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.