The virulence (vir) region of pTiC58 was screened for promoter activities by using gene fusions to a promoterless lux operon in the broad-host-range vector pUCD615. Active vir fragments contained the strongly acetosyringone-inducible promoters of virB, virC, virD, and virE and the weakly inducible promoters of virA and virG. Identical induction patterns were obtained Wvith freshly sliced carrot disks, suggesting that an inducer is released after plant tissue is wounded. Optimal conditions for vir gene induction were pH 5.7 for 50 ,IM acetosyringone or sinapic acid. The induction of virB and virE by acetosyringone was strictly dependent on intact virA and virG loci. An increase in the copy number of virG resulted in a proportional, acetosyringoneindependent increase in vir gene expression, and a further increase occurred only if an inducing compound and virA were present.Pathogenic Agrobacterium tumefaciens strains harbor large (approx. 200-kilobase [kb]) tumor-inducing (Ti) plasmids, which are responsible for crown gall tumor disease on susceptible host plants. Two regions of the Ti plasmids are required for pathogenicity: the T-DNA, which is transferred to and stably maintained in transformed plant cells, and the vir region, which may be involved in surface interactions between bacteria and plant cells and the processing, transfer, and integration of the T-DNA. A third region of Ti plasmids contains the genes for the catabolism of opines. Specific opines produced by transformed plant cells have been used to classify Ti plasmids. Two extensively studied groups of Ti plasmids are those encoding the synthesis and catabolism of octopine, such as pTiAch5, pTiA6, pTiB6S3, and pTi15955, and of nopaline, such as pTiC58 and pTiT37.

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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 ofpathogenic 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 ofthe 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 ofpathogenic organisms, supporting the hypothesis that Ni hyperaccumulation defends S. polygaloides against plant pathogens.