SPATIAL-DISTRIBUTION OF CHITINASES AND BETA-1,3-GLUCANASE TRANSCRIPTSIN BEAN ARBUSCULAR MYCORRHIZAL ROOTS UNDER LOW AND HIGH SOIL PHOSPHATE CONDITIONS

Arbuscular mycorrhizal (AM) fungi extensively colonize the root cortex under low-soil-phosphate (P) conditions, whereas infection is limited under high-P conditions. Fungal growth under both P conditions might be influenced by plant defence-related gene expression. In this study, we used in situ hybridization methods to compare the cellular localiz ation of three defence-related mRNAs in non-infected bean roots and in relation to fungal infection units. In non-infected and infected root s, mRNAs encoding acidic and basic endochitinases were generally most abundant in the vascular cylinder. High-P-grown mycorrhizal roots show ed localized accumulation of the acidic endochitinase mRNA in cortical cells containing arbuscules and in their immediate vicinity (one to f ive cell layers). The pattern of accumulation of the basic endochitina se mRNA was not affected by P or AM fungal infection. At the low P con centration, the beta-1,3-glucanase mRNA accumulated predominantly in t he vascular cylinder of noninfected roots. Suppression of beta-1,3-glu canase mRNA accumulation in these tissues was observed in non-infected roots at the high-P and in mycorrhizal roots at both P concentrations . The observed suppression extends at least several mm from fungal inf ection units, characterizing a systemic effect. Beta-1,3-glucanase mRN A accumulated also around a number of cortical cells containing arbusc ules only at the low P concentration. The localized accumulations of t he endochitinase and beta-1,3-glucanase mRNAs suggest that the encoded proteins might be involved in the control of intraradical fungal grow th.