Cell wall O-acetylation is modified by drought
In this study, we found statistically significant enrichments inO -acetyl ester content of bulk leaf cell walls (AIR) in response to drought stress (Fig. 10b ). In contrast, cell wall monomer composition, which was dominated by galacturonic acid from pectin, changed little over seven days following the cessation of watering (Fig. 10a ). That leaf AIR monosaccharide content was largely insensitive to drought suggests a slower turnover in monosaccharide cell wall polysaccharides than the fast time scales of days observed for changes in volatile emission signatures and cell wall O -acetyl ester content changes (1-7 days). O -acetyl-substituents are present on nearly all cell wall polymers with the exception of cellulose, whereas methyl esters are thought to be primarily associated with pectin (Derbyshireet al., 2007). O -acetyl esterification of plant cell walls is known to play important physicochemical, mechanical, and structural roles that serve to minimize degradation while enhancing intermolecular interactions with other wall polymers (Biely, 2012).  Studies have shown that cell wall O -acetylation of hemicellulose and pectin is critical for proper plant growth and functioning. For example, simultaneous mutations of the acetyl transferase genes TBL32 , TBL33 andTBL29 /ESK1 in Arabidopsis resulted in a severe reduction in xylan O -acetyl level down to 15% that of the wild type, and concomitantly, severely collapsed vessels and stunted plant growth (Yuanet al., 2016). Additional studies demonstrated thatArabidopsis plants with defective ESK1 enzymes have a constitutive drought syndrome and collapsed xylem vessels, low hydraulic conductivity along with low O -acetylation levels in xylan and mannan, low transpiration rates, high water use efficiency, and dwarfism  (Lefebvreet al., 2011; Ramírez et al., 2018). Together with these studies, the observation of enhanced leaf cell wall O -acetylation during drought (Figure 10b ) suggests that polysaccharideO -acetylation is important for the proper functioning of vascular tissues under hydraulic stress.