Scots pine is an ecologically and economically important tree species in Finland because of its heartwood (HW) quality traits that contributed by the pine HW extractives, which is mainly attributed to stilbenes. Pine stilbenes, pinosylvin (PS) and its monomethyl ether, are development and stress inducible metabolites. During HW formation, biosynthesis of pine stilbene takes place in a narrow zone between the HW and the sapwood (SW), namely the transition zone (TZ). Pine stilbenes can also be induced in SW or needles in response to stressors. Despite the importance of pine HW, studies on the molecular development of HW formation from its TZ are scarce. Besides, the timing and type of HW formation in Scots pine has not been satisfactorily described. Scots pine HW extractive content varies between individuals and is highly heritable, thus breeding for high extractives in pine is possible. However, traditional forest tree breeding is time-consuming, particularly for a trait that can only be assessed in mature trees. A solution for early selection in forest tree breeding could be with a genomic approach coupled with bioinformatics analysis. In this work, the transcriptome changes during HW formation were studied in the TZ compared to the SW of mature pine trees. In addition, the stress response transcriptome changes were studied by wounding the stems of pine seedlings.
The timing of Scots pine HW formation was investigated by studying the year-round expression profile of selected transcripts using quantitative RT-PCR. HW formation was initiated in spring and ceased in late autumn. The process was marked by programmed cell death. During HW formation, sucrose was metabolised for stilbene biosynthesis, indicating that pine stilbenes are biosynthesised in situ in the TZ. The pine stilbene biosynthetic pathway is upregulated both during HW formation and in response to stress. Interestingly, distinct transcripts encoding two enzymes acting at the beginning of the pathway were induced during development and stress. This work also showed that the previously characterised PS O-methyltransferase, PMT1, is probably not part of the stilbene pathway. A newly characterised O-methyltransferase, PMT2, turned out to be PS specific, and is strictly coexpressed with stilbene synthase.
Unexpectedly, the resin acid biosynthetic transcripts were not induced in concert with stilbene biosynthesis. The year-round expression study showed that the expression of resin acid biosynthetic transcripts was induced in early spring and ceased later in spring. Resin acid biosynthesis was not induced in response to wounding. Single members of the MYB and NAC transcription factor families were upregulated in the TZ compared to SW, and closely followed the expression of stilbene biosynthesis and its upstream pathways. However, other members of the MYB and NAC families were transiently induced in response to wounding. Similarly, distinct transcripts associated with cell wall modification, water deficit stress and plant defence were induced during development and stress.
This work demonstrated that only little similarity occurred in the transcriptome changes between HW formation and wounding response in pine. Despite stilbene synthase and PMT2 being commonly induced in both conditions, different sets of transcripts were induced, suggesting their physiological roles may be development and stress specific in Scots pine.
