CHOLERA-TOXIN ELEVATES PATHOGEN RESISTANCE AND INDUCES PATHOGENESIS-RELATED GENE-EXPRESSION IN TOBACCO

In animals, plants and fungi, cholera toxin (CTX) can activate signall ing pathways dependent on heterotrimeric GTP binding proteins (G-prote ins). We transformed tobacco plants with a chimeric gene encoding the Al subunit of CTX regulated by a light-inducible wheat Cab-1 promoter. Tissues of transgenic plants expressing CTX showed greatly reduced su sceptibility to the bacterial pathogen Pseudomonas tabaci, accumulated high levels of salicylic acid (SA) and constitutively expressed patho genesis-related (PR) protein genes encoding PR-1 and the class II isof orms of PR-2 and PR-3. In contrast, the class I isoforms of PR-2 and P R-3 known to be induced in tobacco by stress, by ethylene treatment an d as part of the hypersensitive response to infection, were not induce d and displayed normal regulation, In good agreement with these result s, microinjection experiments demonstrated that CTX or GTP-gamma-S ind uced the expression of a PR1-GUS reporter gene but not that of a GLB-G US reporter gene containing the promoter region of a gene encoding the class I isoform of PR-2, Microinjection and grafting experiments stro ngly suggest that CTX-sensitive G-proteins are important in inducing t he expression of a subset of PR genes and that these G-proteins act lo cally rather than systemically upstream of SA induction.