Abstract
Rhizobia are Gram-negative bacteria that can exist either as free-living bacteria or as nitrogen-fixing symbionts inside root nodules of leguminous plants. The composition of the rhizobial outer surface, containing a variety of polysaccharides, plays a significant role in the adaptation of these bacteria in both habitats. Among rhizobial polymers, exopolysaccharide (EPS) is indispensable for the invasion of a great majority of host plants which form indeterminate-type nodules. Various functions are ascribed to this heteropolymer, including protection against environmental stress and host defense, attachment to abiotic and biotic surfaces, and in signaling. The synthesis of EPS in rhizobia is a multi-step process regulated by several proteins at both transcriptional and post-transcriptional levels. Also, some environmental factors (carbon source, nitrogen and phosphate starvation, flavonoids) and stress conditions (osmolarity, ionic strength) affect EPS production. This paper discusses the recent data concerning the function of the genes required for EPS synthesis and the regulation of this process by several environmental signals. Up till now, the synthesis of rhizobial EPS has been best studied in two species, Sinorhizobium meliloti and Rhizobium leguminosarum. The latest data indicate that EPS synthesis in rhizobia undergoes very complex hierarchical regulation, in which proteins engaged in quorum sensing and the regulation of motility genes also participate. This finding enables a better understanding of the complex processes occurring in the rhizosphere which are crucial for successful colonization and infection of host plant roots.
Highlights
Rhizobia comprise a very diverse group of nitrogen-fixing symbiotic bacteria that belong to α and β subclasses of the Proteobacteria, and are members of several genera, including Rhizobium, Sinorhizobium, Mesorhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium and Methylobacterium (α-rhizobia), as well as Burkholderia and Cupriavidus (β-rhizobia) [1]
This review focuses on the genetic control of the biosynthesis of rhizobial EPS, the regulation of this process by different environmental factors and its relationship with other bacterial pathways
Many data concerning the synthesis of rhizobial EPS and regulation of this process by environmental factors have been obtained, especially for the species S. meliloti
Summary
Rhizobia comprise a very diverse group of nitrogen-fixing symbiotic bacteria that belong to α and β subclasses of the Proteobacteria, and are members of several genera, including Rhizobium, Sinorhizobium, Mesorhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium and Methylobacterium (α-rhizobia), as well as Burkholderia and Cupriavidus (β-rhizobia) [1]. The establishment of symbiosis is a very complex process involving a coordinated exchange of signals between the host plant and its microsymbiont, in which flavonoids secreted by plant roots and rhizobial lipochitin oligosaccharides (Nod factors) play key roles [2] Both the bacterial and plant cell surface components participate in this plant-microbe interaction [3,4]. CG-deficient mutants demonstrate an increased production of EPS [9] All these surface polysaccharides play an essential role in the establishment of an effective symbiosis, especially with host plants that form indeterminate-type nodules, such as Trifolium, Pisum, Vicia and Medicago spp. In nitrogen-fixing bacteria which establish symbioses with legumes forming indeterminate-type nodules, EPS is indispensable for successful infection of host plant roots [3]. This review focuses on the genetic control of the biosynthesis of rhizobial EPS, the regulation of this process by different environmental factors and its relationship with other bacterial pathways
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
