Mastering the balance: BAK1's dual roles in steering plant growth and immunity

BAK1 was initially identified as a coreceptor of BRI1 in regulating the brassinosteroid-triggered signaling pathway in Arabidopsis. Over the past two decades, increasing pieces of evidence have demonstrated that BAK1 and its close paralogs form receptor-coreceptor complexes with distinct ligand-binding receptors. Through ligand-induced heterodimerization with receptor-like protein kinases or receptor-like proteins, BAK1 thereby regulates a variety of physiological events such as plant development, immunity, and stress responses. Thus, BAK1 plays a central role in directly governing the trade-offs of multiple signaling pathways. Deciphering the molecular mechanisms underlying how BAK1 coordinates plant growth and defense, with specific emphasis on its coreceptor functions, holds significant potential for future advancements in crop breeding. This review seeks to explore the latest insights into how BAK1 impacts the intricate equilibrium between plant development and immunity, as well as its roles in coordinating the activation of pattern-triggered immunity and effector-triggered immunity or programmed cell death. Furthermore, it offers significant perspectives on why BAK1 has been chosen as a shared component in determining plant growth and defense mechanisms across model plants to horticultural crops.