Stomatal development in diverse species
Stomata are an ancient and nearly ubiquitous feature of land plants and can be found in fossil leaves as early as ~400 mya. Plants often exhibit species-specific distributions and numbers of stomata on their surfaces. In contrast to this diversity in pattern, guard cell morphology has remained quite constant--there are only two classes of stomatal guard cells: the kidney-shaped cells found most plant species and the dumbbell-shaped guard cells found in grasses. We are interested in identifying the genes and developmental rules for stomata across a wide range of species.  We have identified orthologs of many of the stomatal regulators from Arabidopsis and are characterizing their roles in other plants, concentrating primarily on the bHLHs FAMA, MUTE and SPCH in non-vascular plants and in the grasses.

Stomatal development in grasses
Our data from rice and maize provide the first functional evidence that homologs of stomatal genes identified in Arabidopsis can regulate stomatal formation in other plants. This finding is of particular interest when considering the grasses, because both the morphology of guard cells and their tissue distributions differ substantially between Arabidopsis and this group.

The function of FAMA, the gene most closely associated with guard cell differentiation, is conserved between Arabidopsis and grasses. Rice osfama mutants fail to complete the final stage of stomatal differentiation, and OsFAMA can complement the Arabidopsis mutants. This suggests that FAMA acts at a high level in a transcriptional regulatory cascade – in essence specifying the transition to guard cell fate, not the details of guard cell morphology.  In Arabidopsis, SPCH and MUTE control the formation and termination of the self-renewing meristemoids. In the expression and functional studies on the rice and maize homologs of these genes, we found some evidence for their conserved roles in stomatal development; however, there were also some significant differences.

Stomatal development in non-vascular plants
SPCH, MUTE and FAMA are highly similar, yet each is unique required for stomatal development. How did this gene family evolve? We analyzed the bHLH subclass to which these genes belong in representatives of the angiosperms, gymnosperms, lycopods and mosses.  Basal plant groups encode fewer proteins of this subclass, and the features unique to SPCH, MUTE and FAMA (eg. SPCH’s MAPK target domain) are lost.  Moss and lycopod genes are hybrid in structure and in their function in complementation tests, suggesting that as stomatal pattern became more complex, a single stomatal-promoting bHLH duplicated to take on roles at multiple stages.    

Relevant lab publications

Liu T, Ohashi-Ito K, Bergmann DC (2009) Orthologs of Arabidopsis thaliana stomatal bHLH genes and regulation of stomatal development in grasses. Development. 2009 Jul;136(13):2265-76. PMID: 19502487

Funding by NSF, DOE