Root attachment by microbes

Attachment and colonization of legume roots by rhizobia is one of the very early and critical steps in symbiosis. Only upon root-hair attachment can rhizobia colonize and invade plant cells, via infection threads. Legumes tightly regulate attachment and colonization by specific time-dependent signals and the ability to perceive and respond to such signals by rhizobia is a requirement for successful colonization (1).

Rhizobia respond to host-specific signals by activating a specific subset of genes, directed by extracytoplasmic function (ECF) sigma factors which are an important component in transmembrane signal transduction. Root attachment and colonization requires an orchestrated transcription of number of genes coding for polysaccharides (exo-, lipo-polysaccharides and glucomannan), extracellular proteins (rhicadesin and rhizobium adhering proteins), celluose fibrils and secretion systems (2). From our earlier rhizosphere transcriptomics studies, we identified a three-gene operon coding for lipoprotein (LppE), ECF sigma factor (EcfE) and anti-sigma factor (AsfE) to be highly and specifically transcribed during root colonization (3) and only weakly in lab culture which requires supplementing with phenylalanine.

In planta root colonization assays with wild-type harboring lppE-promoter lux fusion showed the spatial activation of lppE, emphasizing an important role of this operon in early stages of the Rhizobium-legume interaction. In vitro studies of lppE-promoter lux fusion induction by phenylalanine in different mutant backgrounds (lppE, ecfE and asfE) demonstrated that EcfE regulates the lppE operon.

Currently, we are employing cutting-edge Lux luminescence assays, confocal imaging, RNA-Seq, ChIP-Seq and Tn-Seq to discover how Rhizobium leguminosarum 3841 coordinates the concerted transcription of multiple genes involved in root attachment and colonization in response to the pea-derived signal.

1. Oldroyd, G.E.D., Murray, J.D., Poole, P.S. & Downie, J.A. (2011) The rules of engagement in legume-rhizobial symbiosis. Annual Review of Genetics 45:119-144.
2. Downie, J.A (2010) The roles of extracellular proteins, polysaccharides and signals in the interactions of rhizobia with legume roots. FEMS Microbiology Reviews 34:150-70.
3. Ramachandran, V., East, A.K., Karunakaran, R., Downie, J.A. & Poole, P.S. (2011) Adaptation of Rhizobium leguminosarum to pea, alfalfa and sugar beet rhizospheres investigated by comparative transcriptomics. Genome Biology 12:R106.