Differential expression of defense-related genes in arbuscular mycorrhiza

Abstract

Differential expression of plant defense related genes may contribute to the control of arbuscular mycorrhiza growth and development. The expression of chitinases, β-1, 3-glucanases, and genes involved in the biosynthesis of isoflavonoid phytoalexins has been studied in these symbiotic associations. Relative to noninfected roots, mycorrhizal roots showed a transient induction of endochitinase activities followed by suppression, under low and high P conditions. The suppression was attenuated under high P. Differential reductions in the steady-state level of mRNAs encoding a basic and an acidic endochitinase was observed. Suppression of β-1, 3-glucanase activities, two glucanase mRNAs, and chalcone isomerase mRNA levels was also observed. In situ localization of several defense-related mRNAs showed a predominant accumulation in the vascular cylinder of mycorrhizal and noninfected roots, under both P conditions. The levels of these mRNAs were systemically suppressed in arbuscular mycorrhizae (AM). Additionally, the accumulation of mRNAs encoding an acidic endochitinase and a β-1, 3-endoglucanase was enhanced in and around cells containing arbuscules, under high and low P conditions, respectively. The differential regulation of defense-related genes may contribute to the observed pattern of intraradical fungal growth. A hypothetical model for the functional roles and regulation of endochitinases and β-1, 3-endoglucanases in arbuscular mycorrhiza is proposed. Key words: arbuscular mycorrhiza, differential gene expression, plant defense genes, chitinases, β-1, 3-glucanases, phosphate.