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。 玄参科中的寄生植物识别由宿主植物的根释放的醌分子以触发吸器、寄生物特异性根器官的发育，所述吸器在宿主附着和入侵中起作用。 吸器的诱导和发展，很容易在体外监测，通过应用主机根分泌物或纯化吸器诱导因子（HIF）的无菌寄生幼苗的根。 由于吸器发育的快速性、高度同步性和对外源HIF的严格依赖性，它是研究地下植物-植物相互作用的分子机制的理想系统，很可能是结构多样的HIF与寄生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。 已经从寄生植物Triphysaria中克隆了两种醌氧化还原酶，其具有与它们直接参与HIF 2，6-二甲氧基苯醌（DMBQ）的感知和初始加工一致的性质。 基于与微生物、真菌和脊椎动物中已充分表征的蛋白质的序列和结构同源性，TvQR 1和TvQR 2可能编码不同的NAD（P）依赖性醌氧化还原酶。 TvQR 1与氧化还原酶家族同源，该家族催化单电子醌还原，产生预测在HIF信号转导中至关重要的半醌中间体。 相反，TvQR 2与催化两个电子还原的解毒醌氧化还原酶同源，从而绕过半醌中间体。 这些实验将验证TvQR 1和TvQR 2催化不同的反应实现不同的生物学functions.TvQR1和TvQR 2都是在没有从头蛋白质合成的情况下由DMBQ转录激活的Triphysaria中的初级应答基因的假设。 负责DMBQ响应性的启动子序列将通过测定转基因拟南芥中的报告融合体来鉴定。 TvQR 1启动子在寄生和非寄生植物的序列比较将阐明寄生虫特异性反应的机制。 因为TvQR 2被预测为清除细胞中被TvQR 1用作HIF底物的氧化醌，所以预测寄生虫根部中这两种酶的相对水平以确定HIF响应性。 用TvQR 1和TvQR 2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转录物丰度。 DMBQ响应和非响应系中蛋白质水平和活性的比较将使TvQR 1和/或TvQR 2与寄生植物中的特化功能相关。 通过了解植物识别和响应其他植物的机制，应该有可能在杂草存在的情况下设计植物的最佳生长。

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.