Chemoreceptors enable the legume symbiont
Sinorhizobium meliloti
to detect and respond to specific chemicals released from their host plant alfalfa, which allows the establishment of a nitrogen-fixing symbiosis. The periplasmic region (PR) of transmembrane chemoreceptors act as the sensory input module for chemotaxis systems via binding of specific ligands, either directly or indirectly.
S. meliloti
has six transmembrane and two cytosolic chemoreceptors. However, only the function of three of the transmembrane receptors have been characterized so far, with McpU, McpV, and McpX serving as general amino acid, short-chain carboxylate, and quaternary ammonium compound sensors, respectively. In the present study, we analyzed the
S. meliloti
chemoreceptor McpT. High-throughput differential scanning fluorimetry assays, using Biolog Phenotype Microarray
TM
plates, identified fifteen potential ligands for McpT
PR
, the majority classified as mono-, di-, and tri-carboxylates.
S. meliloti
exhibited positive chemotaxis toward seven selected carboxylates, namely, α-ketobutyrate, citrate, glyoxylate, malate, malonate, oxalate, and succinate. These carboxylates were detected in seed exudates of the alfalfa host. Deletion of
mcpT
resulted in a significant decrease of chemotaxis to all carboxylates except for citrate. Isothermal titration calorimetry revealed that McpT
PR
bound preferentially to the monocarboxylate glyoxylate, and with lower affinity to the dicarboxylates malate, malonate and oxalate. However, no direct binding was detected for the remaining three carboxylates that elicited an McpT-dependent chemotaxis response. Taken together, these results demonstrate that McpT is a broad range carboxylate chemoreceptor that mediates chemotactic response via direct ligand binding and an indirect mechanism that yet needs to be identified.
IMPORTANCE
Nitrate pollution is one of the most widespread and challenging environmental problems, mainly caused by the agricultural over-application of nitrogen fertilizers. Biological nitrogen fixation by the endosymbiont
Sinorhizobium meliloti
enhances the growth of its host
Medicago sativa
(alfalfa), which also efficiently supplies the soil with nitrogen. Establishment of the
S. meliloti
-
alfalfa symbiosis relies on the early exchange and recognition of chemical signals. The present study contributes to the disclosure of this complex molecular dialogue by investigating the underlying mechanisms of carboxylate sensing in
S. meliloti
. Understanding individual steps that govern
S. meliloti
-alfalfa molecular cross-talk helps in the development of efficient, commercial bacterial inoculants that promote the growth of this most cultivated forage legume in the world and improves soil fertility.