Aerobic fermentation in plant drought response
The upregulation of aerobic fermentation in plants is now recognized as an evolutionarily conserved drought survival strategy in plants, with the amount of acetate produced directly correlating to survival (Kimet al., 2017). Drought-induced acetate accumulation promotesde novo synthesis of the potent phytohormone jasmonic acid (JA) and the acetylation of histone H4, which influences the priming of the JA signaling pathway for plant drought tolerance (Kimet al., 2017). Thus, acetate regulates an epigenetic switch of metabolic flux conversion and hormone signaling by which plants adapt to drought. However, destructive measurements are required to evaluate acetate-linked drought responses, limiting the temporal and spatial scales that can be studied. As a consequence, few studies have reported aerobic fermentation rates in plants during drought due to the current method requirements of destructive sampling followed by offline tissue analysis of acetate content (Dewhirstet al., 2021b). In this study, by directly quantifying real-time leaf emissions rates of MeOH together with volatiles intermediates of aerobic fermentation (acetaldehyde, AA, ethanol, acetone), we suggest that growth and aerobic fermentation responses to drought can be studied in real-time from individual leaves to whole ecosystems. At the onset of drought in poplar, large increases in the fermentation volatiles acetaldehyde, acetic acid, ethanol, and acetone were consistently emitted from poplar branches despite reduced stomatal conductance. This suggests that drought-activation of the aerobic fermentation pathway occurred (Kim et al., 2017; Rasheed et al., 2018), with foliar emissions of methyl acetate (Dewhirst et al. , 2021b) and acetone (Fall 2003, Jardineet al. , 2010) associated with acetate activation to acetyl-CoA (Millerd et al. , 1954).
During aerobic fermentation, acetate formed from the oxidation of acetaldehyde does not lead to Nicotinamide adenine dinucleotide+(NAD+) regeneration, as in the case of ethanol production in anoxic tissues like flooded roots (Kreuzwieser et al. ,1999). However, while NAD+ regeneration is considered a critical aspect of fermentation under anoxia, it may be less important during aerobic fermentation where acetate may be a key respiratory substrate, effectively coupling aerobic fermentation with mitochondrial respiration to help meet high energy demands of the cell (Tadege 1997). However, non-fermentative sources of acetaldehyde may be possible during stress, such as the peroxidation of membranes associated with irreversible damage (Jardine et al. , 2009).
Our study suggests that there are at least two distinct plant sources of atmospheric AA emissions; hydrolysis of O -acetyl groups on the cell wall (Figure 6 ) and the aerobic fermentation pathway (Figures 3, 7 ). AA and MeOH emission patterns of hydrated leaf cell wall isolations (AIR) showed similar temperature sensitivities when compared with physiologically active poplar leaves and branches. Emissions of AA and MeOH increased with temperature, with AA/MeOH tending to slightly increase with temperature but generally remaining below 30%. Similar results were observed at the ecosystem scale, for example in Alabama, where ambient AA and MeOH concentrations and AA/MeOH ratio above a mixed hardwood forest increased with air temperature with AA/MeOH ranging from < 5% during early mornings to high values of 25% in the afternoon. The striking similarities in temperature sensitivities of AA, MeOH, and AA/MeOH emissions from hydrated leaf cell wall material (AIR), leaves, branches, and whole ecosystems provides direct evidence for the cell wall as the main source of foliar MeOH and AA emissions during normal physiological activities. In contrast, drought stress activates a second source of AA emissions via aerobic fermentation, which overwhelms cell wall sources. Together with the decreased MeOH emissions, AA/MeOH ratios increased dramatically (400-3,500%).  However, we caution that the use of the AA/MeOH emission ratio as a plant and ecosystem growth and stress indicator is only realistic if net emissions of AA and MeOH occur under natural conditions. While net uptake of atmospheric MeOH has not been demonstrated to our knowledge, limited studies on AA exchange between plants and the atmosphere suggests that under polluted atmospheres with high AA concentrations in the lower troposphere, net uptake of atmospheric AA can occur (Jardine et al ., 2011).