Conjugation of rhizobial plasmids: preferred niches.

LUIS BAÑUELOS; LAURA CERVANTES; GONZALO A. TORRES TEJERIZO; DAVID ROMERO; LOURDE GIRARD; SUSANA BROM

Seattle

Conferencia; Plasmid Biology 2018; 2018

Rhizobia are a group of bacteria able to establish a symbiosis with theroots of leguminous plants through the formation of "nodules", where thebacteria fix atmospheric nitrogen and provide it, in organic form, to the plant. Inexchange, bacteria receive a carbon source and other nutrients.The genomes of rhizobia usually contain large plasmids in addition to thechromosome. In Rhizobium etli, a symbiont of common bean, informationcarried in the plasmids includes genes required to establish the symbiosis,metabolic functions, lipopolysaccharide biosynthesis, and even informationessential for bacterial growth. Also, some of the plasmids are able to performconjugative transfer.We have previously studied the conjugative transfer ability of plasmidpRet42a from R. etli CFN42. This plasmid contains all elements required fortransfer, and the system is regulated by quorum-sensing, when performing theexperiments in the laboratory.In this work, we have determined the conjugative transfer of plasmidpRet42a in media in the presence of plants, on the surface of the roots andduring the establishment of the symbiosis, using a donor that carries an RFPmarker on the chromosome and a GFP marker on pRet42a, allowing us toemploy different methods to differentiate donor, recipient and transconjugantpopulations (Torres Tejerizo et al., 2015). The results show that conjugativetransfer takes place in the media, and increases in presence of the plant. Thiscould be due to a better availability of nutrients provided by the root exudatesand/or a direct stimulus to the transfer system by specific compounds. Also, wedetected transconjugants on the roots surface, and inside the symbioticstructures. To find out if the transconjugants we observed in the symbioticstructures were being generated there, or if the conjugation took place only inthe media and root surface, we determined the expression levels of transfergenes inside the plant structures, as well as the appearance of transconjugantsin the symbiotic structures in conditions where conjugation was partially ortotally inhibited on the plant exterior. The results strongly suggest thatconjugation may occurr inside the symbiotic structures. This opens a newfunction for nodulation, as a protected niche for the generation of bacterialdiversity.