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Three races of Xanthomonas axonopodis pv. glycines were identified on pustule disease resistant and susceptible soybean cultivars based on virulence phenotype. For race 3, an avrBs3 homolog, avrXg1 was identified that conferred resistance expressed as a hypersensitive response on resistant cultivar Williams 82. Mutations in two predicted functional domains of avrXg1 resulted in gained virulence on Williams 82 and an increase in bacterial population number on susceptible cultivars. Expression of avrXg1 in race 1, that is predicted to confer a nonspecific HR, led to virulence on susceptible cultivars Spencer and PI 520733. Expression of avrXg1 in race 2, that is predicted of carrying avrBs3-like genes, resulted in gained virulence and fitness of pathogen on both resistant and susceptible cultivars. The results demonstrate multifunctions for avrXg1 dependent on pathogen and plant genetic backgrounds.

Tn5-induced mutations in Agrobacterium vitis F2/5 resulted in both altered grape necrosis and tobacco leaf panel collapse phenotypes, suggesting that the underlying mechanisms of the reactions are related. The reaction on tobacco resembles the classical hypersensitive response (HR) caused by several plant pathogenic bacteria in that it is observable within 14 h, is inhibited by treatment of plants with metabolic inhibitors, and results in the inability to recover the pathogen from the necrotic zone. Strains of A. vitis differ with regard to their efficiency of causing the reaction on tobacco. An EcoRI fragment from one mutant, M6, which is necrosis-altered and HR-minus, was cloned and sequenced. Sequence analysis revealed that the Tn5 insertion occurred in a region that shares significant homology with genes involved in long chain fatty acid production by the marine bacteria Shewanella spp. and Moritella marina. Complementation of M6 with a cosmid clone from an F2/5 DNA library restored the tobacco HR and grape necrosis phenotypes.

Agrocin-minus mutants of nontumorigenic Agrobacterium vitis strain F2/5 controlled grape crown gall as well as the wild-type strain, indicating that agrocin is not a major factor in the mechanism of biological control. Relative levels of attachment to grape cells by tumorigenic and biocontrol strains were also measured. Attachment of tumorigenic strains (CG49 and K306) and biological control strains (F2/5 and agrocin-minus mutant 1077) was often reduced when mixtures of the strains were applied. However, high populations (103to 105CFU/ml) of all strains attached following mixed inoculations, suggesting that competition for attachment sites is also not a factor in the mechanism of biological control. Transfer of T-DNA to grape by CG49 was prevented or greatly inhibited in the presence of F2/5 or 1077 as measured by expression of the GUS reporter gene. The Ti plasmid virulence genes, however, were induced by exudates from grape shoots that had been inoculated with F2/5. Sonicated and autoclaved preparations of F2/5 and 1077 did not control crown gall or inhibit T-DNA transfer. Control by F2/5 is specific to grape, since gall formation on tomato, sunflower, and Kalanchoe daigremontiana were not inhibited.

Vitis riparia 'Gloire de Montpellier' (Gloire) was the most resistant Vitis species and clone tested following inoculations with strains of Agrobacterium vitis and A. tumefaciens. Although tumors failed to appear after infection, the pathogen A. vitis multiplied at the same rate in Gloire as in the crown gall-susceptible V. vinifera 'Chasselas'. Root decay was induced by strains of A. vitis on all the investigated Vitis genotypes. Similarly, roots and shoots of all Vitis genotypes produced vir gene-inducing compounds. This fact ruled out the possibility that crown gall resistance is related to the lack of vir gene induction. However, T-DNA from A. vitis strain CG49 transferred to V. riparia cells at a lower rate than to V. vinifera cells, as determined by evaluation of uidA gene (GUS) expression

AbF21-1D3G7C8 does not react with A. tumefaciens biovars 1 or 2, or with other plant pathogens, symbionts, or saprophytes. AbF21-1D3G7C8 is useful for laboratory diagnosis of A. tumefaciens biovar 3, and thus for detection and diagnosis of grapevine crown gall, indexing of propagation wood, and quarantine testing. A. tumefaciens biovar 3 was identified in grapevine cultivars and rootstocks from New York and Washington states by means of the monoclonal antibody