44:	Generation of the Spacing and Anisocytic Patterns of Stomata in Arabidopsis Leaves and Cotyledons.

Authors:	Geisler, Matt(A)Nadeau, Jan(A)Sack, Fred, D.(A)
Affiliations:	(A): Dept. of Plant Biology, The Ohio State University
Presenter:	Geisler, Matt , geisler.7@osu.edu

Wild-type Arabidopsis stomata were studied using dental resin impressions to follow the same cells through time. Two patterns were found, a spacing pattern and an anisocytic pattern. Stomatal precursors include meristemoid mother cells (MMCs) which divide asymmetrically to produce a meristemoid (M), meristemoids which also divide asymmetrically, and guard mother cells (GMC) which divide symmetrically. The key spacing element is that stomata are separated from each other by at least one intervening cell; no higher order spacing pattern could be detected. This pattern is largely established by the oriented placement of asymmetric divisions in one specific class of precursor cells i.e. in MMCs that are located next to a stoma, GMC or meristemoid. Extrinsic cell signaling seems to orient this division so that the new "satellite" meristemoid is always located away from the pre-existing stoma or precursor. About 70% of all stomata develop from satellite meristemoids. Other events, such as the placement of MMCs, or the division of MMCs not next to stomata, appear to be randomly oriented. These results rule out some previous hypotheses for stomatal patterning (lateral inhibition, cell lineage) but reinforce and expand the "oriented divisions" hypothesis. Fewer than half of all stomatal complexes were anisocytic i.e. with three neighbor cells surrounding the stoma and one much smaller than the others. The anisocytic pattern was found to result from a series of asymmetric divisions arranged in an inward spiral that only occurs in the absence of adjacent stomata or precursors. This work establishes the importance of cell-cell communication and asymmetric divisions in patterning Arabidopsis stomata and provide a context for the analysis of mutations defective in patterning.