title: β-amylase-like proteins function as transcription factors in Arabidopsis, controlling shoot growth and development.
abstract: Plants contain β-amylase-like proteins (BAMs; enzymes usually associated with starch breakdown) present in the nucleus rather than targeted to the chloroplast. They possess BRASSINAZOLE RESISTANT1 (BZR1)-type DNA binding domains--also found in transcription factors mediating brassinosteroid (BR) responses. The two Arabidopsis thaliana BZR1-BAM proteins (BAM7 and BAM8) bind a cis-regulatory element that both contains a G box and resembles a BR-responsive element. In protoplast transactivation assays, these BZR1-BAMs activate gene expression. Structural modeling suggests that the BAM domain's glucan binding cleft is intact, but the recombinant proteins are at least 1000 times less active than chloroplastic β-amylases. Deregulation of BZR1-BAMs (the bam7bam8 double mutant and BAM8-overexpressing plants) causes altered leaf growth and development. Of the genes upregulated in plants overexpressing BAM8 and downregulated in bam7bam8 plants, many carry the cis-regulatory element in their promoters. Many genes that respond to BRs are inversely regulated by BZR1-BAMs. We propose a role for BZR1-BAMs in controlling plant growth and development through crosstalk with BR signaling. Furthermore, we speculate that BZR1-BAMs may transmit metabolic signals by binding a ligand in their BAM domain, although diurnal changes in the concentration of maltose, a candidate ligand produced by chloroplastic β-amylases, do not influence their transcription factor function.
authors.affiliation: Department of Biology, ETH Zurich, CH-8092 Zurich, Switzerland.
authors.forename: Heike
authors.initials: H
authors.lastname: Reinhold
authors.forename: Sebastian
authors.initials: S
authors.lastname: Soyk
authors.forename: Klára
authors.initials: K
authors.lastname: Simková
authors.forename: Carmen
authors.initials: C
authors.lastname: Hostettler
authors.forename: John
authors.initials: J
authors.lastname: Marafino
authors.forename: Samantha
authors.initials: S
authors.lastname: Mainiero
authors.forename: Cara K
authors.initials: CK
authors.lastname: Vaughan
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Monroe
authors.forename: Samuel C
authors.initials: SC
authors.lastname: Zeeman
mesh_terms: D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development*; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000378:metabolism*; D001483:Base Sequence; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant / Q000235:genetics; D005936:Glucans / Q000378:metabolism; D006868:Hydrolysis; D008954:Models, Biological; D008969:Molecular Sequence Data; D050505:Mutant Proteins / Q000302:isolation & purification / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D009687:Nuclear Proteins / Q000378:metabolism; D018515:Plant Leaves / Q000235:genetics / Q000254:growth & development; D018520:Plant Shoots / Q000201:enzymology / Q000254:growth & development*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D020218:Response Elements / Q000235:genetics; D015534:Trans-Activators / Q000378:metabolism; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism*; D001614:beta-Amylase / Q000737:chemistry / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D005936:Glucans; D050505:Mutant Proteins; D009687:Nuclear Proteins; D015534:Trans-Activators; D014157:Transcription Factors; C561078:BAM7 protein, Arabidopsis; C561079:BAM8 protein, Arabidopsis; D001614:beta-Amylase