www.elsevier.de/jplph IAA production during germination of Orobanche spp. seeds Slavtcho Slavov a, *, Henry van Onckelen b , Rossitza Batchvarova a , Atanas Atanassov a , Els Prinsen b a Laboratory of Phytopathology, AgroBioInstitute, 8 Dragan Tsankov Blvd., 1164 Sofia, Bulgaria b Laboratory of Plant Biochemistry and Physiology, Department of Biology, Antwerp University, Universiteitsplein 1, B 2610 Antwerp, Belgium Received 11 July 2003; accepted 29 November 2003 Summary Broomrapes (Orobanche spp.) are parasitic plants, whose growth and development fully depend on the nutritional connection established between the parasite and the roots of the respective host plant. Phytohormones are known to play a role in establishing the specific Orobanche–host plant interaction. The first step in the interaction is seed germination triggered by a germination stimulant secreted by the host-plant roots. We quantified indole-3-acetic acid (IAA) and abscisic acid (ABA) during the seed germination of tobacco broomrape (Orobanche ramosa) and sunflower broomrape (O. cumana). IAA was mainly released from Orobanche seeds in host–parasite interactions as compared to non-host–parasite interactions. Moreover, germinating seeds of O. ramosa released IAA as early as 24 h after the seeds were exposed to the germination stimulant, even before development of the germ tube. ABA levels remained unchanged during the germination of the parasites’ seeds. The results presented here show that IAA production is probably part of a mechanism triggering germination upon the induction by the host factor, thus resulting in seed germination. & 2004 Elsevier GmbH. All rights reserved. Introduction Broomrapes (Orobanche spp.) are obligate root parasitic plants. Host specificity is a distinctive feature of these parasitic plants and the identifica- tion of an appropriate host is critical for broom- rapes (Yoder, 1999). Chemical compounds released by the host-plant roots trigger the parasitic seed germination (Boone et al., 1995). All of these compounds isolated from different hosts of Striga and Orobanche are generally known as strigolac- tones (Galindo et al., 2002). In addition to this more or less narrow host specificity, the parasite’s seeds are able to respond to synthetic germination stimulants, strigolactone analogues, termed GR compounds (Thuring et al., 1997; Sugimoto et al., ARTICLE IN PRESS KEYWORDS Broomrapes; Germination; Indole-3-acetic acid; Sunflower; Tobacco *Corresponding author. Tel.: þ359-2-963-5407; fax: þ359-2-963-5408. E-mail address: sbslavov@abi.bg (S. Slavov). 0176-1617/$ - see front matter & 2004 Elsevier GmbH. All rights reserved. doi:10.1016/j.jplph.2003.11.007 Journal of Plant Physiology 161 (2004) 847–853