Mechanistic insights into ligand binding and receptor activation of plant leucine-rich repeat receptor kinases.

Brassinosteroids (BRs) are steroid hormones controlling plant growth and development. How BRs are sensed by the receptor kinase BRI1 and its co-receptors SERK1-3 at the plasma membrane is known in molecular detail. Core downstream signaling components have been uncovered as well. However, the site of BR biosynthesis, the modes of BR transport and the hormone's range of action are only partially understood.

Here, engineered BRI1 receptor proteins fused to fluorescent protein tags are proposed as tools to address these questions. Proteins consisting either of BRI1's extracellular domain (BRI1-ECD) or of the ECD and transmembrane helix (BRI1-ΔKin) cause a drastic growth phenotypes when expressed in Arabidopsis thaliana. Point-mutations that either impair steroid hormone binding or receptor activation are used as genetic controls to study whether these engineered proteins impact BR availability or signaling.

Analysis of our engineered plant lines indicates that the phenotypes might be caused by depletion of the steroid hormone(s) through interaction with the over-expressed ligand-binding domain. As we can localize these proteins to different plant tissues and sub-cellular compartments, we now aim to understand where BRs are synthesized and how their range of action is defined in vivo. In addition, our protein constructs may allow to inhibit BR signaling at distinct growth and developmental stages of the plant.