Research associate University of Wisconsin-Madison Madison, Wisconsin
Nitrogen is a macronutrient for plants and exploring sustainable means of nitrogen acquisition becomes imperative and relevant in the present world. The symbiotic interaction between legumes and rhizobia culminates in the formation of nitrogen-fixing root nodules, which offer a greener alternative to synthetic fertilizers. Rhizobium-legume symbiosis initiates with a molecular dialogue between the two partners. Host-secreted metabolites such as flavonoids induce the expression of nod genes in rhizobia, which causes the production of lipochitooligosaccharides commonly known as Nod factors. The perception of Nod factors by the host initiates a signaling cascade that leads to rhizobial colonization and nodule organogenesis. Nodule development is inhibited by environmental adversities such as extreme temperature, pH, drought, and salinity. Curiously, while high salinity leads to a severe reduction in nodulation, it causes an early boost in the expression of several symbiotic genes in the plant host (hyperinduction). Since the initiation of this symbiosis depends on the physiological status of both partners, we asked whether salinity causes differential accumulation of any metabolite in the host root that could affect the symbiotic behavior of rhizobia. We discovered that salt-stressed roots produce higher amounts of proline, and that proline can induce the expression of nod genes in rhizobia. Such a regulation explains, at least partially, the early boost in gene expression observed in host roots subjected to salt stress. The induction of nod genes by proline further alludes to a more generic response to host exudates, a feature with potential use in agriculture.