FUNGAL PATHOGENS SECRETE AN INHIBITOR PROTEIN THAT DISTINGUISHES ISOFORMS OF PLANT PATHOGENESIS-RELATED ENDO-BETA-1,3-GLUCANASES

Plant endo-beta-1,3-glucanases and chitinases inhibit the growth of so me fungi and generate elicitor-active oligosaccharides while depolymer izing polysaccharides of mycelial walls. Overexpression of the endo-be ta-1,3-glucanases and/or chitinases in transgenic plants provides, in some cases, increased protection against fungal pathogens. However, mo st of the phytopathogenic fungi that have been tested in vitro are res istant to endo-beta-1,3-glucanases and chitinases. Furthermore, some p hytopathogenic fungi whose growth is inhibited by these enzymes are ab le to overcome the effect of these enzymes over a period of hours, ind icating an ability of those fungi to adapt to the enzymes. Evidence is presented indicating that fungal pathogens secrete proteins that inhi bit selective plant endo-beta-1,3-glucanases. A glucanase inhibitor pr otein (GIP-1) has been purified to homogeneity from the culture fluid of the fungal pathogen of soybeans, Phytophthora sojae f. sp. glycines (Psg), and two basic pathogenesis-related endo-beta-1,3-glucanases (E nGL(soy)-A and EnGL(soy)-B) have been purified from soybean seedlings. GIP-1 inhibits EnGL(soy)-A but not EnGL(soy)-B. Moreover, GIP-1 does not inhibit endo-beta-1,3-glucanases secreted by Psg itself nor does G IP-1 inhibit PR-2c, a pathogenesis-related endo-beta-1,3-glucanase of tobacco. Evidence is presented that Psg secretes other GIPs that inhib it other endo-beta-1,3-glucanase(s) of soybean. Furthermore, GIP-1 doe s not exhibit proteolytic activity but does appear to physically bind to EnGL(soy)-A. The results reported herein demonstrate specific inter actions between gene products of the host and pathogen and establish t he need to consider fungal proteins that inhibit plant endo-beta-1,3-g lucanases when attempting to use the genes encoding endo-beta-1,3-gluc anases to engineer resistance to fungi in transgenic plants.