Stomata Density and Guard Cells Movement

The land plants possess waxy cuticle on the leaves and stem which prevents water loss but also interfere with gas exchange. To cope with this problem, plants have evolved pores in the epidermis called stomata. A stoma consists of two symmetrically opposed specialized cells: guard cells that can shrink and swell thus closing or opening the pore. These pores uptake CO2 for photosynthesis and lose water for cooling of plant. Stomata density and guard cells movement is control by network of various hormones and environmental factors. Stomata formation in Arabidopsis thaliana involves at least one asymmetric division as well as a single symmetric division in a specialized epidermal cell lineage (Daszkowska et al., 2009; Le et al., 2014).

Although stomatal density can vary between different species of plants but stomatal spacing is stable and results in stomata being separated from one another by at least one intervening cell (one-celled spacing rule). Numerous signaling components have been implicated in this process (Table1), (Rychel et al., 2010; Serna et al., 2011; Shimada et al., 2011). These include small secreted peptide, subtilisin-like protease, transmembrane receptor-like protein, a family of receptor-like kinases, and several genes of a mitogen activated protein kinase cascade (MAP kinase) five transcription factors that are critical for them. (Lai et al., 2005; Ohashi-Ito and Bergmann, 2006; MacAlister et al., 2007; Pillitteri et al., 2007; Wang et al., 2007).

Loss of function of many of these components shows an increase in stomatal density and loss of patterns of epidermal divisions, which ultimately results in a breakdown of the one-cell spacing rule and the production of adjacent stomata. It is believed that if stomata form adjacent to each other, they cannot function efficiently. On the other hand, overexpression or gain of function in some components reduces stomatal production, indicating that these signaling components inhibit the entry into the stomatal lineage. The identification of several patterning mutants revealed a core group of signaling molecules which include, a protease, a leucine-rich repeat receptor like kinases (LRRRLKs), an LRR receptor-like protein, MAP kinase components, and small cysteine-rich secreted peptides are involved in regulation of stomatal development (Pillitteri et al., 2008).

Stomatal density of young leaves can be varying in response to environmental cues, but the controls of such stomatal development have been not fully elucidated. During last several years, molecular genetics approaches used in the model plant Arabidopsis for identification of genes involved in stomatal development. Once extracellular signaling is recognized by cell surface receptors, the mitogen-activated protein kinase (MAPK) cascade within the cell becomes activated, thereby regulating the transcription factors that promote stomatal development (Shimada et al., 2011). This review summarizes recent research work progress on hormonal regulation and crosstalk during stomatal development with advancement in understanding of negative and positive influence in stomatal developmental events.