Methyl jasmonate induces expression of a novel Brassica juncea chitinase with two chitin-binding domains

We have cloned a 1.3 kb Brassica juncea cDNA encoding BjCHI1, a novel acidic chitinase with two chitin-binding domains that shows 62% identity to Nicotiana tabacum Chia1 chitinase. BjCHI1 is structurally unlike Chia1 that has one chitin-binding domain, but resembles Chia5 chitinase UDA1, the precursor of Urtica dioica agglutinin: however there is only 36.9% identity between them. We propose that BjCHI1 should be classified under a new class, Chia7. The spacer and the hinge region of BjCHI1 are proline-rich, like that of Beta vulgaris Ch1, a Chia6 chitinase with half a chitin-binding domain. Northern blot analysis showed that the 1.3 kb BjCHI1 mRNA is induced by wounding and methyljasmonate (MeJA) treatment but is unaffected by ethylene, salicylic acid (SA) or abscisic acid (ABA). This is the first report on MeJA induction of chitinase gene expression and further suggests that wound-related JA-mediated signal transduction is independent of that involving SA. Western blot analysis using polyclonal antibodies against BjCHI1 showed a cross-reacting band with an apparent molecular mass of 37 kDa in wounded tissues of B. juncea, revealing that, unlike UDA1, BjCHI1 is not cleaved post-translationally at the hinge. Expression of recombinant BjCHI1 in Escherichia coli BL21(DE3) inhibited its growth while crude extracts from E. coli JM109 expressing recombinant BjCHI1 showed chitinase activity. Results from polymerase chain reaction (PCR) suggest that genes encoding chitinases with single or double chitin-binding domains exist in B. juncea.