Differential expression of eight chitinase genes in Medicago truncatula roots during mycorrhiza formation, nodulation, and pathogen infection

Expression of eight different chitinase genes, representing members of five chitinase classes, was studied in Medicago truncatula roots during formati on of arbuscular mycorrhiza with Glomus intraradices, nodulation with Rhizo bium meliloti, and pathogen attack by Phytophthora megasperma f. sp. medica ginis, Fusarium solani f. sp. phaseoli (compatible interactions with root r ot symptoms), Ascochyta pisi (compatible, symptomless), and F. solani f. sp . pisi (incompatible, nonhost interaction). In the compatible plant-pathoge n interactions, expression of class I, II, and IV chitinase genes was enhan ced. The same genes were induced during nodulation. Transcripts of class I and II chitinase genes accumulated transiently during early stages of the i nteraction, and transcripts of the class IV chitinase gene accumulated in m ature nodules, The pattern of chitinase gene expression in mycorrhizal root s was markedly different: Expression of class I, II, and IV chitinase genes was not enhanced, whereas expression of three class III chitinase genes, w ith almost no basal expression, was strongly induced. Two of these three (M tchitinase III-2 and Mtchitinase III-3) were not induced at all in interact ions with pathogens and rhizobia Thus, the expression of two mycorrhiza-spe cific class III chitinase genes can be considered a hallmark for the establ ishment of arbuscular mycorrhiza in Medicago truncatula.