Divergent control of seed germination by cytokinins in weedy broomrapes and witchweeds

Broomrapes (Phelipanche and Orobanche spp.) and witchweeds (Striga spp.) are parasitic weeds that are increasingly threatening crops worldwide. Seeds of these species rely on host-derived signals such as strigolactones to germinate. While cytokinins were also reported as germination inducers of witchweeds, their role during germination of broomrapes remains unexplored. Our study shows that some but not all cytokinins stimulate Striga hermonthica germination independently of strigolactones, and that high concentrations of bioactive cytokinins trigger Striga seedlings to differentiate into fully extruded embryo-like structures. In contrast, cytokinin free bases but not ribosylated or glycosylated conjugates are extremely potent inhibitors of broomrapes germination. Germination inhibition upon the cytokinin signaling inhibitor PI-55 and inhibitor of cytokinin degradation INCYDE suggest that the cytokinin perception and degradation machinery is conserved in parasitic weeds. In Phelipanche ramosa, gene expression analyses combined with targeted quantification of cytokinin contents revealed that strigolactones first induce an increase in ABA catabolism, then a modification of the cytokinin endogenous pool in favor of inactive conjugates. Overall, this study provides valuable insights into the hormonal interplay governing seed germination in broomrapes and witchweeds, paving the way for future studies aimed at developing novel strategies for parasitic weed control.