Strigolactone Hormones and Their Stereoisomers Signal through Two Related Receptor Proteins to Induce Different Physiological Responses in Arabidopsis  

Abstract

Two a/b-fold hydrolases, KARRIKIN INSENSITIVE2 (KAI2) and Arabidopsis thaliana DWARF14 (AtD14), are necessary for responses to karrikins (KARs) and strigolactones (SLs) in Arabidopsis (Arabidopsis thaliana). Although KAI2 mediates responses to KARs and some SL analogs, AtD14 mediates SL but not KAR responses. To further determine the specificity of these proteins, we assessed the ability of naturally occurring deoxystrigolactones to inhibit Arabidopsis hypocotyl elongation, regulate seedling gene expression, suppress outgrowth of secondary inflorescences, and promote seed germination. Neither 5-deoxystrigol nor 4-deoxyorobanchol was active in KAI2-dependent seed germination or hypocotyl elongation, but both were active in AtD14-dependent hypocotyl elongation and secondary shoot growth. However, the nonnatural enantiomer of 5-deoxystrigol was active through KAI2 in growth and gene expression assays. We found that the four stereoisomers of the SL analog GR24 had similar activities to their deoxystrigolactone counterparts. The results suggest that AtD14 and KAI2 exhibit selectivity to the butenolide D ring in the 29R and 29S configurations, respectively. However, we found, for nitrile-debranone (CN-debranone, a simple SL analog), that the 29R configuration is inactive but that the 29S configuration is active through both AtD14 and KAI2. Our results support the conclusion that KAI2-dependent signaling does not respond to canonical SLs. Furthermore, racemic mixtures of chemically synthesized SLs and their analogs, such as GR24, should be used with caution because they can activate responses that are not specific to naturally occurring SLs. In contrast, the use of specific stereoisomers might provide valuable information about the specific perception systems operating in different plant tissues, parasitic weed seeds, and arbuscular mycorrhizae.Strigolactones (SLs) are carotenoid-derived phytohormones that mediate various aspects of plant development in addition to symbiotic and parasitic interactions in the rhizosphere. Originally identified as seed germination stimulants of root-parasitic weeds (Cook et al., 1966(Cook et al., , 1972, SLs have now been implicated in several processes, including inhibition of bud outgrowth to decrease shoot branching, regulation of leaf morphology, regulation of root architecture, control of secondary growth in the cambium, and association of plant roots with symbiotic fungi and nodulating bacteria (for review, see Brewer et al., 2013;Waldie et al., 2014).The isolation of several mutants in pea (Pisum sativum; rms), rice (Oryza sativa; d), petunia (Petunia hybrida; dad), and Arabidopsis (Arabidopsis thaliana; max) that exhibit dwarfism and an increased number of secondary shoots or tillers has enabled the identification of genes involved in four steps in SL biosynthesis and another three involved in signal transduction. The initial step of SL biosynthesis involves the conversion of all-trans-b-carotene into 9-cis-b-carotene by the isomerase D27. The sequential cleavage of the D27 produ...

Destabilization of strigolactone receptor DWARF14 by binding of ligand and E3-ligase signaling effector DWARF3

“…Two of the four possible stereoisomers, designated GR24 5DS and GR24 4DO , have the same 2′R-configuration of the D-ring as found in all physiological SLs. These two isomers, and particularly GR24 5DS , are more active in D14-dependent bioassays than the other two isomers [31]. The same two isomers were selectively capable of stimulating the interaction between D14 and D3, with GR24 5DS being more potent than GR24 4DO ( Figure 5C), thus closely mimicking the effects of GR24 isomers in plants [31].…”

“…These two isomers, and particularly GR24 5DS , are more active in D14-dependent bioassays than the other two isomers [31]. The same two isomers were selectively capable of stimulating the interaction between D14 and D3, with GR24 5DS being more potent than GR24 4DO ( Figure 5C), thus closely mimicking the effects of GR24 isomers in plants [31]. The activity to stimulate the D14-D3 interaction further correlates with the energetically most favorable binding conformation as judged by computational docking (Supplementary information, Figure S7A), and the docking position of the 5DS isomer is most consistent with the GR24 model (Supplementary information, Figure S7B).…”

Nitro-Phenlactone, a Carlactone Analog with Pleiotropic Strigolactone Activities

“…A study on the impact of stereochemical configuration on biological activities demonstrated that natural SLs -with 2'C atom in R configuration -inhibit hypocotyl elongation in Arabidopsis, preferentially through the D14-MAX2 signalling pathway, while the effects of karrikins and ent-GR24 (2'C atom in S configuration) on hypocotyl elongation and their induction of germination in dormant Arabidopsis seeds are mediated by the KAI2-MAX2 pathway (Scaffidi et al, 2014). To explore whether rac-N-PL exerts karrikin-like function in regulating seedling development, we examined the effect of two different concentrations (1 µM and 5 µM) of this compound on hypocotyl length.…”

From little things big things grow: karrikins and new directions in plant development

“…2). Importantly, kai2 mutants retain the ability to respond to exogenous strigolactones, and d14 mutants to karrikins (Waters et al 2012a;Scaffidi et al 2014). By extension, some max2 phenotypes are thus strigolactone related (as mediated by D14), and others are strigolactoneindependent (as mediated by KAI2).…”