Abstract
Brassinosteroids (BRs), plant steroid hormones, play important roles in plant cell elongation and differentiation. To investigate the mechanisms of BR signaling, we previously used the BR biosynthesis inhibitor Brz as a chemical biology tool and identified the Brz-insensitive-long hypocotyl4 mutant (bil4). Although the BIL4 gene encodes a seven-transmembrane-domain protein that is evolutionarily conserved in plants and animals, the molecular function of BIL4 in BR signaling has not been elucidated. Here, we demonstrate that BIL4 is expressed in early elongating cells and regulates cell elongation in Arabidopsis. BIL4 also activates BR signaling and interacts with the BR receptor brassinosteroid insensitive 1 (BRI1) in endosomes. BIL4 deficiency increases the localization of BRI1 in the vacuoles. Our results demonstrate that BIL4 regulates cell elongation and BR signaling via the regulation of BRI1 localization.
Highlights
Steroid hormones are bioactive substances that are widely conserved in eukaryotes[1] and serve as signaling molecules to control growth and development
brassinosteroid insensitive 1 (BRI1) endocytosis is regulated by the guanine nucleotide exchange factor for ADP-ribosylation factor (ARF-GEF) GNOM14, adaptor protein complex-2 (AP-2)[15], and plant-specific TPLATE adaptor complex (TPC), which is associated with key components of clathrin-mediated endocytosis (CME)[16]
BRI1 activation induces the dissociation of its inhibitor BRI1 kinase inhibitor 1 (BKI1) from the plasma membrane and the phosphorylation of BR-signaling kinase 1 (BSK1), a positive regulator of the pathway[10, 11]
Summary
Steroid hormones are bioactive substances that are widely conserved in eukaryotes[1] and serve as signaling molecules to control growth and development. The dwarf phenotype of BRI1-deficient mutants (bri1)[7] suggests that BRI1 plays an important role in plant growth. Brz treatment decreases the BR content[20] and causes a dwarf phenotype in plants, which is similar to the phenotype of BR-deficient mutants. These responses to Brz were used to search in a chemical biology screen for gain-of-function mutants that are resistant to Brz. The first gene identified using this approach was BZR121. We identified BIL4 to be a regulatory factor affecting both endocytosis of the BR receptor BRI1 and BR-mediated plant cell elongation
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