Characterization of the Orobanche–Arabidopsis system for studying parasite–host interactions

Parasitization by Orobanche is a complex process, one that is mediated by host-derived chemical signals that control parasite seed germination and haustorium initiation and one that ultimately results in the union of two plant species. Experiments were conducted to characterize Orobanche parasitization of the model plant Arabidopsis thaliana and to begin to explore the role of host flavonoid metabolism in the interaction. Arabidopsis thaliana stimulated seed germination and allowed tubercle development of O. aegyptiaca and O. ramosa but did not significantly stimulate seeds of O. crenata, O. minor, or O. cernua. However, if Orobanche seeds were artificially stimulated, O. crenata and O. minor successfully established tubercles on A. thaliana. When compared to the recognized crop hosts, Daucus carota and Nicotiana tabacum, A. thaliana stimulated less O. aegyptiaca germination but allowed for formation of equivalent numbers of tubercles. These findings indicate that A. thaliana is not a large-scale producer of germination stimulant but is highly susceptible to the parasite once Orobanche seeds have germinated. Experiments comparing wild-type A. thaliana plants to mutant lines deficient in flavonoid biosynthesis revealed no differences in the ability to stimulate germination or to allow tubercle formation, indicating that host flavonoid production is not essential for Orobanche parasitization. The results of this work support the use of A. thaliana as a valuable host in understanding Orobanche parasitization.Nomenclature: Arabidopsis thaliana L. Heynh., ARBTH, mouseearcress; Daucus carota L. ‘Danver half long’, DAUCS, carrot; Nicotiana tabacum L. ‘Coker 319’, NIOTA, tobacco; Orobanche aegyptiaca Pers., ORAAE, Egyptian broomrape; Orobanche cernua Loefl., ORACE, nodding broomrape; Orobanche crenata Forsk., ORACR, crenate broomrape; Orobanche minor Smith, ORAMI, small broomrape; Orobanche ramosa L., ORARA, branched broomrape.