GSE173197
Salt stress enhances early symbiotic gene expression in Medicago truncatula and induces a stress-specific set of rhizobium-responsive genes
Salt stress is a major agricultural concern inhibiting not only plant growth but also the symbiotic association between legume roots and the soil bacteria rhizobia. This symbiotic association is initiated by a molecular dialogue between the two partners, leading to the activation of a signaling cascade in the legume host and ultimately the formation of nitrogen-fixing root nodules. Here we show that a moderate salt stress increases the responsiveness of early symbiotic genes in Medicago truncatula to its symbiotic partner, Sinorhizobium meliloti, while conversely, inoculation with S. meliloti counteracts salt-regulated gene expression, restoring one-third to control levels. Our analysis of Early Nodulin 11 shows that salt-induced expression is dynamic, Nod-factor dependent, and requires the ionic, but not the osmotic, component of salt. We demonstrate that salt stimulation of rhizobium-induced gene expression requires NSP2, which functions as a node to integrate the abiotic and biotic signals. In addition, our work reveals that inoculation with Sinorhizobium meliloti succinoglycan mutants also hyperinduces ENOD11 expression in the presence or absence of salt, suggesting a possible link between rhizobial exopolysaccharide and the plant response to salt stress. Finally, we identify an accessory set of genes that are induced by rhizobium only under conditions of salt stress and have not been previously identified as being nodulation-related genes. Our data suggests that interplay of core nodulation genes with different accessory sets, specific for different abiotic conditions, function to establish the symbiosis. Together, our findings reveal a complex and dynamic interaction between plant, microbe, and environment. Overall design: There were 4 treatments: +/- NaCl, +/- Rhizobium. Samples were either pouched directly in MNM containing 100 mM NaCl or in MNM at Day 0, and either mock-inoculated or inoculated with Sinorhizobium meliloti Rm1021 at Day 4. Whole roots were harvested and processed for RNA-seq at Day 5 (24 hours after the inoculation treatment). There are 3 biological replicates of each treatment group. Each treatment from a biological replicate includes 21 plants.