Auxin and Root Hair Morphogenesis
Auxin is a potent hormonal effector of root hair development. A plethora of genetic and pharmacological studies have revealed that aberrations in auxin availability or signaling can cause defects in root hair growth and morphology. Recently identified com
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Abstract Auxin is a potent hormonal effector of root hair development. A plethora of genetic and pharmacological studies have revealed that aberrations in auxin availability or signaling can cause defects in root hair growth and morphology. Recently identified components of auxin signaling and auxin transport have been implicated in root hair morphogenesis. The alteration of root hair morphogenesis by auxin also enables this single cell system to serve as an in planta biological marker through which the action mechanism of auxin can be examined.
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Root Hairs Provide an Auxin-Responsive In Planta Single Cell System
Root hair development can be divided into two main stages: fate determination of hair or nonhair cells and hair morphogenesis (Grierson and Schiefelbein 2002). In Arabidopsis, genetic interactions mediating position-dependent hair or nonhair cell fate determination have been well characterized (for recent reviews, see Schiefelbein and Lee 2006; Grierson and Schiefelbein 2008). Downstream of fate determination, the hair-morphogenetic steps (hair initiation, bulge formation, and tip growth) are modulated by hormonal and environmental cues, as well as by developmental cues (Okada and Shimura 1994; Masucci and Schiefelbein 1994, 1996; Katsumi et al. 2000; Peterson and Stevens 2000; Schiefelbein 2000). In particular, auxin and ethylene are potent effectors of root hair morphogenesis. In this chapter, we focus on the role of auxin in root hair development. Although the role of auxin during root hair development is likely to be universal across diverse angiosperm species (Katsumi et al. 2000), most studies have been conducted in Arabidopsis. Numerous studies with Arabidopsis roots have revealed
H.-T. Cho Department of Biology, Chungnam National University, Daejeon, 305-764, South Korea e-mail: [email protected]
Plant Cell Monogr, doi:10.1007/7089_2008_16 © Springer-Verlag Berlin Heidelberg 2008
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that both exogenous and endogenous auxin sources can affect root hair development. Exogenous auxin (2,4-dichlorophenoxyacetic acid) at concentrations as low as 5 nM could enhance root hair elongation in wild-type Arabidopsis (Pitts et al. 1998), and 10 nM indole-3-acetic acid (IAA) restored root hairs in the root hair defective 6 (rhd6) mutant, which has an impairment in hair initiation (Masucci and Schiefelbein 1994). The phenotypes of two auxin-biosynthetic mutants also are consistent with the hypothesis that increased endogenous auxin levels stimulate root hair development. The auxin-overproducing Arabidopsis mutants sur1 (superroot 1) and yucca develop longer root hairs and more plentiful root hairs, because of reduced root epidermal cell length, than do wild-type plants (Boerjan et al. 1995; Zhao et al. 2001). Overexpression of the rice YUCCA1 gene also greatly enhanced the root hair development in rice (Yamamoto et al. 2007). Hormones such as auxin and ethylene do not appear to influence hair or nonhair cell fate determination in the root epidermis, as observed by Masucci and Schiefe
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