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 agains t plant pathogens. Thionins of type I, from the endosperms of wheat (W alpha1, Walpha2, Wbeta) and barley (Balpha, Bbeta), and of type II, fr om barley leaves (BLa, BLb, BLc), have been purified to apparent homog eneity. For a given pathogen, the effective concentration giving 50% i nhibition (EC-50) determined for the different thionins varied over a less than fivefold range. The ranking of the variants according to the ir activity differed among different pathogens, but certain variants, such as Walpha1, Wbeta or Bbeta, tended to be more active than the oth ers. Strains of some bacterial species, such as Clavibacter michiganen sis subsp. sepedonicus or Pseudomonas solanacearum were sensitive in t he 2-3 x 10(-7) M concentration range, whereas the most sensitive fung al pathogens, such as Rosellinia necatrix, Colletotrichum lagenarium a nd Fusarium solani, had EC-50 values in the 1-4 x 10(-6) M range. Thio nins, which were not particularly effective in liquid medium against P hytophthora infestans (EC-50 = 3.9 x 10(-5) M) were more effective tha n the fungicide Ridomil on a molar basis in a drop application assay o n leaf discs from potato.