Plant-Plant Signaling in the Rhizosphere: the Role of Quinone Oxidoreductases in Host Perception by Parasitic Plants

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Plants identify nearby plants by perceiving chemical signals released by roots into the soil. Parasitic plants in the family Scrophulariaceae recognize quinone molecules released by the roots of host plants to trigger the development of haustoria, parasite-specific root organs that function in host attachment and invasion. Haustorium induction and development is readily monitored in vitro by applying host root exudates or purified haustoria inducing factors (HIFs) onto roots of aseptic parasite seedlings. Because haustorium development is rapid, highly synchronous and strictly dependent on exogenous HIFs, it is a fantastic system for investigating molecular mechanisms governing subterranean plant-plant interactions.It is likely that a common redox reaction occurring between structurally diverse HIFs and a parasite receptor initiates the haustorium development signal pathway. Two quinone oxidoreductases have been cloned from the parasitic plant Triphysaria that have properties consistent with them being directly involved in the perception and initial processing of the HIF 2,6-dimethoxybenzoquinone (DMBQ). Based on sequence and structural homologies with well characterized proteins in microbes, fungi and vertebrates, TvQR1 and TvQR2 likely encode different NAD(P) dependent quinone oxidoreductases. TvQR1 is homologous to a family of oxidoreductases that catalyze one-electron quinone reductions, yielding the semiquinone intermediate predicted to be critical in HIF signal transduction. In contrast, TvQR2 is homologous to a detoxifying quinone oxidoreductase that catalyzes two electron reductions, thus bypassing the semiquinone intermediate. These experiments will test the hypothesis that TvQR1 and TvQR2 catalyze distinct reactions fulfilling different biological functions.Both TvQR1 and TvQR2 are primary response genes transcriptionally activated in Triphysaria by DMBQ in the absence of de novo protein synthesis. Promoter sequences responsible for DMBQ responsiveness will be identified by assaying reporter fusions in transgenic Arabidopsis. Sequence comparisons of TvQR1 promoters in parasitic and non-parasitic plants will elucidate the mechanism of parasite specific responsiveness. Because TvQR2 is predicted to cleanse the cells of oxidized quinones used by TvQR1 as HIF substrates, the relative levels of these two enzymes in parasite roots is predicted to determine HIF responsiveness. Western blots probed with TvQR1 and TvQR2 specific antibodies will determine if protein levels reflect transcript abundance. A comparison of protein levels and activities in DMBQ responsive and non-responsive lines will relate TvQR1 and / or TvQR2 to specialized functions in parasitic plants. By understanding the mechanisms that plants use to recognize and respond to other plants, it should be possible to engineer plants for optimal growth in the presence of weeds.