INHIBITION OF BACTERIAL AND FUNGAL PLANT-PATHOGENS BY THIONINS OF TYPE-I AND TYPE-II

Thionins are cysteine-rich, 5 kDa polypeptides which are active against plant pathogens. Thionins of type I, from the endosperms of wheat (Walpha1, Walpha2, Wbeta) and barley (Balpha, Bbeta), and of type II, from barley leaves (BLa, BLb, BLc), have been purified to apparent homogeneity. For a given pathogen, the effective concentration giving 50% inhibition (EC-50) determined for the different thionins varied over a less than fivefold range. The ranking of the variants according to their activity differed among different pathogens, but certain variants, such as Walpha1, Wbeta or Bbeta, tended to be more active than the others. Strains of some bacterial species, such as Clavibacter michiganensis subsp. sepedonicus or Pseudomonas solanacearum were sensitive in the 2-3 x 10(-7) M concentration range, whereas the most sensitive fungal pathogens, such as Rosellinia necatrix, Colletotrichum lagenarium and Fusarium solani, had EC-50 values in the 1-4 x 10(-6) M range. Thionins, which were not particularly effective in liquid medium against Phytophthora infestans (EC-50 = 3.9 x 10(-5) M) were more effective than the fungicide Ridomil on a molar basis in a drop application assay on leaf discs from potato.