Fig. 5.

Summary of behavior-modifying alterations to stomatal bHLHs and a model for the evolution of specialized stomatal bHLH function. (A) Schematic of normal Arabidopsis stomatal progression with WT SPCH, MUTE, and FAMA positioned under the transition that they promote. P’s in purple circles indicate phosphorylation sites. The subgroup Ia-specific C-terminus is in black. Variants investigated in this study are illustrated, with red X’s marking elimination of specific residues. These modified proteins are positioned relative to the transitions that they promote. (B) A plausible evolutionary model supported by sequence comparisons (6) and experimental data from this study. 1, The ancestral subgroup Ia protein may promote both GMC and GC fates by existing in DNA-binding and non–DNA-binding states (here indicated by occlusion with a gray box); after gene duplication, these two functions later become associated with separate proteins (MUTE and FAMA). 2, After another gene duplication, the acquisition of phosphorylation sites changed the subgroup Ia ancestral activity by suppressing the GMC differentiation function and enabling a novel asymmetric division function.