Drought stress affects carbon partitioning between plant primary and secondary metabolism in Scots pine trees

The interplay of the processes controlling carbon partitioning into plant primary and secondary metabolisms, such as respiratory CO₂ release and Volatile Organic Compound (VOC) biosynthesis, is still not fully understood. Pyruvate is a key metabolite which is formed in primary metabolism and acting as substrate in numerous secondary pathways forming many BVOCs, such as isoprene, volatile terpenoids, oxygenated compounds (acetaldehyde, acetic acid), benzenoids and fatty acid oxidation products (several wound VOCs), which can be emitted by plants. Within the ERC project VOCO, we established an innovative analytical setup enabling us to simultaneously measure stable carbon isotope composition of leaf exchanged CO₂ (Infrared Isotope spectroscopy IRIS) together with VOC release (PTR-TOF-MS and GC-MS-C-IRMS). Position specific ¹³C-labeled pyruvate and ¹³CO₂ were applied in tracer experiments to elucidate carbon partitioning at metabolic branching points into VOCs vs. CO₂ in drought stressed Scots pine trees.

We observed treatment specific patterns of VOC emission including volatile terpenoids, oxygenated BVOCs and green leaf volatiles. Tracing ¹³C, we elucidated if compounds were de novo synthesized from ¹³C-labeled pyruvate or ¹³CO₂. Position-specific labeling with [1-13C]-pyruvate and [2-13C]-pyruvate suggested that most VOCs were synthesized from the C2-C3 moiety of pyruvate whereas the C1 position was decarboxylated resulting in ¹³CO₂ release by dark respiration in the light. We observed drought stress related shifts in ¹³CO₂ release and VOC emissions. Our observations suggest that VOC emissions are associated with significant pyruvate C1 decarboxylation which is released to the atmosphere. This novel approach contributes to a better understanding of the metabolic links of processes of primary and secondary plant metabolisms which is relevant for the vegetation-atmosphere exchange of CO₂ and VOCs.