Suppression of Tobacco Basic Chitinase Gene Expression in Response to Colonization by the Arbuscular Mycorrhizal Fungus Glomus intraradices

A differentially displayed cDNA clone (MD17) was isolated from tobacco roots (nicotiana tabacum cv. Xanthi-nc) infected with the arbuscular mycorrhizal (AM) fungus Glomus intraradices. The isolated DNA fragment exhibited a reduced level of expression in response to AM establishment and 90% identity with the 3' noncoding sequence of two basic chitinases (EC 3.2.1.14) from N. tabacum. Northern (RNA) blots and Western blots (immunoblots), probed with tobacco basic chitinase gene-specific probe and polyclonal antibodies raised against the chitinase enzyme, yielded hybridization patterns similar to those of MD17. Moreover, the up-regulation of the 32-kDa basic chitinase gene expression in tobacco roots by (1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH) was less effective in mycorrhizal roots than in nonmycorrhizal controls. Suppression of endogenous basic chitinase (32-kDa) expression was also observed in transgenic mycorrhizal plants that constitutively express the 34-kDa basic chitinase A isoform. When plants were grown with an increased phosphate supply, no suppression of the 32-kDa basic chitinase was obtained. These findings indicate that during the colonization and establishment of G. intraradices in tobacco roots, expression of the basic chitinase gene is down-regulated at the mRNA level.