Three mutations repurpose a plant karrikin receptor to a strigolactone receptor

Abstract

Uncovering the basis of small-molecule hormone receptors’ evolution is paramount to a complete understanding of how protein structure drives function. In plants, hormone receptors for strigolactones are well suited to evolutionary inquiries because closely related homologs have different ligand preferences. More importantly, because of facile plant transgenic systems, receptors can be swapped and quickly assessed functionally in vivo. Here, we show that only three mutations are required to turn the nonstrigolactone receptor, KAI2, into a receptor that recognizes the plant hormone strigolactone. This modified receptor still retains its native function to perceive KAI2 ligands. Our directed evolution studies indicate that only a few keystone mutations are required to increase receptor promiscuity of KAI2, which may have implications for strigolactone receptor evolution in parasitic plants.

KARRIKIN INSENSITIVE2 regulates leaf development, root system architecture and arbuscular‐mycorrhizal symbiosis in Brachypodium distachyon

“…Both karrikins and SLs act through the F‐box protein MAX2 (MORE AXILLARY BRANCHES2), which targets members of the SMXL (SUPPRESSOR‐OF‐MAX2‐1‐LIKE) family of repressor proteins for polyubiquitination and subsequent proteasomal degradation (Nelson et al., 2011; Stanga et al., 2013; Zhou et al., 2013; Jiang et al., 2013; Zhao et al., 2015). Specificity in karrikin and SL response arises both from intrinsic differences in ligand preference between KAI2 and D14 (Scaffidi et al., 2014; Wang et al., 2020; Arellano‐Saab et al., 2021; Yao et al., 2021), and through degradation of specific SMXL proteins that associate with each receptor upon ligand recognition (Soundappan et al., 2015; Wang et al., 2015, 2020; de Saint Germain et al., 2016; Khosla et al., 2020; Zheng et al., 2020). The similar componentry of KAI2‐ and D14‐dependent signalling, the evolutionary conservation of KAI2, and the fact that kai2 phenotypes are opposite to the effects of applying karrikins, collectively have led to the proposal that KAI2 is a receptor for an endogenous butenolide compound(s) called ‘KAI2 ligand’ (KL; Nelson et al., 2011; Conn and Nelson, 2016; Sun et al., 2016; Yao et al., 2021).…”

A novel strigolactone receptor antagonist provides insights into the structural inhibition, conditioning, and germination of the crop parasite Striga

“…Finally, the RG6–ShHTL7 complex did not result in significant structural changes within the receptor compared with apoprotein structures, suggesting RG6 locks ShHTL7 into an inactive open state ( 27 ). Recent studies suggest that flexibility in the lid region, particularly in the αE loop of α/β hydrolase receptors, is key for SL binding and attracting downstream signaling partners ( 28 , 29 , 30 ). Possibly, RG6 reduces this flexibility, which would contribute to the inhibition of downstream signaling.…”

“…The motion of the αE loop and the upper left lid domain are thought to be important in SL signaling ( 28 , 29 , 35 ). Our previous MD simulation findings for a chimeric SL receptor and GR24 indicate that, upon binding to an SL, the αE loop of the protein increases its motion, to facilitate a conformational change and achieve signaling ( 28 ). This analysis reiterates the notion that reducing flexibility of the αE loop should be considered a desirable feature of potent SL antagonists.…”

HTL/KAI2 signalling substitutes for light to control plant germination

“…10 We also know only a few mutations are required to change a KAR receptor into an SL receptor. 37 It is therefore easy to envision how selective pressures on HTL/KAI2 receptors contributed to the evolution of a conserved germination-signalling pathway. Our results emphasize that defining signalling pathways by their ligands and receptors can be confounding when these signals influence conserved downstream effectors that are also modulated by other signalling pathways.…”