Three chitinases have been shown previously to be induced upon various stresses of bean leaves. Time course studies of m R N A accumulation of two of them (P3-and P4-chitinases) have been studied upon virus infection, mercuric chloride treatment and UV irradiation. In alfalfa mosaic virus (AIMV)-infected plants both mRNAs, absent in uninfected bean leaves, become detectable 36 h after inoculation. A maximum level of mRNAs is reached 84 h after inoculation and, whereas the amount of P3-ch m R N A decreases soon after having reached the maximum, the amount of P4-ch m R N A remains at high levels for several days. In mercuric chloride-treated leaves P4-ch m R N A becomes detectable 1-1.5 h after onset of treatment and a maximum level is observed between 6 h and 24 h after treatment; P3-ch m R N A becomes detectable later than P4-ch m R N A in treated leaves and reaches a maximum as late as 18 h after treatment has been applied. UV light also induces the synthesis of both mRNAs but, here again, important differences are observed in the accumulation rate of the two transcripts. The relative amounts of each mRNA induced by the different stresses have been compared. The most effective inducer of P3-ch m R N A is A1MV. In contrast, mercuric chloride induces P4-ch m R N A more efficiently than AIMV or UV light. We have also determined the complete nucleotide sequence of the c D N A encoding P3-chitinase that has been isolated from a c D N A library by using the cucumber lysozyme-chitinase c D N A as a probe. The 1072 bp P3-ch c D N A encodes a mature protein of 268 amino acid residues and the 25 residue NHz-terminal signal peptide of the precursor. Because of its high structural homology to the cucumber and Arabidopsis acidic chitinases as well as to the N-terminal amino acid sequence of the bifunctional lysozyme-chitinase from P. quinquifolia, bean P3-chitinase can be considered to belong to the class III chitinases. Southern blot analysis of bean genomic D N A revealed that P3-chitinase is encoded by a single gene.