pSym nod Gene Influence on Elicitation of Peroxidase Activity from White Clover and Pea Roots by Rhizobia and Their Cell-Free Supernatants. Janet L. Salzwedel. Department of Microbiology, Michigan State University, East Lansing 48824 U.S.A. Frank B. Dazzo. Department of Microbiology, Michigan State University, East Lansing 48824 U.S.A. MPMI 6:127-134. Accepted 19 October 1992. Copyright 1993 The American Phytopathological Society.

The activities of salt-elutable peroxidases from roots of white clover and pea were examined during the early interaction of these legume hosts with strains of Rhizobium leguminosarum in homologous and heterologous combination. Peroxidase-specific activity from clover root hairs began to increase 6 hr after inoculation with R. l. bv. viciae RL300 and was localized over the entire area of their deformations. In contrast, the onset of elicitation of peroxidase activity from root hairs was delayed after inoculation with R. l. bv. trifolii ANU843 and was localized only at the site of infection thread initiation. Three wild-type strains (R. l. bv. trifolii ANU843, R. l. bv. viciae RL300 and 1003) and one hybrid transconjugant strain of R. leguminosarum containing pSym from R. l. bv. viciae 248 (RBL5715) elicited increased specific activity of peroxidases eluted from pea and clover roots in heterologous combination. A comparison of peroxidase activity eluted from pea roots inoculated with ANU843 or its pSym-cured derivative indicated that pSym is required for elicitation of peroxidase on this heterologous host. The level of peroxidase activity elicited by nodE mutants (which have extended host range) is decreased on their new host. An extracellular fraction of RL300 contained flavonoid-dependent, heat-stable, and ethanol-soluble elicitor(s) of peroxidase activity. Treatment of clover seedlings with this cell-free fraction decreased the number of root hairs infected by ANU843. We propose that elicitation of root hair peroxidase may contribute to the infection process in this Rhizobium-legume symbiosis by altering root hair wall structure at sites of incipient penetration.