Abstract

Formation of the appressorium occurs not only on host's plant surface but also on artificial solid substrata such as polycarbonate, indicating that these kinds of substrata possess the plenty of signal(s) triggering differentiation into appressorium. A novel emerging germ tube-specific gene, CBP1 (chitin-binding protein), was found in a cDNA subtractive differential library. CBP1 coded for a putative extracellular protein (signal peptide) with two similar chitin-binding domains at both ends of a central domain with homology to fungal chitin deacetylases. The consensus sequence of the chitin-binding domain found in CBP1 has not been reported in fungi, but is similar to the motif found in plant lectins and plant chitinases classes I and IV. The C-terminus region shows domain rich in Ser/Thr, related extracellular matrix protein such as aggulutinin. CBP1 was disrupted in order to identify its function. Null mutants of CBP1 failed to differentiate appressoria normally on artificial surface but succeeded in normally differentiating appressoria on the plant leaf surface, keeping ability to make legions on host rice plants without significant reduction or delay. Chemical inducers, such as stimulants of cellular signal transduction or plant surface chemicals such as 1,16-hexadecanediol allow the mutants to differentiate the appressoria still on artificial surfaces. Complementation assay using mutated CBP1 indicated that both chitin-binding domains and Ser/Thr rich region required for fulfilling the function of CBP1. To evaluate the expression and localization of CBP1 product, constructs of CBP1 fused with GFP at the inner of the ORF or the upstream region were introduced into M. grisea. The presumable function of CBP1 products during appressorium differentiation will be discussed.
*RIKEN.