Plant Signals to Soil Microbes: Regulators of Rhizosphere Colonization

Abstract

New approaches are needed to identify key plant signals and bacterial genes that control root colonization. Attempts to develop signal-responsive mutants of Rhizobium meliloti and Agrobacterium tumefaciens which can measure the contribution of particular lucerne (Medicago sativa) compounds to root colonization by these bacteria are described. Mutant bacteria showed various growth and competition phenotypes in the lucerne rhizosphere, but wild type Nod, Fix and Vir phenotypes were not altered. Three mutant strains of Rhizobium meliloti were found to require biotin and two were unable to take up stachydrine. The isoflavonoid-responsive mutants in A, tumefaciens grew well in the rhizosphere in single strain tests, but grew poorly in competition with the isogenic wild type. The results suggest that endogenous biotin production is one factor in rhizosphere competence and that stachydrine has a role as regulator of rhizosphere microbes.