Fungal polygalacturonases exhibit different substrate degradation patterns and differ in their susceptibilities to polygalacturonase-inhibiting proteins.

Polygalacturonic acid (PGA) was hydrolyzed by polygalacturonases (PGs) purified from six fungi. The oligogalacturonide products were analyzed by HPAEC-PAD (high performance anion exchange chromatography-pulsed amperimetric detection) to assess their relative amounts and degrees of polymerization. The abilities of the fungal PGs to reduce the viscosity of a solution of PGA were also determined. The potential abilities of four polygalacturonase-inhibiting proteins (PGIPs) from three plant species to inhibit or to modify the hydrolytic activity of the fungal PGs were determined by colorimetric and HPAEC-PAD analyses, respectively. Normalized activities of the different PGs acting upon the same substrate resulted in one of two distinct oligogalacturonide profiles. Viscometric analysis of the effect of PGs on the same substrate also supports two distinct patterns of cleavage. A wide range of susceptibility of the various PGs to inhibition by PGIPs was observed. The four PGs that were inhibited by all PGIPs tested exhibited an endo/exo mode of substrate cleavage, while the three PGs that were resistant to inhibition by one or more of the PGIPs proceed by a classic endo pattern of cleavage.

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 gene as a probe in Northern blot experiments, it is demonstrated that the pgip mRNA accumulates in suspension-cultured bean cells following addition 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 PGIP; the antigenicity of the peptide was enhanced by coupling to KLH. Using the antibodies and the cloned pgip gene fragment as probes in Western 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-labeled goat-anti-rabbit secondary antibodies in EM studies, it has also been demonstrated that, in bean hypocotyls infected with Colletotrichum 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 mechanism of plants that is elicited by signal molecules known to induce plant defense genes.