The general stress response has been extensively studied in Escherichia coli and Bacillus subtilis, in which σ S (RpoS) and σ B (SigB), respectively, were identified as master regulators controlling this response (for recent reviews, Hecker et al., 2007 Klauck et al., 2007). These include desiccation, pH shifts, osmotic stress, carbon starvation in bulk soil and presence of reactive oxygen species during the infection process ( Gibson et al., 2008). Under free‐living conditions in the bulk soil/rhizosphere as well as during invasion, rhizobia are exposed to distinct and fluctuating stresses. The latter symbiotic state represents a well‐balanced physiological interaction: the microsymbiont converts atmospheric nitrogen into ammonium that is provided to the plant host in exchange for carbon and energy generated by photosynthesis ( Prell and Poole, 2006). Rhizobia have different lifestyles either as free‐living bacteria in the soil and as bacteroids in nodules after infection and differentiation. Phylogenetically, they belong to the Proteobacteria, in which most of the currently described rhizobial strains fall into several dispersed lineages within the class of Alphaproteobacteria, and include genera such as Rhizobium, Sinorhizobium, Mesorhizobium, Bradyrhizobium, Azorhizobium and Methylobacterium ( Sawada et al., 2003). This finding was further substantiated by in vitro protein–protein interaction studies which are in line with a partner‐switching mechanism controlling gene regulation triggered by phosphorylation of PhyR. Microarray analysis revealed that PhyR and σ EcfG control congruent regulons suggesting both regulators are part of the same signalling cascade. Remarkably, both mutants triggered aberrant nodules on the different host plants with ectopically emerging roots. Four weeks after infection, soybean nodule development caught up to a large extent whereas most mungbean nodules remained defective even 5 weeks after infection. They induced fewer nodules than the wild type and these nodules were smaller, less pigmented, and their specific nitrogenase activity was drastically reduced 2 or 3 weeks after inoculation. Both mutants had symbiotic defects on the plant hosts Glycine max (soybean) and Vigna radiata (mungbean). Using deletion mutants and phenotypic assays, we showed that PhyR and the ECF σ factor σ EcfG are involved in heat shock and desiccation resistance upon carbon starvation.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |