Arabidopsis thaliana triggers Phelipanche ramosa germination via isothiocyanates under negative strigolactone regulation

Abstract Parasitic weeds are major agricultural threats through fine-tuned chemical interactions with their hosts. We investigated the germination-stimulating metabolites released by Arabidopsis thaliana germinating seeds into the spermosphere, the zone surrounding the seeds and showed that host–parasite chemical recognition can be initiated at the earliest stages of host seed germination. Through co-germination assays, biochemical fractionation of host germinating seed exudates and a mutant approach targeting the glucosinolate (GSL) and strigolactone (SL) pathways, we demonstrated that germinating A. thaliana Col-0 seeds specifically stimulated P. ramosa germination via GSL-derived isothiocyanates (ITCs), not SLs. This activity was MYB28, MYB29-dependent (aliphatic GSLs) and MYB34, MYB51, MYB122-independent (indolic GSLs). These results were confirmed throughout developmental stages of A. thaliana GSL-deficient lines using minirhizotron assays. The primary detected bio-active compound was 4-methylsulfinylbutyl isothiocyanate (4MSOB-NCS, sulforaphane). Additionally, 7-methylsulfinylheptyl isothiocyanate (7MSOH-NCS) was detected as another significant bio-active compound, although it was released in its GSL form and required myrosinase-mediated hydrolysis for activation. By contrast, SL-dependent inhibition of GSLs accumulation in seeds and P. ramosa germination stimulation operated through SLs AtD14 and KARs HTL-3 receptors partially independent on MAX2 integrator. These findings reinforce and complement our views on the major role played by A. thaliana exudated ITC and GSLs in triggering P. ramosa seed germination notably early in the spermosphere of the germinating host seeds. It also reveals a novel regulatory network where SLs indirectly modulate plant-plant interaction chemistry, representing an evolutionary trade-off between pathways. Understanding host–parasite chemical communication offers new avenues for controlling parasitic plants in Brassica crops.