Role of Pectolytic Enzymes and Galacturonic Acid in Citrus Fruit Decay Caused byPenicillium digitatum

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Abstract
Decay of citrus [Citrus sinensis] fruit by P. digitatum was associated with content or activity of galacturonic acid, pectinmethylesterase (PME) and exopolygalacturonase (exo-PG). The pectolytic enzymes endopolygalacturonase or pectin trans-eliminase were not associated with the disease. The hydrolysis of pectin acid by exo-PG resulted in an accumulation of galacturonic acid up to 12 mg/g fresh weight within the lesion. Treatment of thin citrus peel sections with galacturonic acid, PME or exo-PG did not induce cell separation. However, reduction in tissue strength was apparent with serial treatments of galacturonic acid, PME and exo-PG. Each of these 3 constituents caused specific cellular changes which collectively resulted in a weakening of the cell wall. A possible role for each of these constituents in decay of citrus fruit by P. digitatum is discussed.