POLYGALACTURONASE-INHIBITING PROTEIN ACCUMULATES IN PHASEOLUS-VULGARIS L IN RESPONSE TO WOUNDING, ELICITORS AND FUNGAL INFECTION

Polygalacturonase-inhibiting protein (PGIP) is a cell wall-associated protein that specifically binds to and inhibits the activity of fungal endopolygalacturonases. The Phaseolus vulgaris gene encoding PGIP has been cloned and characterized. Using a fragment of the cloned pgip ge ne as a probe in Northern blot experiments, it is demonstrated that th e pgip mRNA accumulates in suspension-cultured bean cells following ad dition of elicitor-active oligogalacturonides or fungal glucan to the medium. Rabbit polyclonal antibodies specific for PGIP were generated against a synthetic peptide designed from the N-terminal region of PGI P; the antigenicity of the peptide was enhanced by coupling to KLH. Us ing the antibodies and the cloned pgip gene fragment as probes in West ern and Northern blot experiments, respectively, it is shown that the levels of PGIP and its mRNA are increased in P. vulgaris hypocotyls in response to wounding or treatment with salicylic acid. Using gold-lab eled goat-anti-rabbit secondary antibodies in EM studies, it has also been demonstrated that, in bean hypocotyls infected with Colletotrichu m lindemuthianum, the level of PGIP preferentially increases in those cells immediately surrounding the infection site. The data support the hypothesis that synthesis of PGIP constitutes an active defense mecha nism of plants that is elicited by signal molecules known to induce pl ant defense genes.