NRT2.4 and NRT2.5 Are Two Half-Size Transporters from the Chlamydomonas NRT2 Family

Light-Dependent Nitrate Removal Capacity of Green Microalgae

In the present study, Chlamydomonas sp. MACC-216 was used to investigate total nitrate removal in TAP medium with sodium nitrate as the sole nitrogen source under several light conditions made up of permuted combinations of three light colors (referred to as blue, red, and white light) and three light intensities (50 µmol m^-2 s^-1, 100 µmol m^-2 s^-1, and 250 µmol m^-2 s^-1). It was observed that nitrate removal efficiency is influenced by light color as well as light intensity. Additionally, Chlamydomonas sp. MACC-216 was cultivated in synthetic wastewater under four light conditions, namely, Blue 250, Blue 125 + Red 125, Red 250, and White 250, where it showed the highest nitrate removal efficiency and nitrate reductase activity under the Blue 125 + Red 125 light condition. To observe the impact of light color on the nitrate removal capacity of Chlamydomonas sp. MACC-216, the expression of five genes participating in nitrate transport and reduction (NRT1, NRT2.1, NRT2.2, NIA, and MCP) was also analyzed; these genes showed the highest expression under the Blue 125 + Red 125 light condition. Based on the above-mentioned findings, the blue + red light combination emerged as a promising light combination for nitrate removal. Hence, our study suggests the importance of the blue + red light combination together with high light intensity, as the optimal light condition for nitrate removal from synthetic wastewater in comparison to other monochromatic lights with high light intensity.

The expression patterns and putative function of nitrate transporter 2.5 in plants

Nitrate transporter 2.5 (NRT2.5) was originally characterized as the transporter for nitrogen (N) limitation. In Arabidopsis, NRT2.5 is expressed mainly under extremely low NO₃^- and N starvation conditions, and this must work in conjunction with NAR2.1. NRT2.5 is expressed both in the roots and leaves in Arabidopsis, poplars, tea trees and cassava. This is also expressed in the seeds of Arabidopsis and wheat. In wheat, NRT2.5 is expressed in the embryo and shell and plays a role in the accumulation of NO₃^- in the seeds. In maize, this is also expressed in silk, cobs and tassel husk leaves. In rice, OsNRT2.5 (also known as OsNRT2.3a) may help the species to remove NO₃^- from the roots to shoots. In addition, NRT2.5 may interact with TGA3, MYC1, LBD37, LBD38, TaNAC2 and other transcription factors and participate in the transmission of NO₃^- signals. The present review summarizes the functions of NRT2.5 in different plant species, which may help plant breeders and molecular biologists to improve crop yield. Abbreviations: NRT, Nitrate transporter; NUE, nitrogen use efficiency; PTR, peptide transporter; NPF, nitrate peptide transporter family; CLC, chloride channel; LAC1/SLAH, slow anion channel-associated 1 homolog 3; LATS, low-affinity transporter systems; HATS, high-affinity transport systems; NNP, nitrate-nitrite-porter; MFS, major facilitator superfamily.

Molecular Characterization of ZosmaNRT2, the Putative Sodium Dependent High-Affinity Nitrate Transporter of Zostera marina L

One of the most important adaptations of seagrasses during sea colonization was the capacity to grow at the low micromolar nitrate concentrations present in the sea. In contrast to terrestrial plants that use H⁺ symporters for high-affinity NO₃⁻ uptake, seagrasses such as Zostera marina L. use a Na⁺-dependent high-affinity nitrate transporter. Interestingly, in the Z. marina genome, only one gene (Zosma70g00300.1; NRT2.1) is annotated to this function. Analysis of this sequence predicts the presence of 12 transmembrane domains, including the MFS domains of the NNP transporter family and the “nitrate signature” that appears in all members of the NNP family. Phylogenetic analysis shows that this sequence is more related to NRT2.5 than to NRT2.1, sharing a common ancestor with both monocot and dicot plants. Heterologous expression of ZosmaNRT2-GFP together with the high-affinity nitrate transporter accessory protein ZosmaNAR2 (Zosma63g00220.1) in Nicotiana benthamiana leaves displayed four-fold higher fluorescence intensity than single expression of ZosmaNRT2-GFP suggesting the stabilization of NRT2 by NAR2. ZosmaNRT2-GFP signal was present on the Hechtian-strands in the plasmolyzed cells, pointing that ZosmaNRT2 is localized on the plasma membrane and that would be stabilized by ZosmaNAR2. Taken together, these results suggest that Zosma70g00300.1 would encode a high-affinity nitrate transporter located at the plasma membrane, equivalent to NRT2.5 transporters. These molecular data, together with our previous electrophysiological results support that ZosmaNRT2 would have evolved to use Na⁺ as a driving ion, which might be an essential adaptation of seagrasses to colonize marine environments.

How Chlamydomonas handles nitrate and the nitric oxide cycle

The green alga Chlamydomonas is a valuable model system capable of assimilating different forms of nitrogen (N). Nitrate (NO3-) has a relevant role in plant-like organisms, first as a nitrogen source for growth and second as a signalling molecule. Several modules are necessary for Chlamydomonas to handle nitrate, including transporters, nitrate reductase (NR), nitrite reductase (NiR), GS/GOGAT enzymes for ammonium assimilation, and regulatory protein(s). Transporters provide a first step for influx/efflux, homeostasis, and sensing of nitrate; and NIT2 is the key transcription factor (RWP-RK) for mediating the nitrate-dependent activation of a number of genes. Here, we review how NR participates in the cycle NO3- →NO2- →NO →NO3-. NR uses the partner protein amidoxime-reducing component/nitric oxide-forming nitrite reductase (ARC/NOFNiR) for the conversion of nitrite (NO2-) into nitric oxide (NO). It also uses the truncated haemoglobin THB1 in the conversion of nitric oxide to nitrate. Nitric oxide is a negative signal for nitrate assimilation; it inhibits the activity and expression of high-affinity nitrate/nitrite transporters and NR. During this cycle, the positive signal of nitrate is transformed into the negative signal of nitric oxide, which can then be converted back into nitrate. Thus, NR is back in the spotlight as a strategic regulator of the nitric oxide cycle and the nitrate assimilation pathway.