PYR/RCAR Receptors Contribute to Ozone-, Reduced Air Humidity-, Darkness-, and CO2-Induced Stomatal Regulation      

Abstract

Rapid stomatal closure induced by changes in the environment, such as elevation of CO2, reduction of air humidity, darkness, and pulses of the air pollutant ozone (O3), involves the SLOW ANION CHANNEL1 (SLAC1). SLAC1 is activated by OPEN STOMATA1 (OST1) and Ca2+-dependent protein kinases. OST1 activation is controlled through abscisic acid (ABA)-induced inhibition of type 2 protein phosphatases (PP2C) by PYRABACTIN RESISTANCE/REGULATORY COMPONENTS OF ABA RECEPTOR (PYR/RCAR) receptor proteins. To address the role of signaling through PYR/RCARs for whole-plant steady-state stomatal conductance and stomatal closure induced by environmental factors, we used a set of Arabidopsis (Arabidopsis thaliana) mutants defective in ABA metabolism/signaling. The stomatal conductance values varied severalfold among the studied mutants, indicating that basal ABA signaling through PYR/RCAR receptors plays a fundamental role in controlling whole-plant water loss through stomata. PYR/RCAR-dependent inhibition of PP2Cs was clearly required for rapid stomatal regulation in response to darkness, reduced air humidity, and O3. Furthermore, PYR/RCAR proteins seem to function in a dose-dependent manner, and there is a functional diversity among them. Although a rapid stomatal response to elevated CO2 was evident in all but slac1 and ost1 mutants, the bicarbonate-induced activation of S-type anion channels was reduced in the dominant active PP2C mutants abi1-1 and abi2-1. Further experiments with a wider range of CO2 concentrations and analyses of stomatal response kinetics suggested that the ABA signalosome partially affects the CO2-induced stomatal response. Thus, we show that PYR/RCAR receptors play an important role for the whole-plant stomatal adjustments and responses to low humidity, darkness, and O3 and are involved in responses to elevated CO2.

Fern Stomatal Responses to ABA and CO2 Depend on Species and Growth Conditions

“…These data suggest a greater flexibility and faster stomatal responsiveness to changes in vapor pressure deficit (VPD) in angiosperms. This could be expected, as the responses to changes in VPD are linked with the biosynthesis and presence of ABA (Bauer et al, 2013;McAdam and Brodribb, 2015) as well as ABA signaling (Merilo et al, 2013), and stomatal ABA responses in angiosperms are faster and stronger than in ferns (Brodribb and McAdam, 2011; Fig. 1, B and C).…”

“…Thus, as in experiments with ABA, growth in the growth room with lower RH tended to reduce the stomatal sensitivity in D. filix-mas and A. filix-femina. As stomatal responses to CO 2 are tightly coupled with ABA responses (Xue et al, 2011;Merilo et al, 2013;Chater et al, 2015), the analogous dampening of responses to ABA and elevated CO 2 found in A. filix-femina and D. filix-mas when grown at low RH suggests that the underlying mechanisms responsible for the effect of growth conditions could be similar and potentially related to ABA levels.…”

Evolution of the Stomatal Regulation of Plant Water Content

“…The dominant role for SnRK2s in this ABA signaling cascade is strongly supported by experimental data. The phenotype of single gene mutants in the key SnRK2 for stomatal responses to ABA, OST1, are profound, displaying no sensitivity to ABA (Mustilli et al, 2002) or VPD (Merilo et al, 2013;Merilo et al, 2015). While other SnRK2-independent signaling pathways for ABA perception and SLAC activation have been suggested (Geiger et al, 2010;Brandt et al, 2012;Pornsiriwong et al, 2017), these are redundant and many either converge or require cross-talk with functional OST1 to activate anion channels (Brandt et al, 2015).…”

“…In angiosperms, however, ABA biosynthesis is extremely rapid (Christmann et al, 2005;Waadt et al, 2014; and can be up-regulated by a drop in leaf turgor over the time frame of minutes (Pierce and Raschke, 1981;. Thus, angiosperms respond to VPD using ABA, as is evidenced by the change in ABA levels in angiosperm leaves in response to step changes in VPD (Bauerle et al, 2004;, while mutants in ABA biosynthetic and signaling genes have compromised stomatal responses to VPD (Xie et al, 2006;Merilo et al, 2013;. The result of this regulation of stomatal responses to VPD by ABA is that, unlike all other vascular land plant clades, the stomatal response to VPD in angiosperms is often hysteretic and unable to be predicted by passive hydraulic processes (O'Grady et al, 1999;.…”

A molecular pathway for CO2 response in Arabidopsis guard cells

“…Elevated CO 2 concentration activates anion channels, resulting in depolarization of plasma membrane in guard cells, causing efflux of solutes and stomatal closure [7][8][9][10] . Earlier studies in Arabidopsis identified several mutants with defects in CO 2 -induced stomatal movements [11][12][13][14][15][16][17][18][19] . However, most of these mutants including abi1-1, abi2-1, gca2, ost1 and slac1 were insensitive to both CO 2 and abscisic acid (ABA), representing components shared by both signalling pathways 5,20 .…”