SOYBEAN ROOTS INFECTED BY GLOMUS INTRARADICES STRAINS DIFFERING IN INFECTIVITY EXHIBIT DIFFERENTIAL CHITINASE AND BETA-1,3-GLUCANASE EXPRESSION

Different species and strains of arbuscular mycorrhizal (AM) fungi sho w different efficiencies of root colonization depending on the genetic background of both symbionts and on the edaphic conditions. It has be en shown previously that the expression of several plant defence-relat ed genes is differentially regulated in AM roots and some of these gen es might be involved in controlling intraradical fungal growth. In thi s study we evaluated the expression of chitinases and beta-1,3-glucana ses in soybean roots infected by two strains of Glomus intraradices Sh enck & Smith exhibiting different infectivities. G. intraradices strai n 26 was shown to be more infective than G. intraradices strain 25. En dochitinase activities were suppressed approx. twofold throughout the time-course in roots colonized by the more infective strain 26, but no t strain 25, compared with uninfected controls. Exochitinase activitie s were threefold higher in roots colonized by the more infective strai n 26 by the end of the experiment, but did not differ from controls in roots colonized by the less infective strain 25. Activities of beta-1 ,3-endoglucanase were not affected in roots colonized by the more infe ctive strain 26, and were suppressed in roots infected by the less inf ective strain 25 at 8 wk after planting (WAP). By contrast, the steady -state levels of a basic beta-1,3-endoglucanase mRNA (EG488) were two to threefold higher at 4 and 6 WAP in roots infected by either strain than in non-infected controls. These data suggest that one factor cont ributing to the differential colonization of plant roots by AM fungi s trains is their differential suppression of defence-related gene expre ssion, especially chitinases.