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Title: Polygalacturonase-inhibiting protein activity in cantaloupe fruit as a function of fruit maturation and tissue origin

item Fish, Wayne

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2004
Publication Date: 1/31/2005
Citation: Fish, W.W. 2005. Polygalacturonase-inhibiting protein activity in cantaloupe fruit as a function of fruit maturation and tissue origin. European Journal of Plant Pathology. 111:67-76.

Interpretive Summary: Information from other plant systems suggests that in cucurbits, polygalacturonase-inhibiting protein (PGIP) may be a key factor in keeping certain fungal fruit pathogens dormant in the immature fruit. Nothing is known about the expression and efficacy of PGIP in cucurbit fruit during the fruit's development and ripening process. Knowledge about the levels of expression of PGIP in various tissues of the fruit during development might ultimately be utilized to maintain dormancy well into postharvest and thus provide a natural, effective, pesticide-free control of postharvest decay. Levels of PGIP activity in netted cantaloupe fruit were measured against polygalacturonase enzymes from three natural pathogens of cantaloupe fruit every five days from flowering through fully ripe. It was observed that as the fruit matured, the levels of PGIP activity decreased in all fruit tissues except the seed cavity. It also appeared that the relative effectiveness of the inhibitor against the individual enzymes changed as the fruit matured. The results are consistent with differential regulation of a family of PGIP genes and might be a key factor in latent fungal infections in cantaloupe fruit.

Technical Abstract: Netted cantaloupe (Cucumis melo var. cantalupensis cv. Magnum 45) were harvested from five to thirty-five days postanthesis. The fruit of each age group were divided into exocarp, outer mesocarp, mid mesocarp, inner mesocarp, placenta, and seed tissues. Each tissue was extracted and assayed for polygalacturonase-inhibiting protein (PGIP) activity against PGs from three fungal pathogens of cantaloupe fruit. The PGIP activity of all tissues except placental was high from the bloom stage through the first week of fruit development but decreased markedly between five and ten days postanthesis. PGIP activity against P. cucurbitae PG remained high and nearly constant in placental tissue throughout fruit development. However, PGIP activity versus F. solani decreased during fruit development to about twenty-five percent of its original level in the five-day old fruit. These observations are consistent with concomitant down-regulation of one Cmpgip gene's expression and up-regulation of a second Cmpgip gene whose product has different binding affinities for the PGs utilized.