Improvement of Salt Tolerance in Rice Plants by Arbuscular Mycorrhizal Symbiosis

Abstract

Rice (Oryza sativa L.) is the most important source of food for more than half of the world population, with salinity having a remarkable negative impact on its productivity worldwide. An important challenge for researchers is to achieve strategies to make rice plants more tolerant and to improve its productivity under salinity, in order to cope with reduced food production due to soil salinization. Several studies have shown that the arbuscular mycorrhizal (AM) symbiosis can alleviate salt stress in different host plant species. In this chapter, we summarize results obtained in relation to the amelioration of salt stress tolerance by the AM symbiosis in a crop of such importance for human nourishment as rice. Results showed that AM rice plants had a higher photochemical efficiency for CO2 fixation and solar energy utilization, and this increases plant salt tolerance by preventing the injury to the photosystems reaction centers and by allowing a better utilization of light energy in photochemical processes. On the other hand, in aerial plant tissues, the AM symbiosis may favor Na+ extrusion from cytoplasm, its sequestration into the vacuole, the unloading of Na+ from the xylem, and its recirculation from photosynthetic organs to roots, through regulation of OsSOS1, OsNHX3, OsHKT2;1 and OsHKT1;5 genes. The AM symbiosis also improves root hydraulic conductivity under saline conditions, and this may be due to enhanced symplastic and apoplastic radial water flow in rice roots, mediated by the induction of specific aquaporin isoforms and also by the additional water-absorbing surface of AM fungal hyphae. In addition to the above effects, the antioxidant capacity of rice plants seems to be also enhanced by the AM symbiosis, as evidenced by a reduced oxidative damage to lipids in AM plants subjected to salinity, by the enhanced activity of some antioxidant enzymes, or by the accumulation of antioxidant compounds such as glutathione.