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Three chitinase isoenzymes, PvChiE, PvChiF, and PvChiC (molecular masses 29, 28, and 27 kD, respectively), were purified from bean (Phaseolus vulgaris L. cv Saxa) roots infected with the funga1 pathogen Fusarium solani f. sp. phaseoli, and their amino acid sequence was partially determined. All sequences from all three isoenzymes exactly matched deduced amino acid sequences of the bean class IV chitinase PvChi4, formerly called PR4. The N terminus of PvChiF mapped to the hinge region, and the N terminus of PvChiC mapped to the catalytic domain of PvChi4. The N terminus of PvChiE was blocked. The appearance of PvChiE, PvChiF, and PvChiC correlated with an increase in protease activity in infected roots, and they could be generated in vitro by mixing extracts with high protease activity with extracts containing high amounts of PvChi4. Extracts from infected roots prepared in the presence of protease inhibitors also contained the processed forms of PvChi4, indicating that processing occurred in planta and not as an artifact of extraction. Processing of PvChi4 was not detected in incompatible interactions with a nonhost strain of F. solani and in symbiotic interactions with Glomus mosseae, and thus may be important only in compatible interactions with F. solani.

Plant roots enter symbiotic as well as pathogenic interactions with fungi in the rhizosphere. We studied the response of bean (Phaseolus vulgaris L. cv. Saxa) roots to infection by the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe and the pathogen Fusarium solani (Mart.) Sacc. f. sp. phaseoli (Burkholder) Snyder & Hansen. In a time‐course study of the symbiotic interaction between bean roots and G. mosseae, covering all stages of mycorrhiza development, we detected little change in the expression of the defence‐related genes chitinase, β‐1,3‐glucanase and phenylalanine ammonia‐lyase compared with non‐mycorrhizal control roots. The only difference observed was a transient increase in chalcone synthase transcripts at later stages of mycorrhizal root colonization. In interactions with the pathogen, a marked induction of chitinase and phenylalanine ammonia‐lyase expression was observed at the level of both the transcripts and enzyme activities. Class I β‐1,3‐glucanase levels strongly increased at the transcript level, but there was little change in the overall β‐1,3‐glucanase enzyme activity. In the non‐host interaction between common bean and Fusarium solani (Mart.) Sacc. f. sp. pisi (Linford) Snyder & Hansen defence responses increased only slightly and transiently, if at all.