Smoke and Hormone Mirrors: Action and Evolution of Karrikin and Strigolactone Signaling

Karrikins and strigolactones are two classes of butenolide molecules that have diverse effects on plant growth. Karrikins are found in smoke and strigolactones are plant hormones, yet both molecules are likely recognized through highly similar signaling mechanisms. Here we review the most recent discoveries of karrikin and strigolactone perception and signal transduction. Two paralogous α/β hydrolases, KAI2 and D14, are respectively karrikin and strigolactone receptors. D14 acts with an F-box protein, MAX2, to target SMXL/D53 family proteins for proteasomal degradation, and genetic data suggest that KAI2 acts similarly. There are striking parallels in the signaling mechanisms of karrikins, strigolactones, and other plant hormones, including auxins, jasmonates, and gibberellins. Recent investigations of host perception in parasitic plants have demonstrated that strigolactone recognition can evolve following gene duplication of KAI2.

TrendsCompelling evidence that KAI2 and D14 are karrikin (KAR) and strigolactone (SL) receptors with hydrolytic activity has been provided through recent genetic, biochemical, and structural studies in several plant species. Ligand hydrolysis promotes protein–protein interactions.The elusive targets of KAR and SL signaling have been discovered. Two clades within the SMXL/D53 family regulate growth processes that are associated with either KAR or SL responses.SL receptors in parasitic weeds, which enable host-triggered germination, evolved from KAI2 paralogs. Other KAI2 paralogs in parasites detect KARs specifically or an unidentified endogenous ligand, KL.KAR and SL signaling mechanisms share features of other plant hormone pathways, including hormone-activated proteolysis of EAR motif-containing proteins that likely interact with transcriptional corepressors.