Neofunctionalization of strigolactones─chemoattractants for host tropism in Orobanchaceaeparasitic plants

Abstract

Strigolactones (SLs) act as both plant growth regulators and as chemoattractants for arbuscular mycorrhizal fungi. Secreted SLs are also sensed by Orobanchaceae root parasitic plants as germination stimulants. Here we focus on the impact of exogenous SLs on Phtheirospermum japonicum, a facultative hemiparasite in the Orobanchaceae. The genome of P. japonicum encodes SL receptor candidates, and its roots exhibit chemotropism toward host-derived SLs. This chemotropism was also observed in the obligate hemiparasite Striga hermonthica but not in non-parasitic plants, suggesting that chemotropism to SLs might be limited to Orobanchaceae parasitic plants. Interestingly, under ammonium-rich conditions, where P. japonicum can grow without hosts, its chemotropism to SLs does not occur. Furthermore, SLs induced asymmetric auxin response in the root epidermis in P. japonicum, leading to asymmetric root growth. Our findings shed new light on the function of SLs as host-derived chemoattractants, providing valuable insights into the mechanisms behind parasitic plant interactions with their hosts.