Host legume‐exuded antimetabolites optimize the symbiotic rhizosphere

Abstract

SummaryRhizobia form symbiotic nodules on host legumes and fix nitrogen for their hosts in exchange for nutrients. In order to establish this mutually beneficial relationship, rhizobia must compete with other soil bacteria in the host legume rhizosphere to colonize plant roots efficiently. A promoter-trap transposon screen in Mesorhizobium tianshanense, a Rhizobium that forms nodules on licorice (Glycyrrhiza uralensis) plants revealed that the expression of msiA, which encodes a putative exporter protein belonging to the LysE family of translocators, is activated by both legume exudates and MsiR, a LysR family transcriptional regulator. Chemical analysis suggests that the msiA-inducing signal in exudates is canavanine, an anti-metabolite present in the seeds and exudates of a variety of legume plants. We show that MsiA serves as a canavanine exporter that is indispensable for canavanine resistance in M. tianshanense. We also show that the expression of MsiA homologues in other rhizobial species is induced by canavanine and is critical for canavanine resistance. Furthermore, rhizobial canavanine resistance is important for root hair adherence as well as for survival in a canavanineproducing legume rhizosphere. Together, these data suggest that host legumes may exude specific antimetabolites into their surroundings to optimize the bacterial population in order to have successful symbiotic events with rhizobia.

Application of Natural Blends of Phytochemicals Derived from the Root Exudates of Arabidopsis to the Soil Reveal That Phenolic-related Compounds Predominantly Modulate the Soil Microbiome

“…For instance, the compound canavanine secreted from the seed coat or roots of leguminous plants acts as an antimicrobial for many rhizosphere bacteria but not for Rhizobia, suggesting that the host plant secretes this compound for selection of the beneficial Rhizobia (13). In the present study we observed a few compounds such as GABA, ferulic acid, salicylic acid, idonic acid, and isoleucine that show a positive correlation with some OTUs while also being negatively correlated with other OTUs.…”

“…Yet when soil nitrogen is not limiting to the plant, this symbiosis does not occur (12). On the other hand, certain legumes release canavanine, an antimicrobial that acts against a broad range of microbes without affecting Rhizobia, which further cultures this beneficial microbe in the rhizosphere of legumes (13). Other plant roots release strigolactones (sesquiterpenes) to attract mycorrhizae, and parasitic plants have benefited from these chemical cues to recognize their host plants (14,15).…”

Plant nodulation inducers enhance horizontal gene transfer of Azorhizobium caulinodans symbiosis island

“…Plants were grown in a plant growth chamber at 28°C with a 12h/ 12h day/night cycle, and sterile nitrogen-free plant nutrient solution (44) was provided. Vermiculite samples (0.5 g) around the roots (after removing 1 cm surface vermiculite) were withdrawn at the time points indicated and resuspended in sterilized H 2 O. HGT events were quantified by determining cfus of transconjugants and recipients on selective plates as described above.…”

“…To confirm whether the ICE Ac was excised and circularized, primers pairs a2/a3 and PCR products were sequenced to confirm the excision positions. To determine the ICE Ac insertion site in transconjugants, arbitrary PCR was performed (44) to amplify the flanking sequences of ICE Ac and the PCR product was then sequenced. To measure the circularization frequency, qPCR was performed by using DNA from cultures of A. caulinodans grown to different time points indicated as templates and RT-a2/RT-a3 as primers.…”

The Quorum Sensing Regulator CinR Hierarchically Regulates Two Other Quorum Sensing Pathways in Ligand-Dependent and -Independent Fashions in Rhizobium etli

“…The deletions in each mutant strain were confirmed by PCR examination and sequence analysis. Each plasmid that constitutively expressed cinI, cinR, raiI, raiR, traI, and traR1-traR2 was constructed by cloning these genes into the pYC12 vector (15), and constructs were introduced into R. etli and E. coli strains by electroporation. Primers used in this study are listed in Table S1 in the supplemental material. AHL bioassays and identification.…”