Karrikins versus strigolactones: A comparative analysis of their effects on seed germination in obligate and facultative Orobanchaceae parasites and fire-responsive species

The karrikin (KAR) and strigolactone (SL) signalling pathways are thought to be evolutionarily related, prompting extensive research on the similarities and differences between KAR- and SL-mediated plant developmental responses. Here, we investigated the ef­fects of plant-derived smoke, KAR1, rac-GR24 (a synthetic SL analogue), and gibberellic acid (GA3) on seed germination in five Oro­banchaceae species, including both obligate and facultative parasites, and in two non-parasitic species known to respond to smoke and KAR1. Our results showed that seeds of the KAR1-responsive Sarcopoterium spinosum and Stachys cretica were insensitive to rac-GR24. Although rac-GR24 promoted germination in all the obligate Orobanche parasites tested (O. anatolica, O. minor, and O. pubescens), it had no effect on the facultative parasites Bellardia trixago and Parentucellia viscosa. Smoke-water and KAR1 did not induce germination in the three Orobanche species, yet significantly increased germination in B. trixago. Additionally, the seeds of S. spinosum, S. cretica, and B. trixago responded positively to GA3, whereas GA3 did not replace the requirement for rac-GR24 to stimulate germination in Orobanche species, except for O. pubescens. Overall, our findings indicate that (I) KARs and SLs are not interchangeable cues for germination in obligate Orobanchaceae parasites or the two fire-responsive non-parasitic species, and (II) KARs may confer an advantage to some facultative parasites in post-fire environments.