Karrikins: A novel plant growth regulator with emerging agronomic applications

Abstract

Karrikins (KARs) are a group of small butenolide organic compounds produced during the combustion of plant material, particularly cellulose, in wildfires. Unlike classical plant hormones that are endogenously synthesized, karrikins are exogenous fire-derived signals that profoundly influence seed germination, seedling development, and plant adaptation to abiotic stresses. The first active compound, KAR1 (3-methyl-2H-furo[2,3-c]pyran-2-one) was identified in 2004 and after that other six karrikin molecules (KAR1–KAR6) have also been characterized. KARs are perceived through the α/β-hydrolase receptor KAI2 (KARRIKIN INSENSITIVE 2) and transduce their signal through MAX2 and SMXL proteins, a pathway strikingly parallel to that of strigolactones (SLs). Despite their structural similarity to SLs, KARs originate from pyrolysis rather than plant biosynthesis and signal ecological contexts in which plant competition is absent. Research across multiple crop species — including tomato, maize, rice, and okra — demonstrates that KARs and smoke water significantly enhance shoot growth, root development, seedling vigor, and tolerance to temperature extremes. KAR signaling modulates interactions with gibberellic acid, abscisic acid, and indole-3-acetic acid to regulate dormancy and germination. Under abiotic stress conditions such as drought, KARs enhance antioxidant defense, stomatal regulation, cuticle formation, and anthocyanin biosynthesis. The ecological role of KARs in post-fire revegetation further underscores their relevance to sustainable agriculture and stress-resilient crop management. This review consolidates current knowledge on karrikin chemistry, signaling mechanisms, hormonal interactions, and agronomic significance, with a focus on future research directions.