The Sulfate Group on the Reducing End Protects Nod Signals of R. Meliloti Against Hydrolysis by Medicago Chitinases

Abstract

Root nodule initiation on leguminous plants is triggered by lipooligosaccharide signals (Nod factors) produced by rhizobia. Different Rhizobium strains produce different sets of Nod factors with specific modifications of the chitooligosaccharide backbone, and these modifications appear to determine host-specificity, as determined in bioassays with pure Nod factors (Lerouge et al., 1990; Spaink et al., 1991; Schultze et al., 1992; Sanjuan et al., 1992; Price et al., 1992). A sulfate group at the O-6 position of the reducing end is required for activity of R. meliloti Nod signals on Medicago. At the same time, the presence of the sulfate group strongly reduces the activity on the non-host plant Vicia sativa. Conversely, non-sulfated Nod factors produced by nodH mutants of R. meliloti are active on Vicia but not on Medicago (Roche et al., 1991). The length of the oligosaccharide chain also influences the activity of these molecules (Schultze et al., 1992). In addition, a highly unsaturated acyl chain was shown to be essential for the biological activity of Nod factors purified from R. leguminosarum by viciae (Spaink et al., 1991).