Karrikin Insensitive 2 (KAI2)

Karrikin Insensitive 2 (KAI2), also known as HYPOSENSITIVE TO LIGHT (HTL), is a plant alpha/beta-hydrolase that functions as a dual receptor and enzyme. It is the primary sensor for smoke-derived karrikins and an unidentified endogenous signal known as KAI2-ligand (KL), which are essential for regulating seed germination, seedling photomorphogenesis, and adaptation to abiotic stresses like drought and salinity. In parasitic weeds such as Striga, KAI2 paralogs have evolved to detect host-derived strigolactones, serving as the molecular trigger for germination and subsequent host infection. This makes the KAI2/HTL receptor system a critical target for agricultural biotechnology, particularly in the development of suicidal germination agents designed to deplete the seed banks of parasitic plants. The signaling mechanism involves the formation of a covalent ligand-receptor adduct that induces a conformational change, allowing the receptor to interact with the SCF-MAX2 ubiquitin ligase complex. This interaction leads to the proteasomal degradation of SMAX1-family transcriptional repressors, thereby activating downstream developmental programs. While primarily studied in the context of plant biology and agriculture, the unique covalent perception mechanism of KAI2/HTL provides a significant model for understanding hormone signaling and developing highly selective chemical regulators.

Other names

HYPOSENSITIVE TO LIGHTHTLDWARF14-LikeD14L

Ligand binding to the catalytic triad (Ser-His-Asp) followed by hydrolysis and the formation of a covalent adduct, which induces a conformational change in the receptor. This change promotes the recruitment of the SCF-MAX2 E3 ubiquitin ligase complex, leading to the polyubiquitination and subsequent proteasomal degradation of SMAX1 and SMXL2 transcriptional corepressors.