a, EPF2 messenger RNA levels in developing 5 DAG (days after germination) cotyledons of WT and ca1 ca4 seedlings, showing induction, at the elevated CO2 concentration in the WT but not ca1 ca4. Levels were normalized to those of the CLATHRIN gene. The insets show the normalized RNA-seq expression of EPF2 exons from an RNA-seq experiment (5 DAG). b–d, MUTE expression correlates with the stomatal development phenotype of the ca1 ca4 mutant. Confocal images showing MUTEpro::nucGFP expression (green) in developing (5 DAG) cotyledons of WT and ca1 ca4 plants (b). Scale bars, 100 µm. Quantitation of MUTEpro::nucGFP-expressing cells in the WT and two independent lines in the ca1 ca4 background, at low and elevated CO2 concentrations (c). d, Stomatal index in WT plants and plants carrying either of two independent mutant alleles of epf2, at low and elevated CO2 concentrations, demonstrating that epf2 mutants show an inversion of the elevated CO2-mediated control of stomatal development. Error bars, mean ± s.e.m., n = 10 in a and n = 20 in c and d. ***, P<0.00005; **, P<0.005; *, P<0.05, using ANOVA and Tukey’s post-hoc test.