Analysis and genetic dissection of stomatal development and patterning in Arabidopsis

Arabidopsis stomata were studies using dental resin impressions to follow the same cells through time. The origin of two patterns were analyzed, the spacing pattern and the anisocytic pattern. Stomatal precursors include meristemoid mother cells (MMCs) which divide asymmetrically to produce a meristemoid (M). The key spacing element is that stomata are separated from each other by at least one intervening cells. This patter is largely established by the extrinsic orientation of asymmetric divisions in neighbor MMCs by stomata or precursors. Other events such as the placement of MMCs, or the divisions of MMCs not next to stomata, appear to be randomly oriented. These results rule out some previous hypotheses for stomatal patterning but reinforce and expand the oriented divisions hypothesis. The anisocytic pattern was found to result from a series of intrinsically oriented asymmetric divisions. Stomatal patterning is violated in too many mouths and four lips mutations of Arabidopsis, which have stomatal clusters in the first formed leaves. The flp mutation affects dorsiventral and cylindrical organs differentially with respect to the frequency of clustering. Tmm has a more complex region-specific phenotype in that some regions lack stomata entirely, other regions have excess stomata, and the flower stalk exhibits an apical to basal gradient from excess to no stomata. Since the frequencies of initiation and clustering can be uncoupled in tmm, these two functions are thought to be under separate region-specific control. The asymmetric division of MMCs producing satellite meristemoids was often disoriented in tmm resulting in meristemoids in contact with pre-existing stomata. In addition, both daughter cells of the asymmetric division often displayed abnormal fates such as the direct conversion of meristemoids into GMCs and the initiation of new asymmetric divisions in sister cells. Wild type neighbor cells that were undifferentiated exhibited a cytoplasmic gradient in relationship to the stoma (or precursor cell). The orientation of this gradient was disrupted in tmm neighbor cells. The TMM gene product may regulate the position of the formative asymmetric division by orienting the polarity of neighbor cells prior to the commitment to a stomatal lineage, perhaps by influencing the establishment or interpretation of cell-cell communication.