Soybean mosaic virus (SMV) was adapted for transgene expression in soybean and used to examine the function of avirulence genes avrB and avrPto of Pseudomonas syringae pathovars glycinea and tomato, respectively. A cloning site was introduced between the P1 and HC-Pro genes in 35S-driven infectious cDNAs of strains SMV-N and SMV-G7. Insertion of the uidA gene or the green fluorescent protein gene into either modified cDNA and bombardment into primary leaves resulted in systemic expression that reflected the pattern of viral movement into uninoculated leaves. Insertion of avrB blocked symptom development and detectable viral movement in cultivar Harosoy, which carries the Rpg1-b resistance gene corresponding to avrB, but not in cultivars Keburi or Hurrelbrink, which lack Rpg1-b. In Keburi and Hurrelbrink, symptoms caused by SMV carrying avrB appeared more quickly and were more severe than those caused by the virus without avrB. Insertion of avrPto enhanced symptoms in Harosoy, Hurrelbrink, and Keburi. This result was unexpected because avrPto was reported to confer avirulence on P. syringae pv. glycinea inoculated to Harosoy. We inoculated Harosoy with P. syringae pv. glycinea expressing avrPto, but observed no hypersensitive reaction, avrPto-dependent induction of pathogenesis-related protein 1a, or limitation of bacterial population growth. In Hurrelbrink, avrPto enhanced bacterial multiplication and exacerbated symptoms. Our results establish SMV as an expression vector for soybean. They demonstrate that resistance triggered by avrB is effective against SMV, and that avrB and avrPto have general virulence effects in soybean. The results also led to a reevaluation of the reported avirulence activity of avrPto in this plant.;Bacterial leaf streak, caused by Xanthomonas oryzae pv. oryzicola, is an important disease of rice. Transposon-mediated mutational analysis of the pathogen using a quantitative assay revealed candidate virulence factors including genes involved in pathogenesis in other phytopathogenic bacteria, virulence factors of animal pathogens, and genes not previously associated with virulence. The reduced virulence mutant, TN38C10, with a disruption in the wxocB gene predicted to play a role in lipopolysaccharide (LPS) synthesis was characterized in this study. TN38C10 caused much shorter lesions than the wild type, but multiplied to a similar extent in inoculated rice leaves. The composition of LPS in TN38C10 and the wild type was characterized, and wxocB was shown to play a role in assembly of the O-chain. The wxocB mutant was partially impaired in extracellular polysaccharide (EPS) production, but characterization of a distinct, EPS-deficient mutant indicated that impairment could not solely account for the virulence deficiency of TN38C10. TN38C10 was more sensitive to osmotic stress but less sensitive to oxidative stress than the wild type, and equally or not as sensitive as the wild type to several phytoalexins. No differences in elicitation of defense-related gene expression in rice were observed between TN38C10 and the wild type. Type III secretion system (T3SS)-mediated protein delivery into rice cells, however, was reduced nearly 4-fold in the wxocB mutant relative to the wild type. These findings indicate that the O-chain of LPS is required for wild-type virulence, and that its requirement likely involves structural and/or regulatory interactions with other virulence components, notably EPS and the T3SS.