KAI2 Can Do: Karrikin Receptor Function in Plant Development and Response to Abiotic and Biotic Factors.

This article focuses on the α/β hydrolase receptor KARRIKIN INSENSITIVE 2 (KAI2) and its role in perceiving a yet unidentified phytohormone, provisionally termed KAI2 ligand (KL), as well as karrikins—smoke-derived butenolide compounds. KAI2-mediated signaling regulates diverse aspects of plant development including seed germination, seedling and adult morphology, and root architecture, while also modulating responses to abiotic stresses such as heat, drought, osmotic stress, and phosphate starvation. Additionally, KAI2 influences biotic interactions by promoting colonization by arbuscular mycorrhizal fungi and contributing to defense against certain bacterial pathogens through stomatal regulation and salicylic acid–dependent pathways. The signaling pathway involves the F-box protein MORE AXILLARY GROWTH 2 (MAX2) and degradation of SUPPRESSOR OF MAX2-LIKE (SMXL) proteins, with emerging evidence of complex cross-talk between KAI2 signaling and other phytohormone pathways including auxin, gibberellin, abscisic acid, ethylene, and salicylic acid. Despite advances, the chemical identity of KL and detailed molecular mechanisms remain to be elucidated.