Traumatic Resin Duct Development, Terpenoid Formation, and Related Synthase Gene Expression in Pinus massoniana Under Feeding Pressure of Monochamus alternatus

Conifer-produced oleoresin is mainly composed of monoterpenoids, sesquiterpenoids, and diterpenoids that can protect conifers against herbivores and pathogens. Pinus massoniana Lamb. is an important economic species in southern China. However, the traumatic resin duct (TD) formation and terpenoid accumulation induced by adult Monochamus alternatus (Coleoptera: Cerambycidae) of P. massoniana have not been clearly reported. Here, we infested P. massoniana with M. alternatus and characterized the induced terpene defenses by microscopy, gas chromatography-mass spectrometry (GC-MS) and GCflame ionization detector (GC-FID), and quantitative reverse transcriptase PCR (qRT-PCR). The induced responses involve TD formation, tissue-specific terpenoid accumulation differentiation, and temporal related gene changes. Formation of TDs was observed in secondary xylem 7 days after feeding. GC-MS and GC-FID results showed terpenoid composition varied in bark and xylem tissue in response to infestation. Following feeding damage, most of the terpenoid synthase genes were up-regulated, which is consistent with the changes in terpenoid constitution. The genes were activated before terpenoid changes were detected indicating that this complex physiological and biochemical process requires time. This study provides evidence of the temporal sequence of changes in terpenoid defenses against herbivory in P. massoniana.