L.), the staple food for more than half of the world's population, is the most salt-sensitive cereal. The need for salt-tolerant rice varieties able to cope with several other stress conditions obviously puts a lot of pressure on breeders who must better comprehend the physiology and genetic control of salt tolerance. In spite of several good reviews recently published, an integrated vision of current information on rice tolerance to salt stress has been lacking. Here we present the most recent data on the salinity effect on rice physiology and stress adaptation, including implications on growth regulation and reproductive development. We have included an inventory of salt tolerance donors available for breeding programs and a comprehensive survey of current work on QTL detection and cloning as well as marker-assisted selection to introgress favorable alleles into elite rice lines. A schematic view of the rice chromosomes on which salt tolerance QTLs and candidate genes are positioned is also included. Finally, we focus on the most promising candidate genes involved in salt stress response. There, we discuss the available knowledge on salt stress signaling and ion homeostasis, LEAs and other stress-induced proteins, genes with unknown function and transcription regulators as well as the present knowledge on the role of post-translational modifications on the modulation of the response to salinity in rice. We conclude by highlighting still missing clues that could help to design better salt tolerant varieties, and we evaluate the significance of the data presented for the future of rice breeding and sustainability of the culture in marginal saline soils.