Light-Dependence of Formate (C(1)) and Acetate (C(2)) Transport and Oxidation in Poplar Trees

Although apparent light inhibition of leaf day respiration is a widespread reported phenomenon, the mechanisms involved, including utilization of alternate respiratory pathways and substrates and light inhibition of TCA cycle enzymes are under active investigation. Recently, acetate fermentation was highlighted as a key drought survival strategy mediated through protein acetylation and jasmonate signaling. Here, we evaluate the light-dependence of acetate transport and assimilation in Populus trichocarpa trees using the dynamic xylem solution injection (DXSI) method developed here for continuous studies of C1 and C2 organic acid transport and light-dependent metabolism. Over 7 days, 1.0 L of [(13)C]formate and [(13)C(2)]acetate solutions were delivered to the stem base of 2-year old potted poplar trees, while continuous diurnal observations were made in the canopy of CO(2), H(2)O, and isoprene gas exchange together with δ(13)CO(2). Stem base injection of 10 mM [(13)C(2)]acetate induced an overall pattern of canopy branch headspace (13)CO(2) enrichment (δ(13)CO(2) +27‰) with a diurnal structure in δ(13)CO(2) reaching a mid-day minimum followed by a maximum shortly after darkening where δ(13)CO(2) values rapidly increased up to +12‰. In contrast, 50 mM injections of [(13)C]formate were required to reach similar δ(13)CO(2) enrichment levels in the canopy with δ(13)CO(2) following diurnal patterns of transpiration. Illuminated leaves of detached poplar branches pretreated with 10 mM [(13)C(2)]acetate showed lower δ(13)CO(2) (+20‰) compared to leaves treated with 10 mM [(13)C]formate (+320‰), the opposite pattern observed at the whole plant scale. Following dark/light cycles at the leaf-scale, rapid, strong, and reversible enhancements in headspace δ(13)CO(2) by up to +60‰ were observed in [(13)C(2)]acetate-treated leaves which showed enhanced dihydrojasmonic acid and TCA cycle intermediate concentrations. The results are consistent with acetate in the transpiration stream as an effective activator of the jasmonate signaling pathway and respiratory substrate. The shorter lifetime of formate relative to acetate in the transpiration stream suggests rapid formate oxidation to CO(2) during transport to the canopy. In contrast, acetate is efficiently transported to the canopy where an increased allocation towards mitochondrial dark respiration occurs at night. The results highlight the potential for an effective integration of acetate into glyoxylate and TCA cycles and the light-inhibition of citrate synthase as a potential regulatory mechanism controlling the diurnal allocation of acetate between anabolic and catabolic processes.