A VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGUS (GLOMUS INTRARADIX) INDUCESA DEFENSE RESPONSE IN ALFALFA ROOTS

Flavonoid accumulation and activities of phenylalanine ammonia-lyase ( PAL), chalcone isomerase (CHI), and chitinase were followed during ear ly colonization of alfalfa roots (Medicago sativa L. cv Gilboa) by ves icular arbuscular (VA) fungi (Glomus infraradix). Formononetin was the only flavonoid detected that showed a consistent increase in the inoc ulated roots. This increase depended only on the presence of the fungu s in the plant rhizosphere; no colonization of the root tissue was req uired. CHI and chitinase activities increased in inoculated roots prio r to colonization, whereas the increase in PAL activity coincided with colonization. After reaching a maximum, activities of all enzymes dec lined to below those of uninoculated roots. PAL inactivation was not c aused by a soluble inhibitor. Our results indicate that VA fungi initi ate a host defense response in alfalfa roots, which is subsequently su ppressed.