Transcriptional control of stomatal development
Five bHLH transcription factors, the three stage-specific regulators SPCH, MUTE and FAMA and their potential partners, the more widely expressed genes ICE1/SCRM and SCRM2 play major roles in stomatal development. A major challenge is to understand how proteins with nearly identical bHLH domains control the different steps in stomatal development.  In animal neural and muscle development, members of the Acheate-Scute and MyoD classes of bHLHs normally specify specific cell fates; promoter-swap experiments indicate that the proteins are interchangeable if expressed in each other’s domains.  However, the same is not true for the stomatal bHLHs.  With the idea that elements of protein structure unique to each stomatal bHLH are critical, we focus on the regulation and activities of SPCH and FAMA, the bHLHs that control initial and final stomatal differentiation stages, respectively.  

SPCH and asymmetric division
SPCH controls the symmetry breaking event that leads to the creation of the stomatal lineage; in its absence, epidermal cells cannot create meristemoids.  How does the presence of SPCH initiate asymmetric divisions?  What are its downstream targets?  We are pursing these questions by determining direct targets of SPCH and monitoring the cell-biological changes induced upon SPCH expression. 

What controls the expression of SPCH itself?  Although we know little about the transcriptional regulation of SPCH, we have shown that MAPK phosphorylation is critical for modulating SPCH protein expression and activity. The coupling of MAPK signaling to SPCH activity provides cell type specificity for MAPK output while allowing the integration of developmental and environmental signals into the production and spacing of stomata.

FAMA and the final differentiation step
FAMA appears to be absolutely required to allow stomatal development.  To date, no mutations or treatments that promote stomatal formation (for example, loss of function mutations in YODA, TMM, and the ERECTA family or overexpression of positive regulators SPCH and MUTE) can overcome the need for FAMA in creating guard cells.  If FAMA is the master regulator of guard cell differentiation, then identifying its targets will let us understand how these highly specialized cells are formed.

Relevant lab publications

Lampard GR*, Macalister CA*, Bergmann DC (2008) Arabidopsis stomatal initiation is controlled by MAPK-mediated regulation of the bHLH SPEECHLESS. Science 2008 Nov 14;322(5904):1113-6. PMID: 19008449* These authors contributed equally to this work

MacAlister CA, Ohashi-Ito K, Bergmann DC (2007) Transcription factor control of asymmetric cell divisions that establish the stomatal lineage. Nature. 2007 Feb 1;445(7127):537-40. PMID: 17183265

Ohashi-Ito K, Bergmann DC (2006) Arabidopsis FAMA controls the final proliferation/differentiation switch during stomatal development. Plant Cell. 2006 Oct;18(10):2493-505. PMID: 17088607

Funding by: NIH, NSF, Terman