SUPPRESSION OF TOBACCO BASIC CHITINASE GENE-EXPRESSION IN RESPONSE TOCOLONIZATION 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 establishme nt and 90% identity with the 3' noncoding sequence of two basic chitin ases (EC 3.2.1.14) from N. tabacum. Northern (RNA) blots and Western b lots (immunoblots), probed with tobacco basic chitinase gene-specific probe and polyclonal antibodies raised against the chitinase enzyme, y ielded 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) w as less effective in mycorrhizal roots than in nonmycorrhizal controls . Suppression of endogenous basic chitinase (32-kDa) expression was al so observed in transgenic mycorrhizal plants that constitutively expre ss the 34-kDa basic chitinase A isoform, When plants were grown with a n increased phosphate supply, no suppression of the 32-kDa basic chiti nase was obtained. These findings indicate that during the colonizatio n and establishment of G. intraradices in tobacco roots, expression of tbe basic chitinase gene is down-regulated at the mRNA level.