Stomatal patterning in plants is an excellent model system for studying molecular mechanisms of cell self-renewal, asymmetric cell division and cell-fate determination. During stomatal development, a precursor cell such as a meristemoid mother cell or a meristemoid undergoes an asymmetric division to produce a new meristemoid and a large sister cell. The new meristemoid may either continue to undergo asymmetric division one to three times, or be directly converted into a guard mother cell, which produces two guard cells by a symmetric division. The large sister cell may either differentiate into a mature pavement cell or regain cell division activity to produce a new meristemoid and a large sister cell (Bergmann and Sack, 2007; Pillitteri and Torii, 2012). In this process, the one-celled spacing pattern rule must be followed (Geisler et al., 2000). This process is regulated by both intercellular and intracellular signaling pathways to make appropriate cell-fate decisions. For example, as the primary receptors of epidermal patterning factors (EPFs), the members of ERECTA (ER) family and their co-receptor TOO MANY MOUTHS (TMM) regulate the initiation and spacing divisions of the stomatal lineage (Nadeau and Sack, 2002; Shpak et al., 2005; Hara et al., 2007; Hunt and Gray, 2009; Abrash and Bergmann, 2010; Sugano et al., 2010; Abrash et al., 2012; Lee et al., 2012). A novel protein, BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL), accumulates in a polarized crescent at the cell periphery to control asymmetric division during stomatal formation (Dong et al., 2009). Several bHLH and MYB transcription factors [FAMA, SPEECHLESS (SPCH), MUTE, ICE1/SCREAM, SCREAM2, FOUR LIPS (FLP) and MYB88] control three consecutive steps of cell specification and differentiation in stomatal development (Lai et al., 2005; Ohashi-Ito and Bergmann, 2006; MacAlister et al., 2007; Pillitteri et al., 2007; Kanaoka et al., 2008). Some cell-cycle regulators such as cyclin B1 (CYCB1), cyclin-dependent kinase B1;1 (CDKB1;1) and cyclin A2 (CYCA2) are implicated in the self-renewal and differentiation of stomatal lineage cells (Boudolf et al., 2004; Xie et al., 2010; Vanneste et al., 2011). In addition, other proteins such as STOMATAL DENSITY AND DISTRIBUTION1 (SDD1), GLUCAN SYNTHASE-LIKE 8 (GSL8) and photoreceptors are involved in stomatal patterning (Berger and Altmann, 2000; Kang et al., 2009; Guseman et al., 2010). More recently, brassinosteroid (BR) was found to regulate stomatal development via BR-INSENSITIVE2 (BIN2)-mediated phosphorylation of YODA (YDA) and SPCH (Gudesblat et al., 2012; Kim et al., 2012). Many of these stomatal regulators play important roles in regulating cell self-renewal, orientation of asymmetric division, physical asymmetric division, and differentiation. However, the regulatory events that connect asymmetric division and cell differentiation are not well understood.
Sterols play significant roles in maintaining cell integration and cell-to-cell communication. They are components of the cell membrane (Simon-Plas et al., 2011) and biosynthetic precursors of steroid hormones, including mammalian androgens (Attard et al., 2009), insect ecdysteroids (Gilbert et al., 2002) and plant BRs (Clouse, 2000). Sterol biosynthesis in plants has been thoroughly investigated within the last decade. The sterol precursor cycloartenol is methylated either once or twice to produce a mixture of sterols, including sitosterol, stigmasterol and campesterol. Campesterol is the precursor of BRs (Clouse, 2000) (Figure 1). Recently, it was demonstrated that the lanosterol pathway of yeast and animals also exists in plants as a minor pathway (Ohyama et al., 2009). Identification of plant sterol mutants has helped to uncover the biological functions of sterols in mediating morphogenesis, cell differentiation, cell polarity and cell patterning (Lindsey et al., 2003; Boutté and Grebe, 2009). Enzymes regulating early sterol biosynthesis, such as squalene epoxidase 1 (SQE1), cycloartenol synthase 1 (CAS1), cyclopropylsterol isomerase 1 (CPI1), sterol 14α–demethylase (CYP51A2), FACKEL (FK), HYDRA 1 (HYD1) and sterol methyltransferases SMT1, SMT2 and SMT3, play additional roles in plant development in addition to their roles in BR biosynthesis (Jang et al., 2000; Schrick et al., 2000, 2002, 2004a; Carland et al., 2002, 2010; Souter et al., 2002; Kim et al., 2005, 2010; Babiychuk et al., 2008; Men et al., 2008; Pose et al., 2009), whereas enzymes catalyzing late stages of sterol biosynthesis, such as DWARF 7 (DWF7), DWF5 and DWF1, mainly control BR biosynthesis (Klahre et al., 1998; Choe et al., 1999, 2000). Although the BR signaling cascade has been well documented, BR-independent roles of sterols mediating plant development are largely unknown.
Figure 1. The sterol biosynthetic pathway in Arabidopsis.
A branch in the pathway results from either one or two methylation steps, producing two major end products, stigmasterol and campesterol. Campesterol is the precursor of brassinosteroids. Enzymes catalyzing these steps are shown in blue, and their corresponding mutants are shown in red (Jang et al., 2000; Schrick et al., 2000, 2002, 2004a; Men et al., 2008).
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