Novel non-canonical strigolactone analogs highlight selectivity for stimulating germination in two Phelipanche ramosa populations

Strigolactones (SLs) are plant hormones exuded in the rhizosphere with a signaling role for the development of arbuscular mycorrhizal (AM) fungi and as stimulants of seed germination of the parasitic weeds Orobanche, Phelipanche and Striga, the most threatening weeds of major crops worldwide. Phelipanche ramosa is present mainly on rape, hemp and tobacco. P. ramosa 2a preferentially attacks hemp while P. ramosa 1 attacks rapeseed. The recently isolated Cannalactone 14 from hemp root exudates has been characterized as a non-canonical SL that selectively stimulates the germination of P. ramosa 2a seeds in comparison with P. ramosa 1. In the present work, we established that (−)-solanacol 5, a canonical orobanchol-type SL exuded by tobacco and tomato, possesses a remarkable selective germination stimulant activity for P. ramosa 2a seeds. We synthesized cannalactone analogs, named (±)-SdL19 and (±)-SdL118 which have an unsaturated acyclic carbon chain with a tertiary hydroxyl group and a methyl or a cyclopropyl group instead of a cyclohexane A-ring, respectively. (±)-SdL analogs are able to selectively stimulate P. ramosa 2a revealing that these minimal structural elements are key for this selective bioactivity. In addition, we showed that (±)-SdL19 is able to inhibit shoot branching in Pisum sativum and Arabidopsis thaliana, and induces hyphal branching in AM fungus R. irregularis, like SLs.