beta-1,3-glucanases are hydrolytic enzymes considered to constitute part of the general array of defense genes induced by pathogen infection in higher plants. We have isolated and characterized two complementary DNA clones, corresponding to new beta-1,3-glucanases from tomato plants (Lycopersicon esculentum) which are expressed upon challenge with citrus exocortis viroid. Amino acid sequence comparison revealed that they are most similar to beta-1,3-glucanases from tobacco, particularly to PR-Q', the unique component of the class III beta-1,3-glucanase. The deduced amino acid sequences of the two tomato beta-1,3-glucanases indicate that, although being highly similar in amino acid sequence, they have different isoelectric points: pI 10.5 for the basic isoform (Tom PR-Q'b) and pI 5.2 for the acidic one (Tom PR-Q'a). The expression of these two beta-1,3-glucanase messenger RNAs (mRNAs) in response to viroid infection and ethephon treatments was examined. mRNAs for these two isoforms are coordinately expressed and induced similarly to mRNAs for other PR proteins, indicating that they are part of a general and coordinate mechanism of response of tomato plants susceptible to viroid infection.