Department of Plant Molecular Biology, Biophore Building, University of Lausanne, Lausanne 1015, Switzerland.
Electron Microscopy Facility, Biophore Building, University of Lausanne, Lausanne 1015, Switzerland.
Plant Physiol. 2021 Feb 25;185(1):196-209. doi: 10.1093/plphys/kiaa016.
Legumes play an important role in the soil nitrogen availability via symbiotic nitrogen fixation (SNF). Phosphate (Pi) deficiency severely impacts SNF because of the high Pi requirement of symbiosis. Whereas PHT1 transporters are involved in Pi uptake into nodules, it is unknown how Pi is transferred from the plant infected cells to nitrogen-fixing bacteroids. We hypothesized that Medicago truncatula genes homologous to Arabidopsis PHO1, encoding a vascular apoplastic Pi exporter, are involved in Pi transfer to bacteroids. Among the seven MtPHO1 genes present in M. truncatula, we found that two genes, namely MtPHO1.1 and MtPHO1.2, were broadly expressed across the various nodule zones in addition to the root vascular system. Expressions of MtPHO1.1 and MtPHO1.2 in Nicotiana benthamiana mediated specific Pi export. Plants with nodule-specific downregulation of both MtPHO1.1 and MtPHO1.2 were generated by RNA interference (RNAi) to examine their roles in nodule Pi homeostasis. Nodules of RNAi plants had lower Pi content and a three-fold reduction in SNF, resulting in reduced shoot growth. Whereas the rate of 33Pi uptake into nodules of RNAi plants was similar to control, transfer of 33Pi from nodule cells into bacteroids was reduced and bacteroids activated their Pi-deficiency response. Our results implicate plant MtPHO1 genes in bacteroid Pi homeostasis and SNF via the transfer of Pi from nodule infected cells to bacteroids.
豆类通过共生固氮（SNF）在土壤氮素有效性中发挥重要作用。由于共生作用对磷（Pi）的需求很高，Pi 缺乏会严重影响 SNF。虽然 PHT1 转运蛋白参与 Pi 向根瘤的摄取，但尚不清楚 Pi 如何从被植物感染的细胞转移到固氮菌。我们假设拟南芥 PHO1 基因同源的 Medicago truncatula 基因，编码一种血管质外体 Pi 外排蛋白，参与 Pi 向固氮菌的转移。在 M. truncatula 中存在的七个 MtPHO1 基因中，我们发现两个基因，即 MtPHO1.1 和 MtPHO1.2，除了根维管束系统外，在各种根瘤区都广泛表达。在 Nicotiana benthamiana 中表达 MtPHO1.1 和 MtPHO1.2 介导了特定的 Pi 外排。通过 RNA 干扰（RNAi）生成了具有根瘤特异性下调 MtPHO1.1 和 MtPHO1.2 的植物，以研究它们在根瘤 Pi 稳态中的作用。RNAi 植物的根瘤中 Pi 含量较低，SNF 减少了三倍，导致地上部分生长减少。尽管 RNAi 植物根瘤中 33Pi 的摄取率与对照相似，但 33Pi 从根瘤细胞向固氮菌的转移减少，固氮菌激活了它们的 Pi 缺乏反应。我们的研究结果表明，植物 MtPHO1 基因通过将 Pi 从被植物感染的细胞转移到固氮菌中，参与了固氮菌的 Pi 稳态和 SNF。
