The Lotus japonicus NPF3.1 Is a Nodule-Induced Gene That Plays a Positive Role in Nodule Functioning

The Lotus japonicus NPF4.6 gene, encoding for a dual nitrate and ABA transporter, plays a role in the lateral root elongation process and is not involved in the N2-fixing nodule development

Plant root development depends on signaling pathways responding to external and internal signals. In this study we demonstrate the involvement of the Lotus japonicus LjNPF4.6 gene in the ABA and nitrate root responding pathways. LjNPF4.6 expression in roots is induced by external application of both nitrate and ABA. LjNPF4.6 promoter activity is spatially localized in epidermal cell layer and vascular bundle structures with the latter pattern being controlled by externally applied ABA. LjNPF4.6 cRNA injection achieves both nitrate and ABA uptake in Xenopus laevis oocytes and the analyses of L. japonicus knock-out insertion mutants confirmed the role played by LjNPF4.6 in root nitrate uptake. The phenotypic characterization of the Ljnpf4.6 plants indicates the role played by LjNPF4.6 in the root program development in response to exogenously applied nitrate and ABA. Based on the presented data, the mode of action of this transporter is discussed.

NPF and NRT2 from Pisum sativum Potentially Involved in Nodule Functioning: Lessons from Medicago truncatula and Lotus japonicus

In addition to absorbing nitrogen from the soil, legumes have the ability to use atmospheric N2 through symbiotic nitrogen fixation. Therefore, legumes have developed mechanisms regulating nodulation in response to the amount of nitrate in the soil; in the presence of high nitrate concentrations, nodulation is inhibited, while low nitrate concentrations stimulate nodulation and nitrogen fixation. This allows the legumes to switch from soil nitrogen acquisition to symbiotic nitrogen fixation. Recently, particular interest has been given to the nitrate transporters, such as Nitrate Transporter1/Peptide transporter Family (NPF) and Nitrate Transporter 2 (NRT2), having a role in the functioning of nodules. Nitrate transporters of the two model plants, Lotus japonicus and Medicago truncatula, shown to have a positive and/or a negative role in nodule functioning depending on nitrate concentration, are presented in this article. In particular, the following transporters were thoroughly studied: (i) members of NPF transporters family, such as LjNPF8.6 and LjNPF3.1 in L. japonicus and MtNPF1.7 and MtNPF7.6 in M. truncatula, and (ii) members of NRT2 transporters family, such as LjNRT2.4 and LjNRT2.1 in L. japonicus and MtNRT2.1 in M. truncatula. Also, by exploiting available genomic and transcriptomic data in the literature, we have identified the complete PsNPF family in Pisum sativum (69 sequences previously described and 21 new that we have annotated) and putative nitrate transporters candidate for playing a role in nodule functioning in P. sativum.

Balancing nitrate acquisition strategies in symbiotic legumes

MAIN CONCLUSION

Legumes manage both symbiotic (indirect) and non-symbiotic (direct) nitrogen acquisition pathways. Understanding and optimising the direct pathway for nitrate uptake will support greater legume growth and seed yields. Legumes have multiple pathways to acquire reduced nitrogen to grow and set seed. Apart from the symbiotic N2-fixation pathway involving soil-borne rhizobia bacteria, the acquisition of nitrate and ammonia from the soil can also be an important secondary nitrogen source to meet plant N demand. The balance in N delivery between symbiotic N (indirect) and inorganic N uptake (direct) remains less clear over the growing cycle and with the type of legume under cultivation. In fertile, pH balanced agricultural soils, NO3- is often the predominant form of reduced N available to crop plants and will be a major contributor to whole plant N supply if provided at sufficient levels. The transport processes for NO3- uptake into legume root cells and its transport between root and shoot tissues involves both high and low-affinity transport systems called HATS and LATS, respectively. These proteins are regulated by external NO3- availability and by the N status of the cell. Other proteins also play a role in NO3- transport, including the voltage dependent chloride/nitrate channel family (CLC) and the S-type anion channels of the SLAC/SLAH family. CLC's are linked to NO3- transport across the tonoplast of vacuoles and the SLAC/SLAH's with NO3- efflux across the plasma membrane and out of the cell. An important step in managing the N requirements of a plant are the mechanisms involved in root N uptake and the subsequent cellular distribution within the plant. In this review, we will present the current knowledge of these proteins and what is understood on how they function in key model legumes (Lotus japonicus, Medicago truncatula and Glycine sp.). The review will examine their regulation and role in N signalling, discuss how post-translational modification affects NO3- transport in roots and aerial tissues and its translocation to vegetative tissues and storage/remobilization in reproductive tissues. Lastly, we will present how NO3-influences the autoregulation of nodulation and nitrogen fixation and its role in mitigating salt and other abiotic stresses.

The small peptide CEP1 and the NIN-like protein NLP1 regulate NRT2.1 to mediate root nodule formation across nitrate concentrations

Legumes acquire fixed nitrogen (N) from the soil and through endosymbiotic association with diazotrophic bacteria. However, establishing and maintaining N2-fixing nodules are expensive for the host plant, relative to taking up N from the soil. Therefore, plants suppress symbiosis when N is plentiful and enhance symbiosis when N is sparse. Here, we show that the nitrate transporter MtNRT2.1 is required for optimal nodule establishment in Medicago truncatula under low-nitrate conditions and the repression of nodulation under high-nitrate conditions. The NIN-like protein (NLP) MtNLP1 is required for MtNRT2.1 expression and regulation of nitrate uptake/transport under low- and high-nitrate conditions. Under low nitrate, the gene encoding the C-terminally encoded peptide (CEP) MtCEP1 was more highly expressed, and the exogenous application of MtCEP1 systemically promoted MtNRT2.1 expression in a compact root architecture 2 (MtCRA2)-dependent manner. The enhancement of nodulation by MtCEP1 and nitrate uptake were both impaired in the Mtnrt2.1 mutant under low nitrate. Our study demonstrates that nitrate uptake by MtNRT2.1 differentially affects nodulation at low- and high-nitrate conditions through the actions of MtCEP1 and MtNLP1.