DIFFERENTIAL EXPRESSION OF DEFENSE-RELATED GENES IN ARBUSCULAR MYCORRHIZA

Differential expression of plant defense related genes may contribute to the control of arbuscular mycorrhiza growth and development. The ex pression of chitinases, beta-1,3-glucanases, and genes involved in the biosynthesis of isoflavonoid phytoalexins has been studied in these s ymbiotic associations. Relative to noninfected roots, mycorrhizal root s showed a transient induction of endochitinase activities followed by suppression, under low and high P conditions. The suppression was att enuated under high P. Differential reductions in the steady-state leve l of mRNAs encoding a basic and an acidic endochitinase was observed. Suppression of beta-1,3-glucanase activities, two glucanase mRNAs, and chalcone isomerase mRNA levels was also observed. In situ localizatio n of several defense-related mRNAs showed a predominant accumulation i n the vascular cylinder of mycorrhizal and noninfected roots, under bo th P conditions. The levels of these mRNAs were systemically suppresse d in arbuscular mycorrhizae (AM). Additionally, the accumulation of mR NAs encoding an acidic endochitinase and a beta-1,3-endoglucanase was enhanced in and around cells containing arbuscules, under high and low P conditions, respectively. The differential regulation of defense-re lated genes may contribute to the observed pattern of intraradical fun gal growth. A hypothetical model for the functional roles and regulati on of endochitinases and beta-1,3-endoglucanases in arbuscular mycorrh iza is proposed.