In agricultural settings, plants are hosts to complex microbial communities. Competition is common within these communities as diverse microbes compete for a limited supply of nutrients, but there are also opportunities for collaboration. In the context of the "arms race" that defines host-pathogen interactions, pathogen-pathogen collaboration represents a potentially powerful mechanism for disease organisms to overcome host resistance strategies, with severe implications for agriculture. As an example, in 2001, cotton bacterial blight (CBB), a disease for which there are resistant cotton varieties, re-emerged across the southern USA. Surprisingly, the disease was observed on CBB-resistant cotton plants. Closer examination revealed the CBB pathogen as well as a second bacterial pathogen on these diseased cotton plants. The co-occurrence led to the hypothesis that the two bacterial pathogens are collaborating to suppress host resistance responses and cause disease. This project tests that hypothesis using genomics and molecular biology approaches. If confirmed, this research will have revealed a new frontier of pathogen strategies to overcome host resistance that will need to be considered in the future.<br/><br/>CBB is caused by Xanthomonas citri pv. malvacearum (Xcm). Xcm and a second pathogen, Pseudomonas syringae (Ps), were co-isolated from cotton plants that are resistant to Xcm. Xcm and Ps may, therefore, be forming a disease complex that suppresses the resistance triggered by Xcm. This research aims to reveal the putative mechanism(s) of collaboration between Xcm and Ps. The first aim will establish experimental systems and assays, adapted from classic plant pathology techniques, to carefully define points of potential interaction between Xcm and Ps. The second aim will seek potential molecular mechanisms of collaboration between Xcm and Ps. For example, Ps may use its repertoire of type III effectors to repress the resistance response normally triggered by Xcm. This hypothesis will be tested by comparing plant defense gene expression after inoculation with Xcm, Ps and co-inoculation with Xcm and Ps. RNA sequencing and differential gene expression analyses will reveal the relevant virulence mechanisms employed by both pathogens and the response triggered within cotton. The final aim will directly test the roles of specific bacterial genes in any interaction by constructing genetic knockouts in Xcm and/or Ps and conducting pathogenicity assays. This project breaks important new ground by considering the effect of multiple bacterial plant pathogens attacking the same host simultaneously. Understanding the rules that define these complex interactions is required to meet the agricultural needs of an increasing population and changing environment.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.