Spatial distribution of chitinases and β‐1,3‐glucanase transcripts in 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 localization of three defence‐related mRNAs in non‐infected bean roots and in relation to fungal infection units. In non‐infected and infected roots, mRNAs encoding acidic and basic endochitinases were generally most abundant in the vascular cylinder. High‐P‐grown mycorrhizal roots showed localized accumulation of the acidic endochitinase mRNA in cortical cells containing arbuscules and in their immediate vicinity (one to five cell layers). The pattern of accumulation of the basic endochitinase mRNA was not affected by P or AM fungal infection. At the low P concentration, the β‐1,3‐glucanase mRNA accumulated predominantly in the vascular cylinder of non‐infected roots. Suppression of β‐1,3‐glucanase 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 infection units, characterizing a systemic effect. Beta‐1,3‐glucanase mRNA accumulated also around a number of cortical cells containing arbuscules only at the low P concentration. The localized accumulations of the endochitinase and β‐1,3‐glucanase mRNAs suggest that the encoded proteins might be involved in the control of intraradical fungal growth.