Poplar and Pathogen Interactions: Insights from Populus Genome-Wide Analyses of Resistance and Defense Gene Families and Gene Expression Profiling

Our understanding of the molecular basis of plant-pathogen interactions is derived mostly from studies of model annual plant species, and until recently, few addressed disease resistance and defense responses in long-lived species such as trees. The release of the Populus genome sequence has permitted extensive genome-wide surveys of gene families and comparative analyses of other sequenced plant genomes. These have revealed striking features for gene families that play key roles in the plant defense response. For example, the NBS-LRR resistance (R)-gene family is expanded compared with other plant genomes, including R-gene subfamilies not previously reported in plants. Some of these genes are clustered on the subtelomeric part of a chromosome that shows segregation distortion in genetic studies. Similar expansion is observed for other genes playing key roles in plant defense such as pathogenesis-related proteins. Among the many pathogens that infect poplar trees, Melampsora spp. fungi, which cause rust diseases in plants, are responsible for considerable damage in poplar plantations. This biotrophic pathogen has attracted recent attention and here we describe molecular insights into the Populus defense response against rust infection. Transcript profiles derived from compatible (susceptible) and incompatible (specific host -resistance) Populus-Melampsora interactions were leveraged to describe molecular changes occurring during defense responses against rust fungi. This highlighted responses that are similar to defense responses of annual plant species, such as up-regulation of transcripts encoding pathogenesis-related proteins. The molecular evidence gathered for the poplar-rust pathosystem indicates a temporal delay in the activation of defense responses between susceptibility and partial or full resistance that is consistent with the signal conversion model described for Arabidopsis. The genome of Melampsora larici-populina, which infects several species of Populus, was recently sequenced and provides a model pathosystem for forest pathology and offers unprecedented opportunities to understand how tree species cope with disease. Ultimately, understanding the molecular basis of Populus rust resistance will greatly improve our understanding of other major diseases of poplar.