The role of phytohormones and light signalling in the guard cell CO2 response

Stomata are formed by a pair of specialized guard cells surrounding a central pore to
regulate plant carbon-water relations via changes in aperture. Stomatal aperture is
regulated by internal and external cues including CO2 concentrations [CO2], light,
hormones such as abscisic acid (ABA). Though ABA and CO2 pathways are shown to
converge to induce stomatal closure the exact mechanism is unknown. In addition, the
photoreceptor phyB is known to regulate stomatal responses to elevated (E) [CO2].
The present study aims to determine the role of phytohormones and photoreceptor
phyB to mediate stomatal closure in response to E[CO2]. Here, we propose that CO2
acts though the ABA antagonist, gibberellins (GAs) and ABA:GA homeostasis
modulates stomatal responses to E[CO2]. In Arabidopsis thaliana, using both GA and
GA biosynthesis inhibitor treatments alongside genetic tools it was demonstrated that
GA can modulate stomatal responses to E[CO2] in both isolated epidermis and whole
leaves. A definitive role for negative regulators of GA responses, DELLA proteins was
not established, and GAs did not act via ROS signaling. Absolute measurement of
ABA and GAs suggests reduction in GAs and not ABA are critical to fine-tune the
E[CO2]-mediated stomatal closure. In Arabidopsis, hypersensitivity of phyB-9 to
E[CO2] could be rescued by GA treatments. phyB-9 has reduced levels of active GA
compared to Col-0 but increased levels of the conjugated form of ABA, ABA-GE.
Perturbations in ABA:GA homeostasis are responsible for the phyB E[CO2] phenotype
as sensitivity is rescued by genetically reducing ABA levels. The photoreceptor phyB
also regulates stomatal responses to E[CO2] in rice. E[CO2] showed no effect on
photosynthetic parameters and had a variable effect on tiller number and flag leaf
measurements and ultimately did not increase rice yield. OsphyB mutants had
significant reductions in yield irrespective of [CO2], demonstrating its critical role in
plant growth and development.