DIFFERENTIAL EXPRESSION OF BEAN CHITINASE GENES BY VIRUS-INFECTION, CHEMICAL TREATMENT AND UV IRRADIATION

Three chitinases have been shown previously to be induced upon various stresses of bean leaves. Time course studies of mRNA accumulation of two of them (P3- and P4-chitinases) have been studied upon virus infec tion, mercuric chloride treatment and UV irradiation. In alfalfa mosai c virus (AlMV)-infected plants both mRNAs, absent in uninfected bean l eaves, become detectable 36 h after inoculation. A maximum level of mR NAs is reached 84 h after inoculation and, whereas the amount of P3-ch mRNA decreases soon after having reached the maximum, the amount of P 4-ch mRNA remains at high levels for several days. In mercuric chlorid e-treated leaves P4-ch mRNA becomes detectable 1-1.5 h after onset of treatment and a maximum level is observed between 6 h and 24 h after t reatment; P3-ch mRNA becomes detectable later than P4-ch mRNA in treat ed leaves and reaches a maximum as late as 18 h after treatment has be en applied. UV light also induces the synthesis of both mRNAs but, her e again, important differences are observed in the accumulation rate o f the two transcripts. The relative amounts of each mRNA induced by th e different stresses have been compared. The most effective inducer of P3-ch mRNA is AlMV. In contrast, mercuric chloride induces P4-ch mRNA more efficiently than AlMV or UV light. We have also determined the c omplete nucleotide sequence of the cDNA encoding P3-chitinase that has been isolated from a cDNA library by using the cucumber lysozyme-chit inase cDNA as a probe. The 1072 bp P3-ch cDNA encodes a mature protein of 268 amino acid residues and the 25 residue NH2-terminal signal pep tide of the precursor. Because of its high structural homology to the cucumber and Arabidopsis acidic chitinases as well as to the N-termina l amino acid sequence of the bifunctional lysozyme-chitinase from P. q uinquifolia, bean P3-chitinase can be considered to belong to the clas s III chitinases. Southern blot analysis of bean genomic DNA revealed that P3-chitinase is encoded by a single gene.