School of Biological Sciences, Washington State University, Pullman, WA, USA.
J Exp Bot. 2018 Oct 12;69(21):5205-5219. doi: 10.1093/jxb/ery289.
Legumes fix atmospheric nitrogen through a symbiotic relationship with bacteroids in root nodules. Following fixation in pea (Pisum sativum L.) nodules, nitrogen is reduced to amino acids that are exported via the nodule xylem to the shoot, and in the phloem to roots in support of growth. However, the mechanisms involved in amino acid movement towards the nodule vasculature, and their importance for nodule function and plant nutrition, were unknown. We found that in pea nodules the apoplasmic pathway is an essential route for amino acid partitioning from infected cells to the vascular bundles, and that amino acid permease PsAAP6 is a key player in nitrogen retrieval from the apoplasm into inner cortex cells for nodule export. Using an miRNA interference (miR) approach, it was demonstrated that PsAAP6 function in nodules, and probably in roots, and affects both shoot and root nitrogen supply, which were strongly decreased in PsAAP6-miR plants. Further, reduced transporter function resulted in increased nodule levels of ammonium, asparagine, and other amino acids. Surprisingly, nitrogen fixation and nodule metabolism were up-regulated in PsAAP6-miR plants, indicating that under shoot nitrogen deficiency, or when plant nitrogen demand is high, systemic signaling leads to an increase in nodule activity, independent of the nodule nitrogen status.
豆科植物通过与根瘤中的类菌体的共生关系固定大气氮。豌豆（Pisum sativum L.）根瘤固定氮后，氮被还原为氨基酸，通过根瘤木质部运出到地上部，通过韧皮部运到根部，以支持生长。然而，涉及氨基酸向根瘤脉管系统移动的机制及其对根瘤功能和植物营养的重要性尚不清楚。我们发现，在豌豆根瘤中，质外体途径是将氨基酸从感染细胞分配到维管束的必要途径，而氨基酸透性酶 PsAAP6 是从质外体中回收氮并将其运入内皮层细胞以进行根瘤输出的关键因子。通过 miRNA 干扰（miR）方法，证明了 PsAAP6 在根瘤中的功能，可能在根中也有功能，并且影响地上部和地下部氮的供应，PsAAP6-miR 植株的地上部和地下部氮供应均显著降低。此外，转运蛋白功能的降低导致根瘤中铵、天冬酰胺和其他氨基酸的水平升高。令人惊讶的是，PsAAP6-miR 植株中的固氮和根瘤代谢被上调，这表明在地上部氮缺乏或植物氮需求较高时，系统信号会导致根瘤活性增加，而与根瘤氮状况无关。
