Pectin, Methanol, and the Growing Plant Cell Wall
The polysaccharide pectin can account for up to 35% of the primary cell wall in dicots and non-grass monocots, and up to 5% of wood tissues (Mohnen, 2008). Newly synthesized pectin in the primary cell wall is known to be highly methyl esterified, with changes in the degree of pectin methylesterification mediated by pectin methylesterases (PME) known to regulate cell wall mechanical properties like elasticity. The degree of pectin methylesterification can have profound impact on physiological processes like tissue morphogenesis and growth as well as numerous biological functions (Levesque-Tremblayet al., 2015). Cell wall synthesis is coupled to changes in cell wall elasticity mediated by pectate formation following pectin demethylesterification (Peaucelleet al., 2012). In Arabidopsis , increases in tissue elasticity in living meristems correlated with pectin demethylesterification (Peaucelleet al., 2011) which is required for the initiation of organ formation (Peaucelleet al., 2008). When pectin demethylation was inhibited, stiffening of the cell walls throughout the meristem was observed which completely blocked the formation of primordia (Peaucelleet al., 2008). Thus, pectin demethylation is a critical process that regulates the direction and speed of cell wall expansion during growth and morphogenesis (Braybrooket al., 2012). Consistent with the view that MeOH emissions from plants into the atmosphere primarily derive from pectin demethylation, numerous studies have revealed that leaf methanol emissions tightly correlate with leaf expansion rates (Hüveet al., 2007) with young rapidly expanding leaves emitting higher fluxes of MeOH than mature leaves (Jardineet al., 2016). Our temperature-controlled gas exchange observations of hydrated leaf bulk cell walls (AIR) provides new direct evidence for pectin demethylation as the dominant source of foliar MeOH emissions. Moreover, purified whole leaf cell walls (AIR) hydrated and placed in a porous Teflon tubes that permit gas exchange showed remarkably similar temperature sensitivities of MeOH and AA emissions (Figure 6 ) as physiologically active leaves (Figure 4 ), branches (Figure 5 ), detached stems (supplementaryFigure S7 ), and whole ecosystems (Figure 8 -9 ), confirming plant cell walls as an important source of these volatile emissions.
In contrast to growth processes, abiotic stress responses may be associated with increased cell wall fortification through a reduction in pectin demethylation rates mediated by pectin methyl esterase inhibitors (PMEI). For example, abiotic stress may lead to the inhibition of pectin demethylation via enhanced expression of PMEI genes known to be involved in abiotic stress tolerance (Anet al., 2008; Hong et al., 2010; Ren et al., 2019; Wang et al., 2020a).  Recent work on drought response of leaf-succulent Aloe vera  reported the drought-induced folding of hydrenchyma cell walls involves changes in pectin esterification (Ahlet al., 2019). It was hypothesized that the cell wall folding process during drought may be initiated by a reduction in pectin de-esterification and its associated MeOH production and Ca+2-complexation, thereby releasing internal constraints on the cell wall. Thus, we suggest that the strong decrease in observed foliar MeOH emissions during water stress (Figs. 2 ,3 ,6 , supplementary S2-5 ) may be related to both gs reductions and reduced cell wall de-methylation rates related to increased PMEI activity. We speculate that reductions in tissue water potential leads to the inhibition of pectin methyl ester hydrolysis, MeOH production, and growth.
Results from the leaf-level environmental response curves (Figure 4 ) are consistent with the view that stomatal regulated leaf MeOH emissions are controlled by light-independent, but highly temperature-dependent production associated with growth processes (Harley et al, 2007). Thus, light and CO2 are assumed to only indirectly influence leaf MeOH emission rates via changes to gs. However, we highlight that reduction of gs at high temperatures in well hydrated leaves was often unable to prevent the temperature increase in MeOH and AA emissions (Figure 4 ). Similarly, reductions in gs during drought were unable to suppress the emissions of fermentation volatiles like AA (Figs. 2 ,3 , supplementary S2 -5 ). While regulated by gs, our observations suggest that the large changes in AA/MeOH ratios during growth and drought stress responses are largely due to changes in production rates, with MeOH production declining and AA production increasing during different phases of the drought response (Figure 3 ).