Acetate as a potential substrate for cell wall O-acetylation
While the mechanisms of methyl esterification of pectin and its de-methylation by PME have been the focus of several studies (Willatset al., 2001; Mohnen, 2008), the mechanisms of howO -acetyl groups are transferred to and from cell wall polymers and their role in the life cycle of a plant are poorly understood. Current biochemical models of cell wall esters assume that carbohydrate monomers are heavily O -acetylated using acetyl-CoA initially in the Golgi apparatus, exported and incorporated into the growing cell wall, and de-esterified on the wall by esterase enzymes at a later point in the life cycle of the cell in support of numerous physiological and biochemical processes. Acetyl transfer activity from acetyl-CoA to xylo oligomers acceptors has been attributed to Golgi localized TBL acetyl transferases (Zhonget al., 2017). Our observations that delivery of13C2-acetate to the transpiration stream of poplar branches and xylem of a whole tree leads to rapid and significant 13C2-labeling ofO -acetyl esters in leaf cell walls isolations (AIR) supports this biochemical mechanism (Table 1 ). 1H-NMR analysis of the acetate released following leaf AIR saponification show that satellite signals corresponding to the13C2-acetate isotopologue were detectable in all three detached branch leaf AIR samples and two of the three whole tree leaf AIR samples which had been treated with 10 mM13C2-acetate via the transpiration stream. In contrast, AIR of leaves labeled with13C2-acetate treated with water instead of NaOD did not show any detectable13C2-acetate in solution, suggesting the acetate was bound to the cell wall material via an ester bond, making it unlikely that the delivered13C2-acetate in the transpiration stream became trapped in the cell wall material, but not esterified.
These results suggest a possible link between the drought-induced increase in foliar AA emissions (e.g. Figure 3 ) and increasedO -acetylation of leaf cell walls (Figure 10b ). Thus, in addition to providing acetate for protein acetylation and defense gene regulation (Kim et al., 2017), the activation of aerobic fermentation during drought may also supply acetyl-CoA used in the Golgi prior to incorporation into the cell wall (Gou et al., 2012; Orfila et al., 2012; de Souza et al., 2014). This hypothesis is consistent with previous studies with microsomal preparations of a potato cell suspension culture that were supplied with14C-acetyl-CoA found radio-labeled acetate in an esterified form on several polysaccharides, including xyloglucan and pectin (Pauly and Scheller, 2000). Although the mechanisms require further investigation, our study is consistent with cell wall methylation andO -acetylation of polysaccharides rapidly responding to environmental conditions, potentially allowing plants the flexibility to dynamically alter growth and defense processes. Our observations are consistent with a coordinated reduction in cell wall de-methyl esterification and growth processes during water stress (resulting in a strong suppression in MeOH production) together with an activation of defense processes including stomatal closure, aerobic fermentation (increasing AA production and emissions), and enhancements in cell wallO -acetylation.