CHARACTERIZATION AND LOCALIZATION OF NEW ANTIFUNGAL CYSTEINE-RICH PROTEINS FROM BETA-VULGARIS

Two novel antifungal proteins, AX1 and AX2, were isolated from leaves of sugar beet infected with Cercospora beticola. AX1 (MW = 5078 +/- 3D ) and AX2 (MW = 5193 +/- 3D) were N-terminally sequenced and identifie d as monomeric, basic proteins consisting of 46 amino acid residues, o f which eight are cysteines. Both AX proteins strongly inhibit growth of C. beticola and other filamentous fungi, but have little or no effe ct against bacteria. Based on primary sequence homology (24 to 46% ide ntity), they are related to the superfamily of gamma-thionins, which h ave been isolated recently from seeds of monocotyledons and Brassicace ae. Specific antibodies were raised against the AX proteins after conj ugation to diphtheria toroid. Using immunoblotting and immunohistology , we detected high concentrations of AX proteins extracellularly in ce ll walls and in globular bodies around necrotic lesions in sugar beet leaves infected with C. beticola, suggesting that AX proteins are invo lved in antifungal defense. Furthermore, AX proteins or serologically related proteins were detected in xylem, stomata, and stomatal cells a s well as in sugar beet styles.