A number of plant pathogens produce pectolytic enzymes whose activity degrades polygalacturonic acid within the host cell wall during the process of infection. This pectolytic cell wall degradation may contribute to pathogenesis by allowing physical penetration of the invading pathogen and perhaps in providing monomeric sugars to support energy metabolism of the pathogen. Pectic fragments produced through the action of pathogen polygalacturonases may, in addition, elicit defense responses in the infected host. Plant-derived inhibitors of pathogen polygalacturonases have been identified in several plant tissues and their presence has been shown to be associated with resistance to pathogen attack. It has been proposed that the presence of these proteinaceous polygalacturonase inhibitors may directly inhibit pathogen colonization and indirectly function to enhance the host defense response. This research will develop the biochemical and molecular genetic basis for critically assessing the role of plant inhibitors of pathogen polygalacturonases in altering pathogen susceptibility in transgenic plants. The specific objectives are to: 1) purify polygalacturonase inhibitors from pear and tomato fruit, 2) raise antibodies to purified polygalacturonase inhibitors and determine N-terminal and internal amino acid sequences, 3) construct appropriate cDNA libraries from pear and tomato and use antibody and oligonucleotide probes to isolate cDNA clones encoding pear and tomato polygalacturonase inhibitors and, 4) characterize the polygalacturonase inhibitor cDNAs by sequence analysis and patterns of mRNA accumulation. The results of these studies will provide the basis for designing strategies of over-expression or anti-sense depression of the polygalacturonase inhibitor cDNAs in transgenic plants. Losses of agricultural products to plant pathogens are substantial. While losses can often be controlled through the application of fungicides, the enhancement of endogenous plant defense systems by molecular genetic means has provided the hope of reducing the dependence upon chemical control of pathogens. The possible role of plant-produced polygalacturonase inhibitors (PGIs) in pathogen defense is widely recognized but has received little attention in recent years. This research will examine the role of PGIs in pathogen resistance. If proven effective in enhancing plant pathogen resistance, molecular manipulation will provide a powerful tool for achieving pathogen resistant crops.
