Gene encoding polygalacturonase inhibitor in apple fruit is developmentally regulated and activated by wounding and fungal infection

A cDNA encoding polygalacturonase-inhibiting protein (PGIP) from mature app le fruit has been cloned and characterized. The open reading frame encodes a polypeptide of 330 amino acids, in which 24 amino acids at the N-terminus comprise the signal peptide. Apple PGIP contains 10 imperfect leucine-rich repeat sequence motifs averaging 24 amino acids in length. In addition to the 1.3 kb PGIP transcript, the cloned cDNA also hybridized to RNA molecule s with sizes of 3.2 and 5.0 kb. Genomic DNA analysis revealed that the appl e PGIP probably belongs to a small family of genes. PGIP transcript levels varied in fruit collected at different maturities, suggesting the gene is d evelopmentally regulated. Very high PGIP transcript levels were detected in decayed areas and the tissue adjacent to the inoculation sites of Penicill ium expansum and Botrytis cinerea. However, no increase in the amount of PG IP transcript in tissue distant from the decayed region was observed. Wound ing on fruit also induced PGIP gene expression but to a much lessser extent when compared with decayed areas. After storage at 0 degrees C for 1 month , the abundance of PGIP transcript in ripe fruit was substantially increase d. The PGIP gene in immature and ripe fruit was rapidly up-regulated by fun gal infections, while in stored fruit the induction was very limited and co ncurred with an increase of fruit susceptibility to fungal colonization. Si nce PGIP gene expression is regulated by fruit development and responds to wounding, fungal infection and cold storage, these observations suggest tha t apple PGIP may have multiple roles during fruit development and stress re sponse.