DNA Homology Group and the Identity of Bradyrhizobial Strains Producing Rhizobitoxine‐Induced Foliar Chlorosis on Soybean

Abstract

A distinctive foliar chlorosis of soybean, Glycine max (L.) Merr., has been traced to nodulation by certain strains of bradyrhizobia that produce the substance rhizobitoxine in the root nodules. Strains of Badyrhyzobiurn nodulating soybean have been classified into three groups by relative DNA homology. To define the possible relationship between chlorosis induction and DNA homology group, 25 bradyrhizobial strains identified by DNA homology group were tested for chlorosis induction on the chlorosis susceptible soybean Line N53‐ 3494. Stationary‐phase broth cultures of bacterial strains were inoculated onto surface‐sterilized seeds planted in vermiculite in Leonard jars. Plants were scored visually for expression of rhizobitoxineinduced foliar chlorosis 35 d after planting. None of the 16 strains in DNA homology Groups 1 and la produced rhizobitoxine‐induced foliar chlorosis. Only strains in DNA homology Group 2 induced chlorosis, but not all of the strains of this group induced chlorosis. Five of the nine strains examined in Group 2 produced chlorosis symptoms in this test. Of the four remaining strains one was previously reported by another investigator to produce mild chlorosis symptoms and another strain was reported to produce rhizobitoxine in vitro. These results indicate that bradyrhizobial strains isolated from soybean in the USA that induce foliar chlorosis are of a distinct DNA group.