PSYM NOD GENE INFLUENCE ON ELICITATION OF PEROXIDASE-ACTIVITY FROM WHITE CLOVER AND PEA ROOTS BY RHIZOBIA AND THEIR CELL-FREE SUPERNATANTS

The activities of salt-elutable peroxidases from roots of white clover and pea were examined during the early interaction of these legume ho sts with strains of Rhizobium leguminosarum in homologous and heterolo gous combination. Peroxidase-specific activity from clover root hairs began to increase 6 hr after inoculation with R. l. by. viciae RL300 a nd was localized over the entire area of their deformations. In contra st, the onset of elicitation of peroxidase activity from root hairs wa s delayed after inoculation with R. l. by. trifolii ANU843 and was loc alized only at the site of infection thread initiation. Three wild-typ e strains (R. l. by. trifolii ANU843, R. l. by. viciae RL300 and 1003) and one hybrid transconjugant strain of R. leguminosarum containing p Sym from R. l. by. viciae 248 (RBL5715) elicited increased specific ac tivity of peroxidases eluted from pea and clover roots in heterologous combination. A comparison of peroxidase activity eluted from pea root s inoculated with ANU843 or its pSym-cured derivative indicated that p Sym is required for elicitation of peroxidase on this heterologous hos t. The level of peroxidase activity elicited by nodE mutants (which ha ve extended host range) is decreased on their new host. An extracellul ar fraction of RL300 contained flavonoid-dependent, heat-stable, and e thanol-soluble elicitor(s) of peroxidase activity. Treatment of clover seedlings with this cell-free fraction decreased the number of root h airs infected by ANU843. We propose that elicitation of root hair pero xidase may contribute to the infection process in this Rhizobium-legum e symbiosis by altering root hair wall structure at sites of incipient penetration.