Barley “uzu” and Wheat “uzu-like” Brassinosteroid Receptor BRI1 Kinase Domain Variations Modify Phosphorylation Activity In Vitro

Abstract

Brassinosteroid insensitive1 (BRI1), a leucine-rich repeat receptor kinase, is responsible for the perception of the brassinosteroid (BR) phytohormone in plants. While recent evidence has implicated a naturally occurring Hordeum vulgare V. (barley) HvBRI1 kinase domain (KD) variant (H857R; "uzu" variation) in increased fungal disease resistance, the impact of the variation on receptor function and thus the mechanism by which disease resistance might be imparted remain enigmatic. Here, the functional implications of the uzu variation as well as the effects of newly identified naturally occurring Triticum aestivum L. (wheat) TaBRI1-KD variants are investigated. Recombinantly produced KDs of wild-type (WT) and uzu HvBRI1 were assessed for phosphorylation activity in vitro, yielding WT K M and V MAX values similar to those of other reports, but the uzu variation delayed saturation and reduced turnover levels. In silico modeling of the H857R variation showed it to be surface-exposed and distal from the catalytic site. Further evaluation of three naturally occurring wheat TaBRI1 variants, A907T, A970V, and G1019R (barley numbering) identified in the A, B, and D subgenomic genes, respectively, highlighted a significant loss of activity for A907T. A907T is located on the same surface as the H857R variation and a negative regulatory phosphorylation site (T982) in Arabidopsis thaliana BRI1. A fourth variation, T1031A (barley numbering), unique to both subgenomic A proteins and localized to the BKI1 binding site, also decreased activity. The outcomes are discussed with respect to the predicted structural contexts of the variations and their implications with respect to mechanisms of action.

Brassinosteroids in Plants: Crosstalk with Small-Molecule Compounds

“…Brassinosteroids (BRs), a new type of plant hormone, have drawn an increased amount of attention. BRs, as a steroidal phytohormone, have been found to be involved in a wide range of physiological processes in plants, including cell elongation, cell division, seed development, flowering, and senescence, as well as both abiotic and biotic stress responses [ 5 , 6 , 7 , 8 ]. In addition, BRs have also been found to interact with other plant hormones to regulate plant growth and development as well as stress resistance.…”