BRASSINOSTEROID-SIGNALING KINASEs: Small Family, Big Functions

Plant cells utilize cell-surface receptors to detect external cues or endogenous signals, thereby triggering appropriate responses that regulate various physiological processes. Receptor-like kinases (RLKs), located on the cell surface, play a pivotal role in recognizing extracellular signal molecules and activating downstream signaling pathways. Receptor-like cytoplasmic kinases (RLCKs), lacking extracellular ligand-binding domains, function as downstream partners of RLKs to transduce the signals to intracellular signaling components such as the mitogen-activated protein kinase (MAPK) cascade. RLCKs are classified into 17 subfamilies, which are further divided into distinct branches based on sequence similarity. Among the RLCKs, the XII-1 members, known as BRASSINOSTEROID-SIGNALING KINASEs (BSKs), play key roles in regulating diverse physiological processes. Given their importance, this review highlights recent advances in understanding the role of BSKs in plant growth, development, and stress responses. We detail the regulation of BSKs' subcellular localization through post-translational modifications and explore the molecular mechanisms by which BSKs modulate signaling pathways by engaging with RLK receptors and MAPK cascades. We also discuss the other signaling pathways regulated by BSKs. Collectively, this review underscores the role of BSKs as potential linker proteins localized at the plasma membrane (PM), functioning downstream of cell surface receptors and upstream of MAPK cascades and other signaling components to orchestrate a multitude of developmental processes and stress responses.