Characterizing the Defense Hierarchy of Populus trichocarpa and its Hybrids

The U.S. Departments of Energy and Agriculture are developing bioenergy crops as a sustainable alternative to traditional fossil fuels. Plant disease threatens this effort by reducing production efficiency of plant feedstocks for bioenergy. In particular, biotrophic Melampsora leaf rust pathogens can increase mortality in Populus trichocarpa and its hybrids, commonly known as poplars. The latter are primary woody feedstocks for bioenergy in many parts of the world. Controlling rust disease in poplar plantations is thus critical to the success of the poplar bioenergy program. Current efforts to control poplar rust focus on plant genetic resistance. However, with novel pathogenic variation generated by the global movement of pathogens and their hybridization, it is increasingly clear that controlling rust disease in long-lived poplar trees depends on our ability to understand and manage not only genes for rust resistance, but also short-lived plant microorganisms that contribute to defense against rust (i.e., defense mutualists). Our proposed research thus seeks to develop an integrative, hierarchical model of P. trichocarpa defense that integrates genetic resistance and defense mutualists. Our overarching hypothesis is that defense against Melampsora rust is biologically degenerate, with major and minor plant resistance genes, plant defense compounds, direct competitors, and defense mutualists within the microbiome each contributing to rust resistance under different circumstances. The specific aims of our study will test the placement of each of these factors in the defense hierarchy. Ultimately, it is our hope that we can develop disease management strategies that harness both resistance genes and naturally occurring defense mutualists of P. trichocarpa. The most difficult challenge will be a strategy that inhibits rust even in the presence of a rust competitor (i.e., the mesophyll-mining eriophyid mite, Schizoempodium mesophyllincola). Our strategies should maximize plant resistance and productivity while minimizing impacts on the surrounding ecological landscape.