Genome wide screening of Ensifer meliloti genes required for rhizosphere colonization. Specificity traits and evolutionary relationships

LAGARES, A.; BECKER, A.; PARISI, G.; DEL PAPA, M.F.; DRAGHI, W. O.; LÓPEZ, J. L.; SALAS, M, E.; SERRANIA, JAVIER; PISTORIO, M.; LOZANO, M. J.

Granada

Congreso; 20th International Congress on Nitrogen Fixation; 2017

Rhizobia are α- and ß-proteobacteria that associate with legumes in symbiosis to fix atmospheric nitrogen. The early stages of these associations have been investigated with special interest since rhizobial competition for host space begins in the rhizosphere before any physical contact between both partners (1). Genetic studies on how rhizobia colonize rhizospheres, however, have been difficult experimentally with few studies addressing this issue (2, 3). Here, by using signature-tagged mutagenesis (STM) (4), we identified more than a hundred Ensifer meliloti genes that are relevant to rhizosphere colonization. The results indicated that rhizobial genes required for an efficient rhizosphere colonization account for nearly 2% of the bacterial genome and that most (ca. 80%) are chromosomally located, pointing to the relevance and ancestral origin of the bacterial ability to colonize plant roots. The identified genes that affected rhizosphere colonization are related to metabolic functions, transcription, signal transduction, motility and chemotaxis, with several ORFs of yet unknown function. Most remarkably, some of those genes were required for the rhizobia to express a preferential colonization of both host (Medicago, Trigonella) and phylogenetically related non-host (Trifolium) plants (specificity), when compared to the colonization of other legumes like pea or soybean. The results helped dating such preferential-colonization phenotype as likely appearing 17 ? 45 MYA, and shed light on the genetic and evolutionary basis of the early interaction of rhizobia with roots. (1) Triplett EW, Sadowsky MJ. 1992. Annu Rev Microbiol. 46: 399-422 (2) Ramachandran et al. 2011. Genome Biol. 12: R106 (3) Barr et al. 2008. FEMS Microbiol Lett 282: 219-27 (4) Pobigaylo et al. 2006. Appl Environ Microbiol. 72: 4329-37 This work was funded by CONICET PIP 2014-0420; the Ministry of Science Technology and Productive Innovation (MinCyT, Argentina) PICT-2012-1719 and PICT-2015-2452; and a bilateral Cooperation Program between MinCyT-DAAD (Deutscher Academischer Austausch Dienst) DA/12/08