Even though the mechanisms controlling intraradical colonization by arbuscular mycorrhizal fungi are not yet known, they may involve the plant defense system. Normally, under appropriate conditions for mycorrhiza formation, e.g. low phosphate (P), the expression of plant defense genes, such as chitinases and b-1,3-glucanases, is suppressed. Under high P conditions, the inhibition of intraradical fungal growth may be due to an attenuation of the suppression and/or induction of specific plant defense genes. If the plant defense system limits intraradical fungal growth under high P conditions, then the application of a plant defense inducer to the roots, such as salicylic acid (SA), might have the same effect as high P. The aim of this work was to evaluate the effects of SA on the intraradical colonization and on the activities of chitinases and b-1,3-glucanases in bean (Phaseolus vulgaris L. cv. Carioca 80-SH) roots inoculated with Glomus clarum or Glomus intraradices, and grown under low and high P conditions. Under low P, the application of SA to bean roots resulted in inhibition of intraradical fungal colonization, to the same levels as observed for high P conditions. This inhibition was even greater under high P conditions. Associated with the inhibition of root colonization, a 10-fold increase in chitinase and reduction of b-1,3-glucanase activities were observed in roots that received SA. Based on the patterns of chitinase and b-1,3-glucanase activities in not-inoculated and inoculated roots, it was not possible to establish a relationship between activities of these hidrolases and intraradical fungal growth.
plant defense response; intraradical colonization; phosphate
