Stereospecific reduction of the butenolide in strigolactones in plants

Stereospecific reduction of 2'S-configured strigolactones by cowpea OPR3 enzymes

Strigolactones (SLs), plant-derived apocarotenoids, serve dual roles as phytohormones and rhizosphere signaling molecules. While exogenous administration of SLs to plants aids in studying their functions, the metabolic destiny of these administered SLs remains poorly elucidated. Our previous research demonstrated that among synthetic SL GR24 stereoisomers administered to cowpea (Vigna unguiculata), 2'-epi-GR24 undergoes selective reduction at the C-3',4' double bond in its D-ring. In this investigation, we isolated proteins from cowpea roots based on SL reducing activity and identified 12-oxophytodienoate reductase 3 homologs (VuOPR3s) as contributors to this reduction. Enzymatic assays conducted with recombinant proteins revealed that VuOPR3s exhibited a preference for reducing activity toward 2'S-configured SLs, including 2'-epi-GR24. This specificity for 2'S-configured SLs was congruent with that observed for orobanchol produced by cowpea and its stereoisomers. These findings suggest that exogenously administered SLs undergo enzymatic stereoselective reduction, underscoring the importance of considering stereospecificity when interpreting data obtained from SL usage.

A Phelipanche ramosa KAI2 protein perceives strigolactones and isothiocyanates enzymatically

Phelipanche ramosa is an obligate root-parasitic weed that threatens major crops in central Europe. In order to germinate, it must perceive various structurally divergent host-exuded signals, including isothiocyanates (ITCs) and strigolactones (SLs). However, the receptors involved are still uncharacterized. Here, we identify five putative SL receptors in P. ramosa and show that PrKAI2d3 is involved in the stimulation of seed germination. We demonstrate the high plasticity of PrKAI2d3, which allows it to interact with different chemicals, including ITCs. The SL perception mechanism of PrKAI2d3 is similar to that of endogenous SLs in non-parasitic plants. We provide evidence that PrKAI2d3 enzymatic activity confers hypersensitivity to SLs. Additionally, we demonstrate that methylbutenolide-OH binds PrKAI2d3 and stimulates P. ramosa germination with bioactivity comparable to that of ITCs. This study demonstrates that P. ramosa has extended its signal perception system during evolution, a fact that should be considered for the development of specific and efficient biocontrol methods.

Initiation of arbuscular mycorrhizal symbiosis involves a novel pathway independent from hyphal branching

The arbuscular mycorrhizal symbiosis is a very common association between plant roots and soil fungi, which greatly contributes to plant nutrition. Root-exuded compounds known as strigolactones act as symbiotic signals stimulating the fungus prior to root colonization. Strigolactones also play an endogenous role in planta as phytohormones and contribute to the regulation of various developmental traits. Structure-activity relationship studies have revealed both similarities and differences between the structural features required for bioactivity in plants and arbuscular mycorrhizal fungi. In the latter case, bioassays usually measured a stimulation of hyphal branching on isolated fungi of the Gigaspora genus, grown in vitro. Here, we extended these investigations with a bioassay that evaluates the bioactivity of strigolactone analogs in a symbiotic situation and the use of the model mycorrhizal fungus Rhizophagus irregularis. Some general structural requirements for bioactivity reported previously for Gigaspora were confirmed. We also tested additional strigolactone analogs bearing modifications on the conserved methylbutenolide ring, a key element of strigolactone perception by plants. A strigolactone analog with an unmethylated butenolide ring could enhance the ability of R. irregularis to colonize host roots. Surprisingly, when applied to the isolated fungus in vitro, this compound stimulated germ tube elongation but inhibited hyphal branching. Therefore, this compound was able to act on the fungal and/or plant partner to facilitate initiation of the arbuscular mycorrhizal symbiosis, independently from hyphal branching and possibly from the strigolactone pathway.