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 successfully under low soil phosphate (P) conditions, whereas infection is delayed or aborted under high P conditions. We evaluated the possibility of an increased expression of endochitinase, beta-1,3-endoglucanase, and phytoalexin pathway enzymes that might contribute to inhibition of intraradical fungal growth under high P concentrations. Under low and high P concentrations, bean mycorrhizal roots showed an early induction of endochitinase activity followed by suppression relative to the noninfected controls. Under low P, maximal suppression occurred during the period of maximal fungal growth rate. Suppression of endochitinase activity was attenuated under high P concentration (up to 1.9-fold) compared with the suppression under low P (up to 3.7-fold). Suppression under both conditions was accompanied by differential reductions in the levels of mRNAs encoding two endochitinase isoforms. Suppression of beta-1,3-endoglucanase activity was also observed at certain stages of the mycorrhiza development under both P concentrations. The levels of two beta-1,3-endoglucanase mRNAs encoding distinct isoforms were differentially suppressed (up to 13-fold) in response to mycorrhiza formation under both P levels. In addition, the chalcone isomerase mRNA level was suppressed (up to 2.4-fold) in mycorrhizal roots under both P concentrations. Our data suggest that there is a general suppression of the plant defense response during VA mycorrhiza development. Possible mechanisms of regulation and functions of endochitinase and beta-1,3-endoglucanase in VA mycorrhiza are discussed.