Abstract
BackgroundRhizobium leguminosarum bv. trifolii is a symbiotic nitrogen-fixing bacterium that elicits nodules on roots of host plants Trifolium spp. Bacterial surface polysaccharides are crucial for establishment of a successful symbiosis with legumes that form indeterminate-type nodules, such as Trifolium, Pisum, Vicia, and Medicago spp. and aid the bacterium in withstanding osmotic and other environmental stresses. Recently, the R. leguminosarum bv. trifolii RosR regulatory protein which controls exopolysaccharide production has been identified and characterized.ResultsIn this work, we extend our earlier studies to the characterization of rosR mutants which exhibit pleiotropic phenotypes. The mutants produce three times less exopolysaccharide than the wild type, and the low-molecular-weight fraction in that polymer is greatly reduced. Mutation in rosR also results in quantitative alterations in the polysaccharide constituent of lipopolysaccharide. The rosR mutants are more sensitive to surface-active detergents, antibiotics of the beta-lactam group and some osmolytes, indicating changes in the bacterial membranes. In addition, the rosR mutants exhibit significant decrease in motility and form a biofilm on plastic surfaces, which differs significantly in depth, architecture, and bacterial viability from that of the wild type. The most striking effect of rosR mutation is the considerably decreased attachment and colonization of root hairs, indicating that the mutation affects the first stage of the invasion process. Infection threads initiate at a drastically reduced rate and frequently abort before they reach the base of root hairs. Although these mutants form nodules on clover, they are unable to fix nitrogen and are outcompeted by the wild type in mixed inoculations, demonstrating that functional rosR is important for competitive nodulation.ConclusionsThis report demonstrates the significant role RosR regulatory protein plays in bacterial stress adaptation and in the symbiotic relationship between clover and R. leguminosarum bv. trifolii 24.2.
Highlights
Rhizobium leguminosarum bv. trifolii is a symbiotic nitrogen-fixing bacterium that elicits nodules on roots of host plants Trifolium spp
Biosynthesis of EPS in R. leguminosarum is a multi-step process requiring the expression of several pss genes, located in the major EPS cluster on the chromosome [12,13]. This region includes pss genes encoding specific glycosyl transferases, epimerases and deacetylases involved in the biosynthesis of EPS repeating units, genes encoding proteins engaged in the polymerization and transport of EPS, and other genes that code for EPS modifying enzymes [12,13]
R. leguminosarum bv. trifolii rosR mutants Recently, we described R. leguminosarum bv. trifolii 24.2 derivatives mutated in the rosR open reading frame (Rt2440 and Rt2472) [23,29]
Summary
Rhizobium leguminosarum bv. trifolii is a symbiotic nitrogen-fixing bacterium that elicits nodules on roots of host plants Trifolium spp. Trifolii is a symbiotic nitrogen-fixing bacterium that elicits nodules on roots of host plants Trifolium spp. Exopolysaccharide (EPS) and lipopolysaccharide (LPS), play an important role in determining the symbiotic competence of rhizobia, root tissue invasion and induction of nitrogen-fixing nodules on host plants forming indeterminate-type nodules, such pss genes encoding specific glycosyl transferases, epimerases and deacetylases involved in the biosynthesis of EPS repeating units, genes encoding proteins engaged in the polymerization and transport of EPS, and other genes that code for EPS modifying enzymes [12,13]. As has been established for R. leguminosarum and Sinorhizobium (Ensifer) meliloti, EPS plays an important role in biofilm development, being the major matrix component [14,15,16,17]. Glycanases PlyA and PlyB secreted via the PrsD-PrsE type I secretion system are responsible for EPS modification and biofilm formation. RapC, RapA1, and RapA2 agglutinins engaged in the adhesion and aggregation of rhizobia are secreted via the PrsD-PrsE type I secretion system [14,21,22]
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