Figure 7.

MPK4 Silencing in Guard Cells Abolishes CO₂-Induced Stomatal Closure.

ht1-2 ghr1-1 double mutant is CO₂ and ABA insensitive and has wild-type-like stomatal conductance.

(A) and (B) Stomatal response to increase in CO₂ concentration (from 400 to 800 ppm) of intact mpk12-4 T1 plants transformed with ProMPK12:AtMIR390a-MPK4 or ProHT1:AtMIR390a-MPK4, respectively. Results from one experiment with several independent transgenic plants marked with numbers are shown. For Col-0 and mpk12-4, and error bars mark se (n = 4 and n = 6, respectively), and data from the same experimental series are presented in (A) and (B).

(C) to (E) Stomatal response of intact plants to increase in CO₂ concentration (from 400 to 800 ppm), reduction in CO₂ concentration (from 400 to 100 ppm), and 5 µM ABA, respectively. Stimulus was applied at time point zero, the experiment was repeated three times, and representative results from one experiment are shown. Error bars indicate se (n = 5).

(F) Model for the role of HT1, MPK12/MPK4, and GHR1 in the regulation of SLAC1 activation in CO₂-induced stomatal closure. PIP2 aquaporins facilitate CO₂ influx into guard cells. The PIP2;1-interacting carbonic anhydrase CA4 (Wang et al., 2016) together with CA1 accelerate CO₂/bicarbonate signal transduction. MPK12/MPK4 and RHC1 form a double negative regulation chain with HT1, controlling GHR1 and then SLAC1 activation in response to CO₂. CO₂ activates MPK4/MPK12 and RHC1, which then inhibit HT1; thus, HT1 no longer inhibits SLAC1 activation by GHR1, resulting in stomatal closure in response to increased CO₂ concentration. Direct HT1-induced inhibition of SLAC1 has not yet been excluded and its contribution would need to be characterized. HT1 can phosphorylate GHR1 and SLAC1. CO₂ and ABA-signaling pathways converge at the level of GHR1 kinase, which can be regulated by ABI2, whereas OST1 is regulated by several PP2Cs (ABI, ABI2, PP2CA, and HAB1).