Bacterial Spot of Tomatoes

 

Pepper plants (Capsicum annuum) containing the Bs2 resistance gene are resistant to strains of Xanthomonas campestris pv vesicatoria (Xcv) expressing the bacterial effector protein AvrBs2. AvrBs2 is delivered directly to the plant cell via the type III protein secretion system (TTSS) of Xcv. Upon recognition of AvrBs2 by plants expressing the Bs2 gene; a signal transduction cascade is activated leading to a bacterial disease resistance response. We have developed a novel pathosystem that consists of epitope-tagged Bs2-expressing transgenic Nicotiana benthamiana plants and engineered strains of Pseudomonas syringae pv tabaci that deliver the effector domain of the Xcv AvrBs2 protein via the TTSS of P. syringae. This pathosystem has allowed us to exploit N. benthamiana as a model host plant to use Agrobacterium tumefaciens–mediated transient protein expression in conjunction with virus-induced gene silencing to validate genes and to identify protein interactions required for the expression of plant host resistance.

In these studies, we have demonstrated that two genes, NbSGT1 and NbNPK1, are required for the Bs2/AvrBs2–mediated resistance responses but that NbRAR1 is not. Protein localization studies in these plants indicate that full-length Bs2 is primarily localized in the plant cytoplasm. Three protein domains of Bs2 have been identified: the N terminus, a central nucleotide binding site, and a C-terminal Leucine-rich repeat (LRR).

Coimmunoprecipitation studies demonstrated that separate epitope-tagged Bs2 domain constructs interact in-trans specifically in the plant cell. Coimmunoprecipitation studies also demonstrated that an NbSGT1-dependent intramolecular interaction is required for Bs2 function.

Finally, Bs2 has been shown to associate with SGT1 via the LRR domain of Bs2. These data suggest a role for SGT1 in the proper folding of Bs2 or the formation of a Bs2-SGT1–containing protein complex that is required for the expression of bacterial disease resistance. During the past three years, we have also been field-testing transgenic tomato plants expressing the Bs2 gene. The results are extremely encouraging and have set the stage for further trials. 

We have also been recently awarded a grant from the Two Blades Foundation to identify additional resistance genes that can be combined with the Bs2 gene to engineer durable resistance into tomato plants. We are extremely excited about the prospects for this approach as this demonstration could be readily applied to other agricultural crops for the molecular breeding of durable resistance in the future. The Two Blades Foundation funds this work.