Fighting The Good Fight: Factors Affecting Pine Defenses Against Mountain Pine Beetle At The Leading Edge Of The Outbreak

Over the course of the current outbreak, mountain pine beetle (MPB; Dendroctonus ponderosae) has devastated millions of hectares of pine forests in western North America.  Lodgepole pine (Pinus contorta) is a primary host for MPB, but since 2006, MPB has spread across northern Alberta, where lodgepole pine hybridizes with jack pine (Pinus banksiana).  We used microsatellite and SNP markers to better characterize this hybrid zone and show that MPB has successfully established in jack pine.  Lodgepole pine is a co-evolved host while jack pine is a new host for MPB, leading us to hypothesize that there are differences in defense responses between species. Stressed trees are considered more prone to successful MPB attack under low or moderate beetle densities; accordingly, we further hypothesized that water limitation will reduce carbon gain, altering carbon partitioning into constitutive and induced defenses.  We are testing these hypotheses in seedlings and mature trees inoculated with the MPB fungal associate Grosmannia clavigera.  Water limitation resulted in increased ABA levels and decreased stomatal conductance and photosynthesis in both species. G. clavigera-induced lesion development was slower in jack pine than lodgepole pine, and further delayed by water deficit. Quantitative RT-PCR revealed that both constitutive and induced defense responses are modulated in pines by water limitation.  Microarray analyses reveal that thousands of genes are invoked in the response of pines to G. clavigera infection, that there are substantial differences in responses of the co-evolved lodgepole and naïve jack pine hosts, and that water limitation greatly alters this transcriptional programme.