Light-Dependence of Formate (C1) and Acetate (C2) Transport and Oxidation in Poplar Trees

Abstract

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 [13C]formate and [13C2]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 CO2, H2O, and isoprene gas exchange together with δ13CO2. Stem base injection of 10 mM [13C2]acetate induced an overall pattern of canopy branch headspace 13CO2 enrichment (δ13CO2 +27‰) with a diurnal structure in δ13CO2 reaching a mid-day minimum followed by a maximum shortly after darkening where δ13CO2 values rapidly increased up to +12‰. In contrast, 50 mM injections of [13C]formate were required to reach similar δ13CO2 enrichment levels in the canopy with δ13CO2 following diurnal patterns of transpiration. Illuminated leaves of detached poplar branches pretreated with 10 mM [13C2]acetate showed lower δ13CO2 (+20‰) compared to leaves treated with 10 mM [13C]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 δ13CO2 by up to +60‰ were observed in [13C2]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 CO2 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.

Fermentation‐mediated growth, signaling, and defense in plants

“…Recently, acetaldehyde emission bursts following light–dark transitions was suggested to not derive from the decarboxylation of pyruvate mediated by PDC, but instead from an intermediate of the photosynthesis‐linked methylerythritol phosphate pathway (MEP) in chloroplasts (Jud et al ., 2016). Consistent with this idea, light/dark emission bursts of acetaldehyde, ethanol, acetic acid, and acetone were inhibited by drought (Jud et al ., 2016; Jardine et al ., 2022a,b), increased with cumulative photosynthesis (the total net amount of CO 2 assimilated during the light period, μmol CO 2 m −2 ), strongly suppressed by removing CO 2 from the atmosphere, and directly incorporated 13 CO 2 assimilated during the light period into 13 C 2 ‐acetyl‐CoA (Jardine et al ., 2012). However, as photosynthetic production of triosephosphates (glyceraldehyde‐3‐phosphate, G3P) can be exported to the cytosol and converted to pyruvate via glycolysis, the role of the pyruvate overflow mechanism in the emission bursts of volatile fermentation products (acetaldehyde, ethanol, acetic acid, acetone, and methyl acetate) following light–dark transitions, as originally proposed by Karl et al .…”

“…3). Toward the goal of developing a comprehensive understanding of whole plant acetate transport and metabolism (Figs 1, 2), future studies could leverage recent advances in continuous nondestructive whole plant 13 C‐labeling of respiratory CO 2 using the dynamic xylem solution injection (DXSI) technique for continuous delivery of 13 C 2 ‐acetate to plant canopies via the transpiration stream (Jardine et al ., 2022b).…”

“…Using online mass spectrometry, temperature‐dependent emissions of acetaldehyde, ethanol, acetic acid, and methyl acetate were quantified in real‐time from individual leaves and branches during drought response in poplar trees (Dewhirst et al ., 2021; Jardine et al ., 2022a,b). Acetic acid emission has also been evaluated at the ecosystem scale using continuous vertical profiles of acetic acid ambient concentrations within and above a tropical forest canopy in South America (Jardine et al ., 2011) and in North America using the technique of eddy covariance where acetic acid emission fluxes are calculated every 30 min from the covariance between above canopy vertical wind speed and ambient acetic acid concentrations (Kim et al ., 2010; Park et al ., 2013).…”

“…In addition to the utilization of acetate fermentation to enhance protein acetylation, the potential for drought‐induced acetate fermentation to enhance acetylation of other biopolymers, such as structural polysaccharides in cell walls, was recently suggested (Jardine et al ., 2022a,b). In poplar trees, drought induced elevated foliar acetic acid emissions together with acetaldehyde, ethanol, and acetone emissions and led to an increase in leaf bulk cell wall O ‐acetylation.…”

“…For example, acetate fermentation under aerobic conditions coupled to acetate‐dependent acetyl‐CoA production was concluded to be very important in lipid biosynthesis in germinating tobacco pollen (Mellema et al ., 2002). Recently, a 13 C 2 ‐acetate solution directly injected into the xylem of poplar trees was shown to be efficiently transported to the canopy via the transpiration stream and utilized as a respiratory substrate in leaves (Jardine et al ., 2022a,b). These observations demonstrated the potential for acetate fermentation under aerobic conditions to contribute to whole plant metabolism and growth.…”