CHARACTERIZATION OF A WHEAT CLASS IB CHITINASE GENE DIFFERENTIALLY INDUCED IN ISOGENIC LINES BY INFECTION WITH PUCCINIA-GRAMINIS

Plant chitinases have been recognized to be involved in defense against pathogens. A class Ib genomic chitinase gene was isolated from a wheat (Triticum aestivum L.) genomic library using a wheat chitinase sequence amplified from DNA by PCR. The nucleotide sequence analysis showed that the wheat chitinase gene (Wchl) contains a 960-bp long coding region with no intron. The derived polypeptide includes a signal peptide, a cysteine-rich domain and a catalytic domain without a carboxy terminal extension. Primer extension experiments identified a single transcription start site 33 bp upstream of the translation initiation site. Southern blot analysis indicated that Wchl is one member of a small gene family in wheat. Northern blot hybridization and histological investigation of wheat leaves infected by Puccinia graminis f. sp. tritici revealed a close correlation between the accumulation of the chitinase transcripts and the inhibition of fungal growth when two isogenic wheat lines (Prelude Sr6 and Prelude sr6) differing at the sr6 locus for race-specific incompatibility with the fungus were studied. The massive accumulation of the chitinase mRNA occurred only in the incompatible sr6/P6 interaction, in parallel with a severe inhibition of mycelial growth in the leaves. In contrast, in the compatible sr6/P6 interaction the fungus grew rapidly throughout the leaf tissue and the chitinase transcripts were not detected. The chitinase mRNA was detectable in non-infected stems but not in non-infected leaves or roots.