Host recognition of Orobanche cumana, the broomrape of cultivated sunflower

 Background and aims: Most of the economically relevant parasitic weeds of the Orobanchaceae have a more or less broad host spectrum within the plant family they are adapted to. The hostreleased compounds inducing their germination were identified as so-called strigolactones and represent four-ring terpenoids derived from the carotenoid metabolism. O. cumana, a fast emerging problem of sunflower cultivation in areas with dry and hot summers, similarly reacts on strigolactones (e.g. GR24), but accepts no other host than Helianthus annuus. However, from sunflower no strigolactone was described yet. Previous reports indicated, that sesquiterpene lactones (STL) may trigger a similar reaction in O. cumana and recently dehydrocostus lactone (DCL) was identified as a first natural germination inductor of this type from sunflower root extracts. However, studies on H. annuus / O. cumana interaction showed that more than one inducing compound is released from sunflower root exudate. We aimed to identify their chemical nature and specificity.  Methodology: Bioassay-guided HPLC purification of root exudate compounds. MS and NMR spectroscopic analysis for structure elucidation. Comparative germination tests.  Key results: HPLC separations of root exudates from hydroponic sunflower cultivation indicated several fractions with stimulating activity for O. cumana seed germination. Besides the pseudoguainolide DCL, we identified the germacranolide costunolide and the xanthanolide tomentosin as bioactive compounds. All three STL are new for H. annuus and were not found in STL-producing glandular trichomes of leaves and flowers. The STL induced germination of O. cumana at an ED50 of ca. 1-7 nM. Concentrations above 1 μM reduced their activity and overdose application inhibited germination irreversibly. This is most likely a consequence of the nucleophilic reactivity known from many cytotoxic effects of STL. In addition, STL from glandular trichomes of aerial parts of sunflower caused similar stimulating effects. The content of less than 1 trichome was sufficient to induce germination of seeds. Simple leave washings with water, simulating natural rain events, strongly induced germination as well. While O. cumana proved higly sensitive for STL from roots or leave leachates of its host, no such effects of STL were found with Phelipanche ramosa in our tests.  Conclusions: O. cumana, unlike other species of the genus reacts on STL signals of its host. At the same time, sunflower broomrape shares with its relatives a similar sensititvity for strigolactones. This indicates a structural alteration in the receptor, necessary for recognition of this new type of germination stimulant. For O. cumana this may have been an evolutionary key step for the host jump to Asteraceae which seemingly lack the production of typical strigolactones.  Contribution to the current knowledge: STL from sunflower were identified as key elements in the interaction with O. cumana. This allows development of new strategies to control this economically important parasitic weed.