Genomics of Disease Resistance in Loblolly Pine

Decades of genetic research and breeding in southern pines, principally focused on loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii var. elliottii), have resulted in germplasm with improved resistance to the fungal pathogens that cause fusiform rust and pitch canker disease. Fusiform rust resistance is regulated by gene-for-gene, or major gene, resistance. Multiple clusters of major fusiform rust resistance genes (Fr genes) have been genetically mapped in loblolly pine. One candidate Fr gene is predicted to encode a TIR-NBS-LRR protein. This suggests a similar model of NLR evolution to that found in model plants where NLR gene clusters evolve through homologous recombination mediated by transposable elements. In contrast, studies of pitch canker resistance have identified no major resistance genes, supporting a model of quantitative resistance. For loblolly pine, candidate genes involved in resistance to fusiform rust and pitch canker are summarized and functional annotation data are provided along with relevant studies from other Pinus spp. Avenues of future research are discussed including advanced genomic technologies for simultaneously characterizing resistance genes in the host and avirulence genes in the pathogen and the development of prediction models of resistance and virulence from heterogeneous data sources.