Brassinolide InducesIAA5, IAA19, and DR5, a Synthetic Auxin Response Element in Arabidopsis, Implying a Cross Talk Point of Brassinosteroid and Auxin Signaling

Abstract

Despite numerous physiological studies addressing the interactions between brassinosteroids (BRs) and auxins, little is known about the underlying molecular mechanisms. We studied the expression of IAA5 and IAA19 in response to treatment with indole acetic acid (IAA) or brassinolide (BL), the most active BR. Exogenous IAA induced these genes quickly and transiently, whereas exogenous BL induced them gradually and continuously. We also found that a fusion of DR5, a synthetic auxin response element, with the GUS (␤-glucuronidase) gene was induced with similar kinetics to those of the IAA5 and IAA19 genes in response to both IAA and BL treatment of transgenic plants. These results suggest that the IAA genes are induced by BL, at least in part, via the activation of the auxin response element. Endogenous IAA levels per gram fresh weight did not increase when seedlings of Arabidopsis wild type (WT) or the BR-deficient mutant det2 were treated with BL. Furthermore, the levels of IAA transcripts were lower in the det2 mutant than in the WT, even though endogenous IAA levels per gram fresh weight were higher in the det2 mutant than in the WT. In conclusion, the lack of evidence for auxin-mediated activation of early auxin-inducible genes in response to BL suggests that the BR and auxin signaling pathways independently activate the transcriptional system of the IAA and DR5-GUS genes.Exogenous application of brassinosteroids (BRs) to plants at nanomolar to micromolar concentrations produces a wide spectrum of physiological effects. These include promotion of cell elongation and division, enhancement of tracheary element differentiation, delaying of abscission, enhancement of gravityinduced bending, promotion of ethylene biosynthesis, and enhancement of stress resistance, as reviewed by Clouse and Sasse (1998) andSasse (1999). A number of BR-deficient mutants have been identified in Arabidopsis, pea (Pisum sativum), and tomato (Lycopersicon esculentum; for review, see Clouse and Feldmann, 1999;Clouse, 2002; Fujioka and Yokota, 2003). These mutants exhibit dwarfism when grown in either light or dark conditions. Many of these mutants also have dark-green leaves, reduced fertility, a prolonged life span, and abnormal skotomorphogenesis. BR-insensitive mutants have been identified in Arabidopsis, pea, tomato, and rice (Oryza sativa; for review, see Clouse and Feldmann, 1999;Clouse, 2002; Fujioka and Yokota, 2003). The molecular mechanisms of BR action, however, remain unclear.It has been suggested that the actions of BRs are related to auxin action (Mandava, 1988;Sasse, 1999). Synergistic interactions between BRs and auxins occur in elongating tissues and cells in dicots (Yopp et al., 1981;Katsumi, 1985;Sala and Sala, 1985) and in monocots (Yopp et al., 1981). Such synergism is also found in the bending responses of dicots (Yopp et al., 1981; Cohen and Meudt, 1983;Meudt, 1987) and monocots (Takeno and Pharis, 1982; Fujioka et al., 1998). Several authors have proposed that BRinduced effects might be mediated via auxin, wi...

Bipartite Promoter Element Required for Auxin Response

“…1A). As previously reported, response dynamics differed somewhat between the two hormones (Nakamura et al, 2003a). Seedlings treated with the natural auxin indole-3-acetic acid (IAA) sustained a similar level of mRNA induction after 1 or 3 h. For seedlings treated Figure 1.…”

“…Growth promotion by auxin or brassinosteroids requires the function of both signaling pathways (Nakamura et al, 2003a(Nakamura et al, , 2003b(Nakamura et al, , 2006Nemhauser et al, 2004). It is less clear if this phenomenon reflects a convergence at the level of transcriptional control .…”

“…It is less clear if this phenomenon reflects a convergence at the level of transcriptional control . To determine whether auxin and brassinosteroids coordinately regulate activity on target gene promoters, we analyzed the regulatory region of SAUR15, one of the best characterized auxin-and brassinosteroid-responsive genes (Gil et al, 1994;Green, 1996, 1997;Nakamura et al, 2003a). SAUR15 is rapidly induced following hormone treatments (Gil et al, 1994;Nakamura et al, 2003a).…”

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

“…Seventeen to 25% of the auxin-induced genes are also activated by BL in microarray analyses (Goda et al, 2004;Nemhauser et al, 2004). Genes responsive to both auxin and BL include those of early auxin-responsive gene families, such as AUX/IAA, SAUR, and GH3, and a putative transcriptional regulator for tropic responses, MSG2/IAA19 (Tatematsu et al, 2004), also responds to both auxin and BL (Nakamura et al, 2003a). The expression of several genes was found to be synergistically promoted by auxin and BL (Nakamura et al, 2003b;Nemhauser et al, 2004).…”

Computer simulation: The imaginary friend of auxin transport biology

“…The biological evidence, acquired through experiments using reporters such as DR5:GFP, suggests that the canals in fact have high auxin concentrations (65) . Kramer (50) has shown that canals with high flux and high concentration can be formed with the addition of auxin influx transporters that actively pump auxin into cells in the canal from the surrounding apoplast, or efflux transporters that pump auxin out of cells bordering the canal towards the cells of the canal.…”