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Unformatted text preview: Brassinosteroids Stimulate Plant Tropisms through Modulation of Polar Auxin Transport in Brassica and Arabidopsis Li Li, a,b Jian Xu, c Zhi-Hong Xu, a and Hong-Wei Xue a,b,1 a National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 200032 Shanghai, China b Partner Group of Max-Planck-Institute of Molecular Plant Physiology on Plant Molecular Physiology and Signal Transduction, 200032 Shanghai, China c Department of Molecular Genetics, Utrecht University, 3584 CH Utrecht, The Netherlands Brassinosteroids (BRs) are important plant growth regulators in multiple developmental processes. Previous studies have indicated that BR treatment enhanced auxin-related responses, but the underlying mechanisms remain unknown. Using 14 C-labeled indole-3-acetic acid and Arabidopsis thaliana plants harboring an auxin-responsive reporter construct, we show that the BR brassinolide (BL) stimulates polar auxin transport capacities and modifies the distribution of endogenous auxin. In plants treated with BL or defective in BR biosynthesis or signaling, the transcription of PIN genes, which facilitate functional auxin transport in plants, was differentially regulated. In addition, BL enhanced plant tropistic responses by promoting the accumulation of the PIN2 protein from the root tip to the elongation zone and stimulating the expression and dispersed localization of ROP2 during tropistic responses. Constitutive overexpression of ROP2 results in enhanced polar accumulation of PIN2 protein in the root elongation region and increased gravitropism, which is significantly affected by latrunculin B, an inhibitor of F-actin assembly. The ROP2 dominant negative mutants ( 35S-ROP2-DA/DN ) show delayed tropistic responses, and this delay cannot be reversed by BL addition, strongly supporting the idea that ROP2 modulates the functional localization of PIN2 through regulation of the assembly/reassembly of F-actins, thereby mediating the BR effects on polar auxin transport and tropistic responses. INTRODUCTION Since the identification of plant steroids more than two decades ago, brassinosteroids (BRs), such as brassinolide (BL), have turned out to be essential plant growth regulators in multiple developmental processes (Clouse and Sasse, 1998). Studies using BR synthesis and signalingdeficient Arabidopsis thaliana mutants, such as de-etiolated2 ( det2 ), diminuto1 ( dim1 ), and BR-insensitive1 ( bri1 ), that display altered photomorphogenesis (Chory et al., 1991; Takahashi et al., 1995; Clouse et al., 1996) have revealed profound effects of BRs on cell elongation, organogenesis, and reproductive growth (Clouse, 2002)....
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