PRODUCTION AND EXCRETION OF NOD METABOLITES BY RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII ARE DISRUPTED BY THE SAME ENVIRONMENTAL-FACTORS THAT REDUCE NODULATION IN THE FIELD

Lipooligosaccharides (Nod metabolites) have been shown to be essential for the successful nodulation of legumes. In strains of Rhizobium leg uminosarum bv. trifolii, Nod metabolites were detected predominantly w ithin the cell and to a lesser extent in the periplasmic space and the growth medium. The production, and in particular the excretion, of No d metabolites was restricted by a range of environmental conditions wh ich are associated with poor nodulation in the field. Lowering the med ium pH from 7.0 to 5.0, reducing the phosphate concentration from 1 mM to 5 muM KH2PO4, and lowering the incubation temperature from 28 to 1 8-degrees-C affected the number and relative concentrations of the Nod metabolites made. The form and concentration of the nitrogen source a ffected the relative concentrations of the Nod metabolites produced an d excreted. KNO3 concentrations of >10 mM did not affect cell growth r ate but substantially reduced the number of Nod metabolites released. Environmental stresses differentially altered Nod metabolite productio n and excretion in the same strain carrying different introduced nod r egions. Strain ANU845(pWLH1) produced and excreted comparatively fewer Nod metabolites at pH 5.0 and at 18-degrees-C than strain ANU845(pRI4 003). The excretion but not the production of Nod metabolites by strai n ANU845(pRtO32) was dependent on the presence of both nodI and nodJ. Tn5-induced nodI and nodj mutants did not accumulate any metabolites e ither outside the cell or within the outer membrane or periplasmic spa ce. Recognition that Nod metabolite accumulation is a complex system o f production and excretion, with each component responding differently to changes in environmental conditions, has many consequences, both a t the molecular level and in the field. The ability of different strai ns to produce and release Nod metabolites is likely to be a major dete rminant of nodule occupancy and should be considered when screening st rains suitable for adverse environments.