CHITOSANASE AND CHITINASE ACTIVITIES IN TOMATO ROOTS DURING INTERACTIONS WITH ARBUSCULAR MYCORRHIZAL FUNGI OR PHYTOPHTHORA-PARASITICA

New chitosanase acidic isoforms have been shown in Glomus mosseae-colo nized tomato roots and their induction, together with the previously d escribed mycorrhiza-related chitinase isoform, has been further corrob orated in plants colonized with another Glomus species (G. intraradice s), as well as in tomato roots colonized in vitro by Gigaspora rosea. The induction of these chitosanase isoforms appears as a specific resp onse to the arbuscular mycorrhizal (AM) symbiosis, and does not corres pond to unspecific defence mechanisms, since these isoforms were not i nduced by the pathogen Phytophthora parasitica. Analysis by isoelectro focusing showed two closely migrating chitinase isoforms, specific to mycorrhizal plants colonized either with G. mosseae or G. intraradices , and their isoelectric points were estimated to be 4.5 and 4.7. The e stimated molecular mass of chitosanases was 20 kDa, and after isoelect rofocusing, the chitosanase activities were detected along the acidic pH range (6.5-3.5). Constitutive and induced isoforms were also invest igated during a time-course study. In some experiments, chitin and chi tosan were embedded together as substrates in polyacrylamide gels with the aim of studying the capacity of some isoforms to display both chi tinase and chitosanase activities. In extracts from plants colonized w ith either G. mosseae or G. intraradices, some constitutive chitinases and the previously described mycorrhiza-related chitinase isoform, ap peared to display chitosanase activity, while this bifunctional charac ter was not found for the chitinases from non-mycorrhizal tissue, nor in Phytophthora-infected plants. These results suggest some diversity in the chitinase activities concerning substrate specificity in mycorr hizal plants. The possible implications of these observations in the f unctioning of the symbiosis is discussed.