Identification of Arabidopsis BAK1-Associating Receptor-Like Kinase 1 (BARK1) and Characterization of its Gene Expression and Brassinosteroid-Regulated Root Phenotypes

Brassinosteroids (BRs) activate the BRI1 and BAK1/SERK3 membrane receptor complex, which leads to a wide range of changes in gene expression, plant growth and development. As an initial step to elucidate additional roles of BAK1, we cloned a BAK1-binding protein, BAK1-Associating Receptor-Like Kinase 1 (BARK1), and characterized its gene expression and root phenotypes. BARK1 is a putative membrane LRR-RLK (leucine-rich repeat receptor-like kinase) protein that specifically binds to BAK1 and its homologs. Careful examination of BARK1 expression using transgenic plants expressing a green fluorescent protein (GFP) reporter under the control of the native BARK1 promoter (BARK1p::GFP) revealed that this gene is ubiquitously expressed in most plant tissues, and shows especially strong expression in the xylem vasculature of primary and lateral roots as well as in mature pollen. Interestingly, the expression of the BARK1 gene was increased in the BR biosynthetic loss-of-function mutant, det2, and a loss-of-function mutant of BR signaling, bak1-3. In contrast, this gene was down-regulated in the bzr1-1D plant, which is a BR signal gain-of-function mutant. BARK1-overexpressing transgenic plants clearly enhanced primary root growth in a dose-dependent manner, and their roots were hypersensitive to BR-induced root growth inhibition. In addition, both the number and density of lateral roots were dramatically increased in the BARK1 transgenic plants in a dose-dependent manner. Together with observations that ARF (AUXIN RESPONSE FACTOR) genes are up-regulated in the BARK1 overexpressor, we suggest that the BARK1 overexpressor phenotype with more lateral roots is partly due to the increased expression of ARF genes in this genetic background. In conclusion, BAK1-interacting BARK1 protein may be involved in BR-mediated plant growth and development such as in lateral roots via auxin regulation.