Minisymposium 25: Pathogen virulence mechanisms

Abs # 46001: The Xanthomonas campestris pv. vesicatoria effector protein XopX contributes to pathogen virulence and suppresses host defense in Nicotiana benthamiana

Presenter:	Morales, Christina Q, cmorales@berkeley.edu
Authors	Morales, Christina Q (A)   Metz, Matthew  (B)   Dahlbeck, Douglas  (A)   Al-Sady, Bassem  (A)   Staskawicz, Brian J (A)
Affiliations:	(A): University of California, Berkeley (B): Bureau for Economic Growth, Agriculture & Trade

The ability of a plant pathogen to incite disease on a particular host plant is the rare exception, as most pathogens are recognized by a wide range of plant species and are resistant to infection. This phenomenon has been referred to as non-host resistance. Our initial objective in investigating the molecular basis of non-host resistance was to determine whether this type of resistance is also specified by the recognition of bacterial effector proteins by the infected non-host plant. We employed Xanthomonas campestris pv. vesicatoria (Xcv), the causal agent of bacterial spot disease on pepper and tomato, as our model system to isolate bacterial effector proteins that are recognized by the non-host plant, Nicotiana benthamiana (Nb). Strains of Xcv are unable to cause disease on Nb, but elicit a non-host hypersensitive cell death reaction when inoculated into Nb. A genomic library of Xcv was constructed in the wide host range cosmid pLAFR3 and conjugated into Xanthomonas campestris pv. campestris (Xcc) strain 8004. Transconjugants were inoculated into Nb leaves and screened for the production of a non-host necrotic reaction. Two overlapping cosmid clones were identified, both containing a novel gene, xopX, that encodes the 74kD effector protein. XopX contributes to the fitness of Xcv on pepper and tomato plants, as site-directed gene knock-outs in Xcv were less virulent in the normally susceptible pepper and tomato hosts. Unexpectedly, Agrobacterium-mediated transient expression of XopX in Nb produced detectable protein levels but no necrotic reaction was seen. However, the necrotic reaction was observed in Agrobacterium-mediated transient expression of XopX in Nb when co-inoculated with either Xcv∆XopX or Xcc, which lacks XopX. The necrotic reaction is Type III Secretion System (TTSS)-dependent. In addition, XopX is translocated from Xcv into the plant cell by measuring increased levels of cAMP that result from the translocation of a calmodulin-dependent xopX:adenyl cyclase translational fusion. Transgenic XopX-expressing Nb plants were also compromised in basal host defense as these plants were more susceptible to bacterial infection. These results suggest that XopX is an important bacterial effector protein that targets basic innate immunity in plants and contributes to pathogen virulence. Current work includes determining the in planta targets of XopX and identifying other bacterial effectors necessary for the causing the necrotic reaction.