NICKEL HYPERACCUMULATION DEFENDS STREPTANTHUS POLYGALOIDES (BRASSICACEAE) AGAINST PATHOGENS

The Ni-hyperaccumulating annual, Streptanthus polygaloides, may contain as much as 16,400 ppm Ni (dry weight) in its tissues. The function of Ni hyperaccumulation is not known. We tested the hypothesis that one function of Ni hyperaccumulation in S. polygaloides is defense against pathogens. Growth of pathogenic organisms on Ni-hyperaccumulating plants (averaging 5,630 ppm Ni, produced by growing plants on high-Ni soil) was compared to pathogen growth on nonhyperaccumulating plants (averaging 124 ppm Ni, produced by growing plants on low-Ni soil). Plants containing hyperaccumulated Ni were more slowly infected by a powdery mildew (Erysiphe polygoni) than low-Ni plants. Two strains of the bacterial pathogen Xanthomonas campestris pv. campestris (one a genetically engineered bioluminescent strain) grew in low-Ni plants but not high-Ni plants. Growth of X. campestris pv. campestris was markedly inhibited by Ni concentrations of 400 ppm in artificial media. Growth of the fungus Alternaria brassicicola, which was necrotrophic on S. polygaloides, also was inhibited on high-Ni leaves relative to low-Ni leaves. These results demonstrated negative effects of hyperaccumulated Ni on a taxonomically wide range of pathogenic organisms, supporting the hypothesis that Ni hyperaccumulation defends S. polygaloides against plant pathogens.