The glabrous feature of rice is considered as a favorite agronomic trait for rice farmers because it has greater packing capability of rice grains and produces less dust that causes itching effect on farmers. Glabrous rice lacks trichomes on leaves and glumes (Khush, et al.2001). Most rice cultivars in America are glabrous and recognized as an important germplasm resource in breeding due to its high yield, good quality, and wide compatibility in crossing with other rice varieties (Guoet al.1999, Luo et al.2000). Trichomes are derived from aerial epidermal cells and serve various protective purposes such as insect herbivore resistance, freezing tolerance, and shade of UV irradiation (Ishida et al. 2008). There are two distinct types of trichomes developed on leaves of monocot plants. One is macrohairs on silica cells, the other is microhairs along the stomata cells (Khush, et al. 2001). So far, a number of glabrous mutants have been identified in many plant species, including Arabidopsis, tomato, cotton, and maize (Machado et al.2009, Moose et al.2004, Rerie et al.1994, Yang et al.2011). However, the molecular mechanism underlying trichome development has only been intensively investigated in Arabidopsis.
In Arabidopsis, trichome development has been used as a model system to study the cell fate determination and shown to be regulated by a complex gene network (Ishida et al.2008). A homeodomain-leucine zipper protein GLABRA2 (GL2) and an R3 Myb protein TRIPTYCHON (TRY) play essential roles in trichome initiation and hairless cell differentiation (Rerie et al. 1994, Schellmann et al. 2002). The expression of GL2 and TRY are regulated by the WD-repeat/bHLH/MYB complex including TRANSPARENT TESTA GLABRA1 (TTG1), GLABRA3 (GL3)/ENHANCER OF GLABRA3 (EGL3) and GLABRA1 (GL1). Epidermal cells expressing the GL2 protein are able to differentiate into trichome cells. The TRY protein expressed in trichome cells, however, can move into neighboring cells and compete with GL1 for binding to GL3/EGL3 to repress the GL2 expression. The TRY mediated down regulation of the GL2 expression inhibits trichome formation in neighboring cells (Ishida, et al. 2008). Actually, factors able to modulate this gene network affect the trichome development. Previous studies on mutants defective in the biosynthesis and/or signaling of gibberellins, salicylic acid, jasmonic acid, and cytokinin have showed that phytohormones are involved in trichome initiation (Gan et al.2006, Gan et al.2007b, Perazza et al.1998, Traw and Bergelson 2003, Zhou et al.2011). It has been turned out that roles of these phytohormones in trichome development are mediated by their effect on the expression or activity of the components of the WD-repeat/bHLH/MYB complexes. Roles of gibberellins and cytokinins in trichome initiation are mainly dependent on C2H2 transcription factors including GIS1, GIS2, ZFP5 and ZFP8. These transcription factors are able to promote the GL1 expression (Gan et al.2007a, Maes et al.2008, Perazza, et al.1998, Zhou, et al.2011). In addition, JAZ proteins, the key components in the JA signaling pathway, have been shown to interact with bHLH transcription factors (GL3, EGL3 and TT8) and MYB transcription factors (MYB75 and GL1) (Qi et al.2011). The JA-induced destruction of JAZ proteins results in releasing the transcriptional function of the WD-repeat/bHLH/MYB complex and activating downstream events of trichome initiation. Furthermore, recent studies have shown that the microRNA156 targeted gene SPL9 could bypass the function of GL1 and directly binds to promoters of TCL1 and TRY to activate their expression (Yu et al.2010).
In contrast to the sophisticated mechanisms revealed in Arabidopsis, little is known about the molecular mechanisms of trichome development in other plants. It has been noted that a couple of homeodomain-leucine zipper proteins, which are specifically expressed in epidermal cells, are essential in differentiation of epidermal cells. Outer Cell Layer 4 (OCL4), a maize HD-ZIP transcription factor, has been suggested to involve in the repression of macrohair differentiation (Vernoud, et al. 2009), and a HD-Zip protein in tomato, Woolly (Wo) that interacts with Cyclin B2, plays an essential role for trichome formation and embryonic development (Yang et al. 2011). In addition, another subfamily of the homeobox gene, known as WUS-like homeobox genes (WOX), may also play roles in division or differentiation of epidermal cells. Pressed Flower (PRS) is involved in activation of the proliferation of marginal cells. It has been observed that multicellular bulges with trichomes formed on stems and epidermal cells outgrow on sepals of 35S:PRS transgenic plants (Matsumoto and Okada 2001). Moreover, Narrow sheath 1 (NS1) and Narrow sheath 2 (NS2), which are duplicated relatives of PRS in maize, have been suggested to play a role in a lateral domain of shoot apical meristems (Nardmann et al.2004). In addition, OsWOX3 has been found to repress the expression of OsYAB3, which is required for cell differentiation during rice leaf development (Dai et al.2007).
Previous study showed that macrohairs on the leaf blade are greatly reduced in the maize macrohairless 1 (mhl1) mutant (Moose, et al.2004). A major QTL controlling macrohairs in Teosinte has been found to locate near the maize gene MHL1 (Lauter et al.2004). In rice, previous genetic analysis has identified a couple of loci that control trichome development. For example, gl regulates glabrous leaf and hull traits, Hl
were related to long hair development on rice leaves and Hg may be responsible for the extreme long hairs on auricles and glumes (Nagao et al.1960). However, no gene controlling these traits has been cloned in rice as yet. Recently, glabrous leaf and hull mutant (gl1) has been reported to locate within a 54-kb region at the short arm of chromosome 5 (Li et al.2010, Wang et al.2009, Yu et al.1995), but the gene has not been identified yet. Here, we report the identification and characterization of the Glabrous Rice 1 (GLR1), which controls the trichome development in rice. Our work extends an insight into the molecular mechanism of trichome development in rice. The identification and characterization of GLR1 will facilitate breeders to develop elite glabrous rice varieties via marker-assisted-selection and genetic modification approaches.