Exploring Pinus nigra's induced defense arsenal against Diplodia sapinea through genes and metabolic pathway analysis

The European black pine (Pinus nigra J. F. Arnold) is a conifer of high economic and ecological importance, and a potential alternative to several forest tree species in Central Europe in response to global climate warming. Unfortunately, the fungus Diplodia sapinea (Fr.) Fuckel is causing severe damages and world-wide economic losses to this and other Pinus host species. The lack of genomic resources and the scarce knowledge of the tree´s molecular defense mechanisms limit any breeding perspectives. Here, we report the results of a controlled infection experiment in which the transcriptomic and metabolomic profiles of mock and infected P. nigra saplings from two provenances (Austrian and Corsican) were compared over a period of 21 days. This combined approach suggests that P. nigra response to D. sapinea infection is activated 21 days post infection when key plant defense signaling hormones such as jasmonic acid, abscisic acid and salicylic acid were increased. This concurred with a high differential gene expression, including important plant defense related pathways, leading to the induction of several phytoalexins and defense related proteins. Furthermore, we show that some of these responses are provenance specific. Finally, this study identifies the key genes and metabolic pathways involved in the P. nigra defense response to D. sapinea. It provides a solid basis for further characterization of the variation of this response in natural populations with varying susceptibility, as a prerequisite for association studies to better elucidate the resistance mechanism and to select reproductive material.