Mechanisms of stomatal development: an evolutionary view

Plants conquered land more than 400 million years ago. In the fossil record, the appearance of these pioneer species is contemporaneous with the appearance of structures on their surfaces called stomata. Each stoma (plural, stomata) consists of paired epidermal guard cells, a pore between them and an airspace in the photosynthetic mesophyll tissue subtending it. The function of stomata is to regulate gas exchange between the plant and its surroundings. On short timescales (minutes to hours), the opening and closing of the stomatal pore by turgor-driven changes in guard cell shape is a key regulatory step in maintaining water and carbon dioxide balance. Work from many laboratories has defined the intracellular signal transduction cascades that mediate changes in pore size in response to hormone and environmental signals [1].The current view is that stomata arose only once during evolution [2]. In early land plants, stomatal density was low [3]. During intervening millennia, the stomatal density (SD, number of stomata/unit leaf area) increased, probably in response to reduced aerial CO2 concentration [4]. The stomatal complex has been fine-tuned by several innovations including recruitment of neighboring subsidiary cells to facilitate stomatal opening/closing, relocation of stomatal complexes under protective epidermal cells and incorporation of multiple asymmetric cell divisions in precursors to create a variety of stomatal distributions. Despite the variation, the basic core structure has remained unchanged: two guard cells flank the stomal pore. In nearly all species, two stomata are separated at least by one non-stomatal cell, an arrangement thought to be essential for efficient opening and closing. Stomata are located on aerial organs including leaves, stems, flowers, fruits and seeds and they develop gradually during organ growth such that young organs have fewer total stomata than mature organs, though SD often decreases as the neighboring epidermal cells expan