Signalling and responses to strigolactones and karrikins

• Strigolactone hormones and karrikin smoke signals are similar signalling molecules.
• A common mechanism both perceives and distinguishes strigolactones and karrikins.
• A new protein (D53) has been discovered as a key target for strigolactone signalling.
• Downstream signalling involves transcriptional and post-translational mechanisms.
• Multiple members of receptor and target protein families allow multiple outputs.

Strigolactone (SL) and karrikin (KAR) signalling control many aspects of plant growth and development through similar mechanisms employing related α/β-fold hydrolase-receptors and a common F-box protein named MORE AXILARY GROWTH2 (MAX2) in Arabidopsis or DWARF3 (D3) in rice. D3 mediates SL-dependent ubiquitination and proteolysis of DWARF53 (D53) protein, thought to be involved in the control of gene expression, while a related protein SUPPRESSOR OF MAX2-1 (SMAX1) is implicated in the response to KAR in Arabidopsis. Different members of the D53/SMAX1 multigene family likely mediate different responses in plant growth and development. Analysis of responses to SL or KAR has identified many genes regulated by these compounds. Crosstalk with other signalling systems including light, hormones and abiotic stress has also been identified. Here we critically analyse how to progress towards a clearer understanding of the targets and functions of the SL and KAR signalling systems.