Cloning and expression analysis of two putative papaya genes encoding polygalacturonase-inhibiting proteins

Polygalacturonase-inhibiting proteins (PGIPs) are naturally occurring plant inhibitors that are able to retard the activity of fungal polygalacturonases (PGs) on pectin, and the manipulation of PGIP levels or the transfer of specific PGIP genes could reduce plant tissue decay. Because there is no information about PGIPs from papaya, even though this fruit is highly susceptible to fungal infection, two papaya PGIP genes were cloned, and their expression patterns were followed in different organs and tissues at different developmental stages. The Cppgip4 and Cppgip6 sequences share many features with other PGIPs. These genes were ubiquitously expressed in different organs and tissues and were more abundant in fruit pulp and peel. Both transcripts peaked when the fruit were still growing in size and then decreased at a late stage of development. A further reduction was observed during ripening, as both genes decreased significantly within 9 days after harvest. The down-regulation of PGIP genes during ripening was correlated to the decreased inhibitory activity of papaya protein extract against fungal pectinase from Aspergillus niger, and although the enzymatic assay did not provide the specific activity of each gene product, the finding suggests that protection against fungal PGs was impaired during ripening.