Terpenoid Accumulation and Terpene Synthase Gene Expression Are Coordinately Modulated in Calabrian Pine (Pinus nigra subsp. laricio) Following Infestation by the Pine Processionary Moth (Thaumetopoea pityocampa)

Calabrian pine (Pinus nigra subsp. laricio) is an ecologically and economically important Mediterranean conifer increasingly exposed to the pine processionary moth (PPM; Thaumetopoea pityocampa). We investigated temporal differences in needle terpene metabolism between saplings maintained in a control enclosure and saplings exposed to prolonged PPM infestation in a separate, otherwise identical enclosure. Three-year-old saplings underwent semi-natural infestation for 195 days, and needles were sampled at 0, 21, 84, and 195 days post-infestation. Solvent-extractable monoterpenoids and diterpene resin acids (DRAs) were quantified through gas chromatography–mass spectrometry, while the transcript abundances of mono- and diterpene synthase (MTPS and DTPS, respectively) genes were analyzed through quantitative real-time PCR. During the whole experimental period, the terpenoid blend remained unchanged, qualitatively speaking, in both control and PPM-infested saplings, whereas the two experimental groups showed marked time-dependent quantitative differences among each other in terms of individual compound and transcript abundances. In PPM-infested plants, β-pinene, α-pinene, α-terpineol, and (−)-β-phellandrene accumulated mainly at 84 and 195 days, with changes in their putatively associated Pnl MTPS transcripts accompanying or preceding metabolite accumulation. Conversely, δ-3-carene and Pnl MTPS3 transcript abundance declined strongly. Abietane-type DRAs and dehydroabietic acid accumulated at later stages, together with higher Pnl DTPS1 transcript abundance. Multivariate analyses showed earlier divergence of TPS transcript profiles than metabolite profiles. Thus, Calabrian pine saplings infested by PPM exhibit selective, temporally structured reprogramming of mono- and diterpenoid pathways, rather than a generalized terpenoid induction.