Karrikin perception and signalling

Abstract

Summary Karrikins (KARs) are a class of butenolide compounds found in smoke that were first identified as seed germination stimulants for fire‐following species. Early studies of KARs classified the germination and postgermination responses of many plant species and investigated crosstalk with plant hormones that regulate germination. The discovery that Arabidopsis thaliana responds to KARs laid the foundation for identifying mutants with altered KAR responses. Genetic analysis of KAR signalling revealed an unexpected link to strigolactones (SLs), a class of carotenoid‐derived plant hormones. Substantial progress has since been made towards understanding how KARs are perceived and regulate plant growth, in no small part due to advances in understanding SL perception. KAR and SL signalling systems are evolutionarily related and retain a high degree of similarity. There is strong evidence that KARs are natural analogues of an endogenous signal(s), KAI2 ligand (KL), which remains unknown. KAR/KL signalling regulates many developmental processes in plants including germination, seedling photomorphogenesis, and root and root hair growth. KAR/KL signalling also affects abiotic stress responses and arbuscular mycorrhizal symbiosis. Here, we summarise the current knowledge of KAR/KL signalling and discuss current controversies and unanswered questions in this field.

Phytohormones in a universe of regulatory metabolites: lessons from jasmonate

“…The KARRIKIN INSENSITIVE 2 receptor, which perceives smoke-derived compounds (i.e. karrikins) that promote seed germination, is thought to perceive an endogenous ligand, but the identity of this compound(s) remains elusive ( Waters and Nelson 2023 ). Another example is the cytochrome P450 KLUH/KLU, which in Arabidopsis is involved in the production of a mobile growth-promoting factor that is likely distinct from known phytohormones ( Anastasiou et al 2007 ).…”

Volatile communication in plants relies on a KAI2-mediated signaling pathway

“…Karrikins are not endogenously produced by plants but are bioactive compounds of smoke, which stimulate the gemination of seeds across more than 1200 plant species from more than 80 genera ( 15 , 16 ). They also regulate numerous plant developmental processes unrelated to fires, including ethylene-dependent root growth ( 18 ), in addition to their important roles in biotic and abiotic responses ( 19 ). Karrikins are perceived by the karrikin insensitive2 (KAI2) receptor ( 14 , 20 ), for which most angiosperms have one or more copies of the encoding gene(s).…”

Karrikin signalling: impacts on plant development and abiotic stress tolerance

“…There is a great deal of potential chemical diversity that can emerge from the oxidative cleavage of carotenoids, whether via enzymic or non-enzymic means, and searching this chemical space is gradually resulting in the discovery of new bioactive hormone-like compounds ( Wang et al , 2019 ). However, to date, no definitive evidence has been found to indicate an apocarotenoid origin for KLs, nor have any clues on a biosynthetic route been uncovered via genetic approaches ( Yao et al , 2018 ; Waters and Nelson, 2023 ). Indeed, rather than being a single compound, ‘KL’ may constitute several compounds of differing origins and chemical structure, unified by their capacity to signal through a KAI2-like α/β-hydrolase.…”

“…Such signals include KARs and synthetic butenolides, effects of which can be determined empirically, as well as unknown KL compounds, whose influence is inferred through mutant analysis. To complement other comprehensive reviews in this area ( Varshney and Gutjahr, 2023 ; Waters and Nelson, 2023 ), this review will focus on the roles of KAR–KL signalling in plant growth responses and stress tolerance.…”