Crosstalk between strigolactone and KAI2/KL signalling in plant growth and stress responses

Strigolactones (SLs) and karrikins (KARs) are signalling molecules of distinct origin: SLs are produced through plant metabolism, whereas KARs arise from the combustion of plant carbohydrates and are thought to mimic an endogenous ligand, whose chemical identity remains unknown. Despite these differences, they share structural similarities and partially overlapping signalling pathways, which are central to understanding their crosstalk as well as their distinct vs shared effects. These similarities extend from structural features and sensing mechanisms to downstream signal transduction. Accordingly, SL and KAR pathways are not entirely independent; rather, they converge, interact, and mutually regulate one another. However, advancements as well as the interpretation of the existing literature are constrained by two unresolved issues. First, the effects of KARs are rightly tested against the effects of loss-of-function mutations of their receptor KAI2; but this does not allow discriminating against the effects of its endogenous, so far uncharacterized ligand. Second, the synthetic compounds most routinely used as analogues of SLs lack receptor specificity, with rac-GR24 activating both D14 (the proper SL receptor) and KAI2. This is a confounding factor that is often not considered when attributing phenotypes to either pathway. Drawing on current research, this paper reviews similarities and differences between the two pathways, with particular emphasis on their cooperative and regulatory roles in shoot, leaf and root development. It also reviews their functions and especially interactions under abiotic stresses such as drought, salinity, low-light conditions, nutrient limitation, and suboptimal temperatures, systematically examining the level of confidence and criticalities for each dataset. Finally, it discusses whether and how natural or synthetic KAR-related molecules may be used to enhance crop resilience under adverse environmental conditions.