Deviating from the Beaten Track: New Twists in Brassinosteroid Receptor Function

Abstract

A key feature of plants is their plastic development tailored to the environmental conditions. To integrate environmental signals with genetic growth regulatory programs, plants rely on a number of hormonal pathways, which are intimately connected at multiple levels. Brassinosteroids (BRs), a class of plant sterol hormones, are perceived by cell surface receptors and trigger responses instrumental in tailoring developmental programs to environmental cues. Arguably, BR signalling is one of the best-characterized plant signalling pathways, and the molecular composition of the core signal transduction cascade seems clear. However, BR research continues to reveal new twists to re-shape our view on this key signalling circuit. Here, exciting novel findings pointing to the plasma membrane as a key site for BR signalling modulation and integration with other pathways are reviewed and new inputs into the BR signalling pathway and emerging “non-canonical” functions of the BR receptor complex are highlighted. Together, this new evidence underscores the complexity of plant signalling integration and serves as a reminder that highly-interconnected signalling pathways frequently comprise non-linear aspects which are difficult to convey in classical conceptual models.

Importance of tyrosine phosphorylation for transmembrane signaling in plants

“…In turn, BIN2 controls the localization and stability of the transcription factors BRASSINAZOLE RESISTANT 1 (BZR1) and BZR2 through phosphorylation, thereby impairing the expression of BR-responsive genes [ 52 ]. Deactivation of BIN2 by BSU1 leads to stabilization and re-localization of BZR1/BZR2 to the nucleus, and eventually to induction of BR-responsive genes [ 53 , 54 ]. ( Figure 1A )…”

Engineering Chimeras by Fusing Plant Receptor-like Kinase EMS1 and BRI1 Reveals the Two Receptors’ Structural Specificity and Molecular Mechanisms

“…In Arabidopsis thaliana, the BRASSINOS-TEROID INSENSITIVE 1 (BRI1) LRR-RLK is found to perceive the plant growth hormones brassinosteroids (BRs); when BRI1 binds with BRs, an intracellular signaling cascade can be initiated, inducing the accumulation of non-phosphorylated forms of two transcription factors, BRI1 EMS SUPPRESSOR 1 (BES1) and BRASSINAZOLE RESISTANT 1 (BZR1), in the nucleus [3][4][5][6][7][8]. Activated BES1 and BZR1 can then mediate the expression of thousands of downstream responsive genes [9][10][11]. The EXCESS MICROSPOROCYTES1 (EMS1) LRR-RLK and its small protein ligand TAPETUM DETERMINANT1 (TPD1) are required for determining the fate of tapetal cells via a signaling transduction pathway shared with BRI1 [12][13][14][15][16].…”

Computational modeling and quantitative physiology reveal central parameters for brassinosteroid-regulated early cell physiological processes linked to elongation growth of theArabidopsisroot

“…The canonical sequence of BR perception and signal transduction, which also leads to cell elongation, is mediated by the plasma membrane (PM)-resident, nanoscale-organized receptor kinase brassinosteroid-insensitive 1 (BRI1) and its co-receptor BRI1-activating kinase 1 (BAK1) as central elements (Bücherl et al, 2013(Bücherl et al, , 2017Lv and Li, 2020;Wolf, 2020). The binding of BR to the recep-tor´s extracellular domain results in the re-arrangement of several BRI1-associated proteins.…”

“…Brassinosteroids(BRs)are plant steroid hormones that regulate a great variety of physiological and developmental processes including elongation growth as well as environmental adaptations ( Müssig et al, 2002 ; Clouse, 2002 ; Lv and Li, 2020 ; Wolf, 2020 ). To achieve this, BR signal transduction is closely linked with a multitude of other signaling pathways ( Lv and Li, 2020 ).…”