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 ).