Are diurnal patterns of stomatal movement the result of alternating metabolism of endogenous guard cell ABA and accumulation of ABA delivered to the apoplast around guard cells by transpiration?

Abstract

Abscisic acid (ABA) prevents opening of closed stomata and causes open stomata to close. A dual-source model is proposed linking ABA to diurnal stomatal movements. Darkness would favour guard cell biosynthesis of endogenous ABA and disfavour ABA catabolism. At first light, xanthophyll cycling, isomerization of ABA precursors, and activation of a cytochrome P450 mono-oxygenase (CytP450) would deplete endogenous guard cell ABA. The NADPH-requiring CytP450 would be activated by elevated O2 and reduced CO2 concentrations resulting from mesophyll photosynthesis. An increased O2-to-CO2 ratio would limit the Calvin cycle in guard cells, diverting NADPH produced by photosynthetic electron transport to the cytosol where, along with elevated O2, it would activate CytP450. Depletion of endogenous ABA would liberate guard cells to extrude protons and accumulate the ions and water needed to increase guard cell turgor and open stomata. By midday, stomata would be regulated by steady-state concentrations of ABA delivered to the apoplast around guard cells by transpiration. In temperate conditions, ABA would reach concentrations high enough to trigger ion efflux from guard cells, but too low to defeat the accumulation of sugars used to maintain opening. In dry conditions, ABA would reach effective concentrations by midday, high enough to trigger ion efflux and inhibit sugar uptake, reducing apertures for the rest of the day. At sunset, conditions would again favour biosynthesis and disfavour catabolism of endogenous guard cell ABA. The model can be used to reconcile proposed cellular mechanisms for guard cell signal transduction with patterns of stomatal movements in leaves.

Diurnal Variation in Gas Exchange: The Balance between Carbon Fixation and Water Loss

“…Such a process could only occur over longer time scales, as high rates of photosynthesis are not associated with low g s ; however, decreases in g s often are seen toward the end of the diurnal period, despite environmental conditions being similar to morning conditions . In most species, the synchronized decrease over the course of the day of A and g s is potentially under the control of the same negative feedback (Vialet-Chabrand et al, 2017), which could be explained by the slow catabolism of ABA toward the end of the diurnal period (Tallman, 2004). Figure 1 illustrates the relative coordination between A and g s as well as the decreases in g s and A toward the end of the diurnal period.…”

The PYL4 A194T Mutant Uncovers a Key Role of PYR1-LIKE4/PROTEIN PHOSPHATASE 2CA Interaction for Abscisic Acid Signaling and Plant Drought Resistance

“…A194T OE plants, because diurnal stomatal movements are linked to ABA concentration via its effect on ion and sugar fluxes (Tallman, 2004).…”

Fern and Lycophyte Guard Cells Do Not Respond to Endogenous Abscisic Acid

“…In addition to closing stomata during drought, ABA is a vital component in a number of networked guard cell-signaling cascades that regulate stomatal aperture in response to a wide range of environmental stimuli (Schroeder et al, 2001;Galvez-Valdivieso et al, 2009;Wilkinson and Davies, 2009;Lee and Luan, 2012). Much of the mechanistic understanding of ABA-induced guard cell closure and membrane-specific biochemistry comes from ABA signaling and synthesis mutants (Imber and Tal, 1970;Lemichez et al, 2001;Mustilli et al, 2002;Assmann, 2003;Tallman, 2004;Xie et al, 2006;Geiger et al, 2010). Stomata in these mutants are unresponsive to environmental stimuli (Koornneef et al, 1984;Mustilli et al, 2002), remaining open during leaf and soil dehydration, resulting in their classic wilty phenotype (Leymarie et al, 1998;Young et al, 2006).…”