The preference for nitrate over chloride through regulation of transporters is a fundamental feature of plant ion homeostasis. We show that Medicago truncatula MtNPF6.5, an ortholog of Arabidopsis thaliana AtNPF6.3/NRT1.1, can mediate nitrate and chloride uptake in Xenopus oocytes but is chloride selective and that its close homologue, MtNPF6.7, can transport nitrate and chloride but is nitrate selective. The MtNPF6.5 mutant showed greatly reduced chloride content relative to wild type, and MtNPF6.5 expression was repressed by high chloride, indicating a primary role for MtNPF6.5 in root chloride uptake. MtNPF6.5 and MtNPF6.7 were repressed and induced by nitrate, respectively, and these responses required the transcription factor MtNLP1. Moreover, loss of MtNLP1 prevented the rapid switch from chloride to nitrate as the main anion in nitrate-starved plants after nitrate provision, providing insight into the underlying mechanism for nitrate preference. Sequence analysis revealed three sub-types of AtNPF6.3 orthologs based on their predicted substrate-binding residues: A (chloride selective), B (nitrate selective), and C (legume specific). The absence of B-type AtNPF6.3 homologues in early diverged plant lineages suggests that they evolved from a chloride-selective MtNPF6.5-like protein.

中文翻译：

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MtNPF6.5 介导苜蓿根部的氯离子吸收和硝酸盐偏好


通过调节转运蛋白而优先选择硝酸盐而不是氯化物是植物离子稳态的基本特征。我们发现，蒺藜苜蓿MtNPF6.5 是拟南芥AtNPF6.3/NRT1.1 的直系同源物，可以介导非洲爪蟾卵母细胞对硝酸盐和氯化物的吸收，但具有氯化物选择性，并且其密切同源物 MtNPF6.7 可以转运硝酸盐和氯化物但对硝酸盐有选择性。相对于野生型， MtNPF6.5突变体显示出大大降低的氯含量，并且MtNPF6.5表达被高氯抑制，表明MtNPF6.5在根氯吸收中的主要作用。 MtNPF6.5和MtNPF6.7分别被硝酸盐抑制和诱导，这些反应需要转录因子 MtNLP1。此外，在提供硝酸盐后，MtNLP1 的丢失阻止了硝酸盐饥饿植物中主要阴离子从氯离子快速转变为硝酸盐，从而深入了解硝酸盐偏好的潜在机制。序列分析根据预测的底物结合残基揭示了 AtNPF6.3 直系同源物的三种亚型：A（氯化物选择性）、B（硝酸盐选择性）和 C（豆科植物特异性）。早期分化植物谱系中缺乏 B 型 AtNPF6.3 同源物，表明它们是从氯选择性 MtNPF6.5 样蛋白进化而来的。