Stomatal development in Arabidopsis.

Stomata consist of two guard cells around a pore and act as turgor-operated valves for gas exchange. Arabidopsis stomata develop from one or more asymmetric divisions followed by the symmetric division of the guard mother cell. Stomatal number is partly a function of the availability of smaller epidermal cells that are competent to divide asymmetrically. Stomata are spaced apart from each other by at least one neighbor cell. Pattern generation may involve cell-cell signaling that transmits spatial cues used to orient specific classes of asymmetric divisions. TOO MANY MOUTHS may function in receiving or transducing these cues to orient asymmetric divisions. TMM also is a negative or positive regulator of entry into the stomatal pathway, with the direction of the response dependent on organ and location. STOMATAL DENSITY AND DISTRIBUTION1 is a negative regulator of stomatal formation throughout the shoot and encodes a processing protease that may function in intercellular communication. FOUR LIPS apparently controls the number symmetric divisions at the guard mother cell stage. In some organs, such as the hypocotyl, the placement of stomata may be coordinated with internal features and involves genes that also regulate root hair and trichome formation. Other mutations affect guard cell morphogenesis, cytokinesis, and stomatal number in response to carbon dioxide concentration. The molecular analysis of stomatal development promises advances in understanding intercellular signaling, the control of the plane and polarity of asymmetric division, the specification of cell fate, and the regulation of cell differentiation and shape.