Haustoria develop on roots of Orobanchaceae in response to host factors, both chemical and tactile (Atsatt et al. 1978; Riopel and Timko 1995) . The first haustorium-inducing factor (HIF) to be identified was 2,6-dimethoxy-1,4-benzo-quinone (DMBQ) (Chang and Lynn 1986) (Fig. 2). DMBQ is a common component of plant cell walls and has been observed in at least 48 genera belonging to 29 plant families (Handa et al. 1983). Due to its electrophilic, oxidant nature, DMBQ has allelopathic, mutagenic, carcinogenic and cytotoxic characteristics (Brambilla et al. 1988). Cellular damage results from the redox cycling between quinone and semiquinone states, giving rise to reactive oxygen species (Testa 1995).
In fact, it is the redox cycling of quinones to their semiquinone forms that has been hypothesized to induce haustorium development. In DMBQ induction of Striga seedlings, addition of spin-trap chemicals, such as cyclopropyl benzoqui-none (CPBQ) and tetrafluorobenzo-1,4-quinone (TFBQ), has been shown to inhibit haustorium development (Smith et al. 1996; Zeng et al. 1996) . Further, the HIFs
active in inducing Striga seedlings had a narrow range of redox potentials (Smith et al. 1996), and it was shown that phenolics must be converted to quinone forms before they become active HIFs (Kim et al. 1998). The hypothesis put forward by this work on Striga is that the free radical associated with redox cycling between the oxidized and reduced forms is the signal that initiates haustorium development.
Genes upregulated in Triphysaria roots soon after DMBQ exposure included two NAD(P)H-dependent quinone oxidoreductases, TvQR1 and TvQR2 (Matvienko et al. 2001a, b) . In Triphysaria there is rapid transcriptional induction of both TvQR1 and TvQR2 as a primary response to DMBQ treatment. TvQR1 exhibits homology to a family of zeta-crystallins and catalyzes a one-electron reduction of quinone to semiquinone, providing a free radical consistent with the redox signaling hypothesis (Fillapova, Petite, Yoder, unpublished). TvQR2 is related to a class of detoxifying enzymes, such as human liver DT-diaphorase, and catalyzes a two-electron reduction of quinones (Wrobel et al. 2002) . We hypothesize that TvQR1 and TvQR2 act antagonistically in that TvQR1 generates free radicals and TvQR2 detoxifies them. We propose that if the activity of TvQR1 is greater than TvQR2, haustorium development proceeds; if the activity of TvQR2 is greater, no haustoria form. Haustorium development in this model is proposed to be regulated by the relative activities of two counteracting enzymes.
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