Conclusions and prospects
Although plants are known to activate growth suppression and defense
signaling during abiotic stress, the biochemical, physiological, and
ecological mechanisms involved are under intense investigation. Of
particular importance is the development of non-destructive in
situ methods that are able to characterize the onset of reversible and
irreversible phases of drought stress including alterations in growth
and defense balances and their associated changes in leaf
CO2 and H2O gas exchange fluxes.
Moreover, although cell walls constitute the majority of plant biomass,
little information exists on the impact of abiotic stress on the degree
of cell wall methylation and O -acetylation, which can change
biomass properties and value as a renewable source of biofuel or
bioproducts. Therefore, understanding the extent to which these changes
occur in woody tissue as it develops could help to understand
recalcitrance of biomass when harvested, and the extent to which this is
driven by exposure of trees to abiotic stress.
In this study, we show that foliar AA/MeOH emissions ratios are a
sensitive indicator of the balance between plant growth and defense
during drought which can be studied in real-time from individual leaves
to whole ecosystems. We identified the active growth phase associated
with rapid biomass accumulation and high rates of leaf gas exchange as
highly enriched in MeOH emissions relative to AA. Temperature
sensitivity studies of MeOH and AA emissions from isolated leaf cell
wall preparations (AIR) showed that these volatiles can be released at
high rates directly from hydrated AIR with AA/MeOH ratios and their
temperature sensitivities similar to emission observations from
physiologically active leaves, branches, and ecosystems under
non-drought stressed conditions.
However, drought stress was discovered to activate the reversible
suppression of MeOH emissions, potentially linked directly to the
inhibition of pectin demethylation and suppression of cell wall
expansion and growth. Continued exposure to drought conditions lead to
numerous coordinated leaf physiological and biochemical changes
including reduced stomatal conductance, the suppression of net
photosynthesis and transpiration, and the activation of aerobic
fermentation. The simultaneous suppression of MeOH emissions and
increase in AA emissions during drought resulted in large increases in
AA/MeOH foliar emissions ratios from 400-3,500%. While current methods
require destructive sampling, our observations suggest that real-time AA
emissions (together with other fermentation volatiles like acetaldehyde,
ethanol, and acetate) may represent a new highly sensitive technique to
evaluate aerobic fermentation-linked defensive processes during abiotic
stress. Moreover, results from13C2-acetate labeling of the
transpiration stream followed by 1H-NMR analysis of
the acetate content of leaf cell wall preparations demonstrates that
leaf cell wall O -acetylation is highly dynamic may derive from
aerobic fermentation via acetate activation to acetyl-CoA. Critical to
the understanding of the roles of cell wall esters in plant abiotic
stress responses is to elucidate their functional role(s) during drought
including impacts on growth, hydraulics, defense signaling, and carbon
and energy metabolism. Rapid changes to cell wall methyl andO -acetyl ester content during drought may allow plants to quickly
respond to environmental signals potentially critical for survival
during climate extremes.