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

植物通过感知根部释放到土壤中的化学信号来识别附近的植物。玄参科寄生植物识别寄主植物根部释放的苯醌分子，以触发吸器的发育，吸器是寄生植物特有的根器官，在寄主附着和入侵中发挥作用。通过将寄主根分泌物或纯化的吸器诱导因子(HIF)应用于无菌寄生虫幼苗的根上，可以很容易地在体外监测吸器的诱导和发育。由于吸器发育迅速、高度同步且严格依赖于外源HIF，它是研究地下植物与植物相互作用的分子机制的理想系统，很可能是结构不同的HIF与寄生虫受体之间发生的共同氧化还原反应启动了吸器发育信号通路。从寄生植物三叶草中克隆了两个苯醌氧化还原酶，它们的性质与它们直接参与HIF 2，6-二甲氧基苯醌(DMBQ)的感知和初始加工有关。根据与微生物、真菌和脊椎动物中已知的蛋白质序列和结构的同源性，TvQR1和TvQR2可能编码不同的依赖于NAD(P)的苯醌氧化还原酶。TvQR1与催化单电子对苯二酚还原的氧化还原酶家族同源，产生在缺氧诱导因子信号转导中起关键作用的半喹酮中间体。相反，TvQR2与一种解毒的苯醌氧化还原酶同源，该酶催化两个电子还原，从而绕过了半喹酮中间体。这些实验将验证TvQR1和TvQR2催化不同的反应实现不同的生物学功能的假设。TvQR1和TvQR2都是在没有从头合成蛋白质的情况下被DMBQ转录激活的主要反应基因。负责DMBQ响应的启动子序列将通过分析转基因拟南芥中的报告融合来鉴定。对寄生植物和非寄生植物中TvQR1启动子的序列进行比较，将有助于阐明寄生植物对寄生植物的特异性应答机制。由于TvQR2被预测为清除TvQR1用作HIF底物的细胞中的氧化醌，因此这两种酶在寄生虫根中的相对水平被预测为决定HIF反应的决定因素。用TvQR1和TvQR2特异性抗体探测的蛋白质印迹将确定蛋白质水平是否反映了转录的丰度。对DMBQ反应和非反应品系的蛋白质水平和活性的比较将把TvQR1和/或TvQR2与寄生植物中的特殊功能联系起来。通过了解植物识别和响应其他植物的机制，应该有可能设计出在杂草存在的情况下优化生长的植物。

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.