Sunflower (Helianthus annuus L.) response to broomrape (Orobanche cernua Loefl.) parasitism: induced synthesis and excretion of 7-hydroxylated simplecoumarins

The interaction of the parasitic plant Orobanche cernua with resistant and susceptible cultivars of Helianthus annuus L. was investigated. Using diffe rent bioassays to evaluate the early stages of the parasite life cycle (ger mination, attachment, penetration, and establishment), differences were obs erved between O. cernua-resistant and O. cernua-susceptible sunflower varie ties. Germination of O. cernua seeds in the presence of resistant sunflower roots was approximately half that of germination in the presence of suscep tible roots, and germinated seeds displayed enhanced browning symptoms. Par asite radicles or host-tissue around the contact point turned brown after O . cernua attachment to sunflower roots, especially in the resistant varieti es. These observations suggested the possible accumulation of toxic compoun ds as a defence strategy in the resistant sunflower varieties. Sunflower 7- hydroxylated simple coumarins may play a defensive role against O. cernua p arasitism by preventing successful germination, penetration and/or connecti on to the host vascular system. This hypothesis is supported by the followi ng data: (i) coumarins inhibited the in vitro germination of O. cernua seed s induced by the strigol analogue GR(24) and caused a browning reaction in germinated seeds and (ii) resistant sunflowers accumulated higher levels of coumarins in roots and excreted greater amounts than susceptible varieties in response to O. cernua infection.