PHB3 interacts with BRI1 and BAK1 to mediate brassinosteroid signal transduction in Arabidopsis and tomato

Summary Brassinosteroids (BRs) are plant hormones that are essential in plant growth and development. BRASSINOSTEROID‐INSENSITIVE 1 (BRI1) and BRI1 ASSOCIATED RECEPTOR KINASE 1 (BAK1), which are located on the plasma membrane, function as co‐receptors that accept and transmit BR signals. PROHIBITIN 3 (PHB3) was identified in both BRI1 and BAK1 complexes by affinity purification and LC‐MS/MS analysis. Biochemical data showed that BRI1/BAK1 interacted with PHB3 in vitro and in vivo . BRI1/BAK1 phosphorylated PHB3 in vitro . When the Thr‐80 amino acid in PHB3 was mutated to Ala, the mutant protein was not phosphorylated by BRI1 and the mutant protein interaction with BRI1 was abolished in the yeast two‐hybrid assay. BAK1 did not phosphorylate the mutant protein PHB3 T54A . The loss‐of‐function phb3 mutant showed a weaker BR signal than the wild‐type. Genetic analyses revealed that PHB3 is a BRI1/BAK1 downstream substrate that participates in BR signalling. PHB3 has five homozygous in tomato, and we named the closest to AtPHB3 as SlPHB3.1. Biochemical data showed that SlBRI1/SlSERK3A/SlSERK3B interacted with SlPHB3.1 and SlPHB3.3. The CRISPR‐Cas9 method generated slphb3.1 mutant led to a BR signal stunted relatively in tomatoes. PHB3 is a new component of the BR signal pathway in both Arabidopsis and tomato.