Strigolactones and karrikins: Butenolide signals shaping plant development and stress adaptation

Strigolactones (SLs) and karrikins (KARs) are small butenolides but originate from distinct sources and serve different roles. Strigolactones, derived from carotenoids, are synthesized endogenously and regulate shoot branching, root growth, leaf senescence and reproductive development. Karrikins, on the other hand, are exogenous smoke-derived molecules that trigger seed germination and postfire seedling vigour. Despite this diversity of origin, SLs operate through the DWARF14 (D14) receptor, while KARRIKIN INSENSITIVE 2 (KAI2) is assumed to be the karrikin receptor, both working through many overlapping pathways. These signalling receptors work as complexes with the F-box protein MORE AXILLARY GROWTH 2 (MAX2) which assists pathway activation via repressor degradation mediated by SMXL/D53 proteins. Beyond developmental modulation, SLs have recently been uncovered as pivotal mediators in plant abiotic stress responsive pathways. The plant thus becomes tolerant to drought and salinity, as well as nutrient limitation, through stomatal modulation by SLs, root system architecture and antioxidant defence adjustments. KAR/KAI2 signalling influences a wide range of developmental responses, including cuticle formation, antioxidant defence and seedling establishment under stress. It helps to regulate osmotic adjustment and seed germination in response to stress conditions. This crosstalk involves interactions with abscisic acid (ABA), auxin, gibberellins, cytokinins (CKs), ethylene and other redox molecules such as reactive oxygen species (ROS) and nitrous oxide (NO) to regulate adaptive responses under stress conditions. This review covers recent breakthroughs in SL and KAR biology, with a particular focus on their developmental functions, roles in stress and potential applications for engineering crop resilience.