Strigolactones, karrikins and beyond

The plant hormones strigolactones are synthesized from carotenoids and signal via the α/β hydrolase DWARF 14 (D14) and the F-box protein MORE AXILLARY GROWTH 2 (MAX2). Karrikins, molecules produced upon fire, share MAX2 for signalling, but depend on the D14 paralog KARRIKIN INSENSITIVE 2 (KAI2) for perception with strong evidence that the MAX2-KAI2 protein complex might also recognize so far unknown plant-made karrikin-like molecules. Thus, the phenotypes of the max2 mutants are the complex consequence of a loss of both D14-dependent and KAI2-dependent signalling, hence, the reason why some biological roles, attributed to strigolactones based on max2 phenotypes, could never be observed in d14 or in the strigolactone-deficient max3 and max4 mutants. Moreover, the broadly used synthetic strigolactone analog rac-GR24 has been shown to mimic strigolactone as well as karrikin(-like) signals, providing an extra level of complexity in the distinction of the unique and common roles of both molecules in plant biology. Here, a critical overview is provided of the diverse biological processes regulated by strigolactones and/or karrikins. These two growth regulators are considered beyond their boundaries, and the importance of the yet unknown karrikin-like molecules is discussed as well.