Analysis of Quorum-Sensing-Dependent Control of Rhizosphere-Expressed ( rhi ) Genes in Rhizobium leguminosarum bv. viciae

Abstract

The rhi genes of Rhizobium leguminosarumbiovar viciae are expressed in the rhizosphere and play a role in the interaction with legumes, such as the pea. Previously (K. M. Gray, J. P. Pearson, J. A. Downie, B. E. A. Boboye, and E. P. Greenberg, J. Bacteriol. 178:372–376, 1996) therhiABC operon had been shown to be regulated by RhiR and to be induced by addedN-(3-hydroxy-7-cis-tetradecenoyl)-l-homoserine lactone (3OH,C14:1-HSL). Mutagenesis of a cosmid carrying the rhiABC and rhiR gene region identified a gene (rhiI) that affects the level of rhiAexpression. Mutation of rhiI slightly increased the number of nodules formed on the pea. The rhiI gene is (likerhiA) regulated by rhiR in a cell density-dependent manner. RhiI is similar to LuxI and other proteins involved in the synthesis of N-acyl-homoserine lactones (AHLs). Chemical analyses of spent culture supernatants demonstrated that RhiI produces N-(hexanoyl)-l-homoserine lactone (C6-HSL) andN-(octanoyl)-l-homoserine lactone (C8-HSL). Both of these AHLs induced rhiA-lacZand rhiI-lacZ expression on plasmids introduced into anAgrobacterium strain that produces no AHLs, showing thatrhiI is positively regulated by autoinduction. However, in this system no induction of rhiA or rhiI with 3OH,C14:1-HSL was observed. Analysis of the spent culture supernatant of the wild-type R. leguminosarum bv. viciae revealed that at least seven different AHLs are made. Mutation ofrhiI decreased the amounts of C6-HSL and C8-HSL but did not block their formation, and in this background the rhiI mutation did not significantly affect the expression levels of the rhiI gene orrhiABC genes or the accumulation of RhiA protein. These observations suggest that there are additional loci involved in AHL production in R. leguminosarum bv. viciae and that they affect rhiI and rhiABC expression. We postulate that the previously observed induction of rhiA by 3OH,C14:1-HSL may be due to an indirect effect caused by induction of other AHL production loci.

Halogenated furanones from the red alga, Delisea pulchra, inhibit carbapenem antibiotic synthesis and exoenzyme virulence factor production in the phytopathogen Erwinia carotovora

“…Gram-negative bacteria can engage in AHL dependent phytopathogenic [16] and phytosymbiotic [17] relationships with terrestrial plants. E. carotovora for example produces extracellular enzymes required for the infection of a variety of terrestrial plant species.…”

Quorum sensing and swarming migration in bacteria

“…Thirdly, signal exchange between different rhizobia in the rhizosphere is based on the production of quorum sensing signal molecules such as AHLs allowing the whole population to initiate a concerted action once a critical concentration has been reached e.g. [43,[220][221][222][223][224][225]. Recently, a new class of quorum sensing molecules involved in symbiotic gene regulation was identified in Bradyrhizobium japonicum (Fig.…”

An integrated view of biofilm formation in rhizobia

“…Quorum-sensing mechanisms control numerous functions in rhizobia, including exopolysaccharide production (Marketon et al, 2003;Hoang et al, 2004;Glenn et al, 2007), motility and nitrogen fixation (Hoang et al, 2004(Hoang et al, , 2008, and nodulation (Cubo et al, 1992;Rodelas et al, 1999;Daniels et al, 2002;Hoang et al, 2004), all of which are related to symbiosis. The studies cited in this section show clearly that Mesorhizobium is one of the genera of bacteria in which quorum sensing plays an important role in biofilm formation, attachment, colonization, and nodulation of legumes.…”