Waking up parasitic witchweed: Signaling pathways in Striga germination

In Africa, the parasitic weed, Striga hermonthica, infects major food crops which results in devastating yield losses for over 100 million subsistence farmers. Because Striga is an obligate parasite, Striga seed germinate exclusively upon sensing the hormone, strigolactones (SL), which are emitted by plant hosts. Striga has modified its response to signals that would typically promote germination of seeds from non-parasitic plants. To elucidate the molecular mechanisms underpinning Striga germination, the Lumba lab investigates how SLs are perceived and transduced by signalling components from Striga. We had previously identified a group of strigolactone receptors from Striga (ShHTL), whose counterparts in non-parasitic plants like Arabidopsis (AtKAI2/HTL) are poor receptors of SLs. To determine which residues have contributed to the ability of ShHTL to perceive SLs, we have systemically switched residues in AtKAI2 to their respective identities in ShHTL7. Our biochemical and functional analyses have converged to three key residues that likely contribute to SL perception. To elucidate the SL signal transduction pathway, we used a heterologous system of expressing ShHTL receptors in Arabidopsis to identify downstream components. By integrating genetic, physiological and transcriptomic data, we demonstrate that they function through the F-box protein, AtMAX2 and a protein with similarity to a heat shock protein, AtSMAX1. Finally, we unmasked a surprising ability of ShHTL receptors to bypass the requirement of an essential hormone for germination.