Symposium I: New Advances & Insights in Plant Biology

Abs # 10002: Leaf Polarity and Meristem Formation in Arabidopsis

Presenter:	Barton, M. Kathryn , barton@andrew2.stanford.edu
Authors	Barton, M. Kathryn  (A)
Affiliations:	(A): Carnegie Institution, Dept. Plant Biology

The upper (adaxial) and lower(abaxial) domains of the leaf develop with distinct morphologies reflecting the specialized functions of these domains. For instance, the adaxial domain contains tightly packed, chlorophyll rich cells in the palisade layer specialized for photosynthesis while the abaxial domain contains the loosely packed cells of the spongey mesophyll layer that allow for gas exchange. In addition, ability to form axillary meristems as well as the competence to form ectopic meristems resides in the adaxial leaf domain. The developmental events that specify ad- and abaxial domains of the leaf are thought to occur early in leaf development while the leaf primordium is still closely associated with the meristem. These events are thought to be mediated by members of several classes of transcription factors (these include class III HD-ZIP proteins, YABBY/FIL proteins, and KANADI proteins). At early stages, the leaf primordium appears to lack ad/abaxial polarity - adaxial and abaxial transcripts co-occur throught the primordium. Only later are the ad- and abaxial transcripts restricted to their appropriate domains. Analysis of dominant mutations in two members of the HD-ZIP class III genes PHABULOSA and PHAVOLUTA may yield insight into the mechanism by which adaxial and abaxial transcripts become limited to subdomains of the leaf. Dominant mutations in PHB cause the transformation of abaxial domains of the leaves into adaxial domains and also cause ectopic and increased levels of PHB mRNA accumulation. Interestingly, these dominant mutations are predicted to simultaneously affect binding of a steroid ligand to the PHB protein and to interfere with binding of a microRNA to the PHB mRNA. Experiments aimed at determining the relative contributions of each of these potential modes of regulation will be presented.