Fungal pathogens secrete an inhibitor protein that distinguishes isoforms of plant pathogenesis-related endo-β-1,3-glucanases

Authors

  • Kyung-Sik Ham,

    1. Complex Carbohydrate Research Center and Department of Biochemistry and Molecular Biology, University of Georgia, 220 Riverbend Road, Athens, GA 30602-4712, USA
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  • Sheng-Cheng Wu,

    1. Complex Carbohydrate Research Center and Department of Biochemistry and Molecular Biology, University of Georgia, 220 Riverbend Road, Athens, GA 30602-4712, USA
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  • Alan G. Darvill,

    1. Complex Carbohydrate Research Center and Department of Biochemistry and Molecular Biology, University of Georgia, 220 Riverbend Road, Athens, GA 30602-4712, USA
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  • Peter Albersheim

    Corresponding author
      *For correspondence (fax +1 706 542 4412; e-mail palbersh@ccrc.uga.edu).
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*For correspondence (fax +1 706 542 4412; e-mail palbersh@ccrc.uga.edu).

Abstract

Plant endo-β-1,3-glucanases and chitinases inhibit the growth of some fungi and generate elicitor-active oligosaccharides while depolymerizing polysaccharides of mycelial walls. Overexpression of the endo-β-1,3-glucanases and/ or chitinases in transgenic plants provides, in some cases, increased protection against fungal pathogens. However, most of the phytopathogenic fungi that have been tested in vitro are resistant to endo-β-1,3-glucanases and chitinases. Furthermore, some phytopathogenic fungi whose growth is inhibited by these enzymes are able to overcome the effect of these enzymes over a period of hours, indicating an ability of those fungi to adapt to the enzymes. Evidence is presented indicating that fungal pathogens secrete proteins that inhibit selective plant endo-β-1,3-glucanases.A glucanase inhibitor protein (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-β-1,3-glucanases (EnGLsoy-A and EnGLsoy-B) have been purified from soybean seedlings. GIP-1 inhibits EnGLsoy-A but not EnGLsoy-B. Moreover, GIP-1 does not inhibit endo-β-1,3-glucanases secreted by Psg itself nor does GIP-1 inhibit PR-2c, a pathogenesis-related endo-β-1,3-glucanase of tobacco. Evidence is presented that Psg secretes other GIPs that inhibit other endo-β-1,3-glucanase(s) of soybean. Furthermore, GIP-1 does not exhibit proteolytic activity but does appear to physically bind to EnGLsoy-A. The results reported herein demonstrate specific interactions between gene products of the host and pathogen and establish the need to consider fungal proteins that inhibit plant endo-β-1,3-glucanases when attempting to use the genes encoding endo-β-1,3-glucanases to engineer resistance to fungi in transgenic plants.

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