Biotic Stress/Applied Plant Bio
CS-18-1 - Modulation of symbiotic signaling between nitrogen-fixing Rhizobia and their legume hosts by moderate salt stress
Monday, July 16
3:18 PM - 3:38 PM
As our climate warms, the problem of salinity is increasing, affecting plants in both agricultural and natural settings. The symbiosis between legumes and nitrogen-fixing bacteria, a key part of sustainable agriculture, is especially sensitive to salt stress. We found that Medicago truncatula plants experiencing a moderate salt stress reduce nodulation by 95%, but shoot biomass and root length are unaltered. Under these conditions, we found that most infections were blocked in the epidermis. Symbiosis is the result of signaling between microbial endosymbiont and plant host. The first rhizobial signal, Nod factor, induces a signaling cascade within the plant, leading to activation and induction of transcription factors (TF’s), which regulate gene expression, including that of Early Nodulin 11 (ENOD11). Surprisingly, we found that moderate salt stress causes a hyperinduction of ENOD11 by the rhizobial symbiont, despite a decrease both in infection and primordium formation. Using qRT-PCR and RNAseq, we found a set of genes, including ERN1 and Nodule Pectate Lyase, that are coregulated with ENOD11. Although NSP2, a direct regulator of ENOD11, was not differently regulated in these conditions, its function was required for ENOD11 hyperinduction by salt. Curiously, ENOD11 expression strongly increased in mutants lacking NSP2 function, but not in mutants of the DMI3 CCamKinase that functions prior to NSP2, suggesting that NSP2 may receive another input in addition to Nod factor. Sinorhizobium meliloti also produces a second signal, the exopolysaccharide succinoglycan, which is required for infection. S. meliloti mutants that either fail to produce or overproduce succinoglycan hyperinduce ENOD11 in the presence or absence of salt in a Nod factor-dependent process. Together, these data demonstrate regulation of the early symbiotic signaling pathway by salt and implicate the exopolysaccharide succinoglycan and the NSP2 transcription factor as key nodes in the intersection between these two signaling pathways.
Sanhita Chakraborty – University of Vermont