Expression of Nodulation Genes in Rhizobium and Acid-Sensitivity of Nodule Formation

Abstract

The formation of nitrogen-fixing nodules on roots of leguminous plants by Rhizobium spp. involves a complex interaction between host plant and symbiont. Successful nodulation requires the coordinated expression of several nodulation (nod) genes in the bacteria. The expression of these genes is induced by flavonoid compounds present in root exudates of host plants. Growth of Rhizobium spp. and formation of nodules on roots of leguminous plants is known to be adversely affected by low pH and factors associated with soil acidity, but the basis of this acid-sensitivity is poorly understood. We consider that poor induction of nodulation gene expression in Rhizobium is a major factor contributing to the acid-sensitivity of nodulation formation. At low pH, induction of nod gene expression in R. leguminosarum biovar trifolii is markedly reduced in the presence of flavone-inducer. Furthermore, inducibility of nod gene expression in R. leguminosarum bv. trifolii is also affected by a net reduction in the concentration of nod gene-inducing factors present in the root exudates of clover seedlings grown in acidic conditions.