Brassinosteroids Stimulate Plant Tropisms through Modulation of Polar Auxin Transport in Brassica and Arabidopsis

Abstract

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.

Inhibition of Brassinosteroid Biosynthesis by Either a dwarf4 Mutation or a Brassinosteroid Biosynthesis Inhibitor Rescues Defects in Tropic Responses of Hypocotyls in the Arabidopsis Mutant nonphototropic hypocotyl 4

“…We also found that treatment with 10 nM BL did not significantly affect gravitropism of etiolated hypocotyls under our experimental conditions (data not shown). Li et al (2005) also found that BR promotes polar auxin transport through gene expression of PIN-FORMED (PIN) family proteins in agreement with a previous observation by Bao et al (2004). This seems important because it suggests that BR deficiency caused by dwf4 or Brz treatment may lead to reduction of polar auxin transport.…”

“…A synergistic positive interaction between BR and auxin was reported for gravitropism of partially deetiolated bean (Phaseolus vulgaris) hypocotyl sections (Meudt, 1987) and dark-grown maize (Zea mays) primary roots (Kim et al, 2000). Recently, Li et al (2005) have also shown promotive effects of BR on root and hypocotyl gravitropism in Arabidopsis. They observed gravitropic curvature of roots and hypocotyls of dark-grown seedlings that were simultaneously irradiated with unilateral white light from above.…”

Arabidopsis phosphatidylinositol monophosphate 5-kinase 2 is involved in root gravitropism through regulation of polar auxin transport by affecting the cycling of PIN proteins

“…Genetic analyses have revealed the critical roles of the influx carrier AUX1 and efflux carrier PIN proteins in gravity response. Mutation in AUX1 abolishes root gravitropic curvature [36]; similarly, pin2 seedlings are agravitropic, whereas PIN2 overexpression results in an enhanced tropistic response [37,38]. The lateral relocation of PIN3 in response to gravity redirects the auxin flux and mediates tropism in Arabidopsis [39].…”

Higher plants use LOV to perceive blue light

“…Unfortunately, it is unknown whether this protein is also required for phototropism [27]. Growth responses such as phototropism require auxin and brassinosteroids [28][29][30][31] while there is no evidence suggesting that these hormones are involved in the control of chloroplast movements. For chloroplast movement responses the nucleus is not even required [20].…”