Transcriptome profiles of hybrid poplar (Populus trichocarpa × deltoides) reveal rapid changes in undamaged, systemic sink leaves after simulated feeding by forest tent caterpillar (Malacosoma disstria).

• Poplar has been established as a model tree system for genomic research of the response to biotic stresses. This study describes a series of induced transcriptome changes and the associated physiological characterization of local and systemic responses in hybrid poplar (Populus trichocarpa × deltoides) after simulated herbivory. • Responses were measured in local source (LSo), systemic source (SSo), and systemic sink (SSi) leaves following application of forest tent caterpillar (Malacosoma disstria) oral secretions to mechanically wounded leaves. • Transcriptome analyses identified spatially and temporally dynamic, distinct patterns of local and systemic gene expression in LSo, SSo and SSi leaves. Galactinol synthase was strongly and rapidly upregulated in SSi leaves. Genome analyses and full-length cDNA cloning established an inventory of poplar galactinol synthases. Induced changes of galactinol and raffinose oligosaccharides were detected by anion-exchange high-pressure liquid chromatography. • The LSo leaves showed a rapid and strong transcriptome response compared with a weaker and slower response in adjacent SSo leaves. Surprisingly, the transcriptome response in distant, juvenile SSi leaves was faster and stronger than that observed in SSo leaves. Systemic transcriptome changes of SSi leaves have signatures of rapid change of metabolism and signaling, followed by later induction of defense genes.