Regulation of cation transporter genes by the arbuscular mycorrhizal symbiosis in rice plants subjected to salinity suggests improved salt tolerance due to reduced Na+ root-to-shoot distribution

Rice is a salt-sensitive crop whose productivity is strongly reduced by salinity around the world. Plants growing in saline soils are subjected to the toxicity of specific ions such as sodium, which damage cell organelles and disrupt metabolism. Plants have evolved biochemical and molecular mechanisms to cope with the negative effects of salinity. These include the regulation of genes with a role in the uptake, transport or compartmentation of Na+ and/or K+. Studies have shown that the arbuscular mycorrhizal (AM) symbiosis alleviates salt stress in several host plant species. However, despite the abundant literature showing mitigation of ionic imbalance by the AM symbiosis, the molecular mechanisms involved are barely explored. The objective of this study was to elucidate the effects of the AM symbiosis on the expression of several well-known rice transporters involved in Na+/K+ homeostasis and measure Na+ and K+ contents and their ratios in different plant tissues. Results showed that OsNHX3, OsSOS1, OsHKT2;1 and OsHKT1;5 genes were considerably upregulated in AM plants under saline conditions as compared to non-AM plants. Results suggest that the AM symbiosis favours Na+ extrusion from the cytoplasm, its sequestration into the vacuole, the unloading of Na+ from the xylem and its recirculation from photosynthetic organs to roots. As a result, there is a decrease of Na+ root-to-shoot distribution and an increase of Na+ accumulation in rice roots which seems to enhance the plant tolerance to salinity and allows AM rice plants to maintain their growing processes under salt conditions.