Defense-Related Transcript Accumulation in Phaseolus vulgaris L. Colonized by the Arbuscular Mycorrhizal Fungus Glomus intraradices Schenck Smith.

Accumulation of mRNAs for the defense-related genes phenylalanine ammonia-lyase (PAL), chalcone synthase, chitinase (CHT), glucanase, and hydroxyproline-rich glycoprotein were examined in roots of dark red kidney bean (Phaseolus vulgaris L. cv Moncalm) colonized by the arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith. In three separate experiments the root length colonized ranged from 28 to 55% by 28 d after planting and inoculation. RNA blot analysis revealed little change in the accumulations of PAL, chalcone synthase, CHT, glucanase, and hydroxyproline-rich glycoprotein transcripts from the 28-d mycorrhizal roots compared to the uninoculated controls. Normalizing the ratios of defense-related transcript accumulation against RNA pools regarded as being constitutively expressed, actin mRNA, 25S rRNA, and 18S rRNA, indicated that changes in the ratios of up to 20% occur according to the RNA pool used for normalization. In situ hybridizations of colonized roots using probes for PAL and CHT showed that accumulations of both transcripts occurred only in arbusculated cells. Both young, finely branched arbuscules and older, clumped arbuscules displayed PAL and CHT message accumulations. The PAL and CHT mRNA accumulations were greater in cortical cells containing young arbuscules than in cells containing clumped arbuscules. Intercellular hyphae and vesicles elicited no response.