SUPPRESSION OF ENDOCHITINASE, BETA-1,3-ENDOGLUCANASE, AND CHALCONE ISOMERASE EXPRESSION IN BEAN VESICULAR-ARBUSCULAR MYCORRHIZAL ROOTS UNDER DIFFERENT SOIL PHOSPHATE CONDITIONS

Vesicular-arbuscular (VA) mycorrhizal fungi colonize the root cortex s uccessfully under low soil phosphate (P) conditions, whereas infection is delayed or aborted under high P conditions. We evaluated the possi bility of an increased expression of endochitinase, beta-1,3-endogluca nase, and phytoalexin pathway enzymes that might contribute to inhibit ion of intraradical fungal growth under high P concentrations. Under l ow and high P concentrations, bean mycorrhizal roots showed an early i nduction of endochitinase activity followed by suppression relative to the noninfected controls. Under low P, maximal suppression occurred d uring the period of maximal fungal growth rate. Suppression of endochi tinase activity was attenuated under high P concentration (up to 1.9-f old) compared with the suppression under low P (up to 3.7-fold). Suppr ession under both conditions was accompanied by differential reduction s in the levels of mRNAs encoding two endochitinase isoforms. Suppress ion of beta-1,3-endoglucanase activity was also observed at certain st ages of the mycorrhiza development under both P concentrations. The le vels of two beta-1,3-endoglucanase mRNAs encoding distinct isoforms we re differentially suppressed (up to 13-fold) in response to mycorrhiza formation under both P levels. In addition, the chalcone isomerase mR NA level was suppressed (up to 2.4-fold) in mycorrhizal roots under bo th P concentrations. Our data suggest that there is a general suppress ion of the plant defense response during VA mycorrhiza development. Po ssible mechanisms of regulation and functions of endochitinase and bet a-1,3-endoglucanase in VA mycorrhiza are discussed.