Maize Seed Chitinase is Modified by a Protein Secreted by Bipolaris zeicola

Authors: Naumann, Todd; Wicklow, Donald; Kendra, David

Submitted to: Physiological and Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/16/2009
Publication Date: 11/1/2009
Citation: Naumann, T.A., Wicklow, D.T., Kendra, D.F. 2009. Maize Seed Chitinase is Modified by a Protein Secreted by Bipolaris zeicola. Physiological and Molecular Plant Pathology. 74(2):134-141.

Interpretive Summary: Field corn is exposed to numerous microorganisms including many filamentous fungi. In order to protect themselves from invasion by filamentous fungi, corn plants have mechanisms of detection and defense. Some specialized filamentous fungi are able to overcome plant defenses and cause disease. Detailed information about how plants detect filamentous fungi and defend themselves and the strategies that specialized filamentous fungi use to overcome plant defenses is lacking. This manuscript reports the discovery that a corn protein called ChitA is modified by a protein (Bz–cmp) produced by a specialized filamentous fungus that is a known pathogen of corn. ChitA was purified from corn seed and Bz–cmp was purified from the fungus. Incubation of purified ChitA with purified Bz–cmp resulted in conversion of ChitA to a modified form (ChitA–m). The biochemical and biological properties of ChitA–m were defined. The results demonstrate that ChitA is modified by a pathogenic, filamentous fungus and is likely to play an important role in corn defense. The results also suggest that ChitA, previously reported by others as a protein with antifungal activity, may play a more complex role in detection and defense against specialized filamentous fungi. The basic science knowledge generated by this research contributes to an improved understanding of interactions between filamentous fungi and field corn. This knowledge helps guide corn breeders to develop hybrids with improved resistance to fungal infection, mycotoxin contamination, and improved yields.

Technical Abstract: Plants contain defense mechanisms that prevent infection by most fungi. Some specialized fungi have the ability to overcome plant defenses. The Zea mays (maize) seed chitinase ChitA has been previously reported as an antifungal protein. Here we report that ChitA is converted to a modified form by a protein secreted by the ear–rot pathogen Bipolaris zeicola. Chitinase zymograms demonstrated that ChitA chitinase activity was absent in protein extracts from diseased seeds of ears that had been inoculated with B. zeicola and that these protein extracts contained an active biomolecule capable of removing ChitA chitinase activity from protein extracts of asymptomatic seeds from uninoculated ears. We determined that this activity is due to a secreted protein we term Bipolaris zeicola chitinase modifying protein (Bz–cmp). Biochemical experiments demonstrated that Bz–cmp modified ChitA retains catalytic activity but is impaired for binding to chitin. Plate diffusion assays demonstrated that Bz–cmp modified ChitA retains concentration dependent antifungal activity. These findings demonstrate that ChitA is targeted for modification by a secreted protein from a phytopathogen suggesting that ChitA plays an important role in defense against fungal invasion. The plate diffusion assay results suggest that ChitA is not simply an antifungal protein but may participate in defense against fungal invasion by some other mechanism.