A GAL4-like protein is involved in the switch between biotrophic and necrotrophic phases of the infection process of Colletotrichum lindemuthianum oncommon bean

Random insertional mutagenesis was conducted with the hemibiotrophic fungus Colletotrichum lindemuthianum, causal agent of common bean anthracnose. Ni ne mutants that were altered in their infection process on the host plant w ere generated. One of these, H433 is a nonpathogenic mutant able to induce necrotic spots on infected leaves rapidly. These spots are similar to those observed during the hypersensitive reaction. Cytological observations show ed that the development of the mutant H433 is stopped at the switch between the biotrophic and the necrotrophic phases. This mutant carries two indepe ndent insertions of the transforming plasmid pAN7-1. Complementation studie s using the wild-type genomic regions corresponding to the two insertions s howed that one is responsible for the H433 phenotype. Sequencing analysis i dentified a single open reading frame that encoded a putative transcription al activator belonging to the fungal zinc cluster (Zn[II](2)Cys(6)) family. The corresponding gene was designated CLTA1 (for C. lindemothianum transcr iptional activator 1). Expression studies showed that CLBA1 is expressed in low amounts during in vitro culture. Targeted disrupted strains were gener ated, and they exhibited the same phenotype as the original mutant H433. Co mplementation of these disrupted strains by the CLTA1 gene led to full rest oration of pathogenicity. This study demonstrates that CLTA1 is both a path ogenicity gene and a regulatory gene involved in the switch between biotrop hy and necrotrophy of the infection process of a hemibiotrophic fungus.