Physiological and molecular effects of brassinosteroids on Arabidopsis thaliana

Abstract

We examined the effects of brassinosteroids on Arabidopsis thaliana (L.) Henyh. ecotype Columbia in order to develop a model system for studying gene regulation by plant steroids. Submicromolar concentrations of two brassinosteroids, brassinolide and 24-epibrassinolide, stimulated elongation of Arabidopsis peduncles and inhibited root elongation, respectively. Furthermore, brassinolide altered the abundance of specific in vitro translatable mRNAs from peduncles and whole plants of Arabidopsis. Root elongation in the auxininsensitive Arabidopsis mutant axrl was inhibited by 24-epibrassinolide but not by 2,4-D, indicating an independent mode of action for these growth regulators in this physiological response.Brassinosteroids are plant growth-promoting natural products with structural similarities to insect and animal steroid hormones (Mandava 1988). Brassinolide (BR), the first brassinosteroid to be structurally characterized (Grove et al. 1979), was originally isolated from Brassica napus pollen and was shown to induce cell elongation and proliferation at nanomolar concentrations in bean, rice, and many other plants. Subsequent work identified other brassinosteroids with similar structures from a broad range of plants including rice, maize, mustard, and wheat (Mandava 1988). The physiological effects and agricultural applications of brassinosteroids have been widely studied, and there is growing evidence that these compounds have properties characteris-

A Putative Role for the Tomato Genes DUMPY and CURL-3 in Brassinosteroid Biosynthesis and Response

“…The L. pimpinellifolium mutant cu-3 was investigated for sensitivity to BR using a screen adapted from Clouse et al (1993). Seeds were sterilized as above and rinsed three times in a large volume of sterile water.…”

“…To screen for BR insensitivity among the dwarf mutants, the vertical Petri plate root elongation assay previously used by Clouse et al (1993Clouse et al ( , 1996 to identify BR-insensitive mutants in Arabidopsis was adapted for use in tomato. Wild-type Arabidopsis root elongation is severely inhibited by medium containing 0.5 m 24-epibrassinolide (Clouse et al, 1993).…”

“…A BR-insensitive Arabidopsis mutant, termed BR insensitive 1 (bri1), was identified by the ability of mutant plants to elongate primary roots in the presence of BR concentrations inhibitory to wild-type root elongation (Clouse et al, 1993(Clouse et al, , 1996. Several alleles of bri1 have been identified in independent screens (Kauschmann et al, 1996;Li and Chory, 1997;Clouse and Feldmann, 1999).…”

“…Therefore, application of exogenous BR was used as a screen to identify potential deficient mutants by rescue to wild-type phenotype. Furthermore, BRs are known to inhibit primary root elongation in tomato (Takatsuto et al, 1983;Roddick, 1994), and we adapted the Arabidopsis root elongation assay (Clouse et al, 1993) to tomato in an attempt to identify a BRinsensitive mutant . We present physiological, biochemical, and molecular evidence that the L. esculentum mutant dumpy (dpy) is likely to result from BR deficiency, while the Lycopersicon pimpinellifolium mutant curl-3 (cu-3) is not rescued by BR treatment and is insensitive specifically to BRs in the root inhibition assay.…”

Recombinant Brassinosteroid Insensitive 1 Receptor-Like Kinase Autophosphorylates on Serine and Threonine Residues and Phosphorylates a Conserved Peptide Motif in Vitro

“…In an attempt to identify components of the BR signal transduction pathway, we screened for BR insensitivity based on the ability of mutant Arabidopsis seedlings to elongate roots in the presence of BR concentrations inhibitory to wild-type root elongation (Clouse et al, 1993). This analysis uncovered a mutant brassinosteroid-insensitive1 (bri1) that conferred pleiotropic phenotypic effects, including severely dwarfed stature, reduced apical dominance, delayed flowering and senescence, male sterility, and nearly complete insensitivity to BRs in a variety of assays (Clouse et al, 1996).…”