# Spinach SoNRT3 Interacts with SoNRT2a to Improve Low-Nitrogen Tolerance via Nitrate Uptake and Root Growth

## TL;DR

This study identifies a spinach protein, SoNRT3, that improves plant growth under low-nitrogen conditions by enhancing nitrate uptake and root development.

## Contribution

The novel contribution is the discovery of SoNRT3's role in low-nitrogen tolerance and its interaction with SoNRT2a in spinach.

## Key findings

- SoNRT3 overexpression increases root growth and nitrate uptake under low-nitrate conditions.
- SoNRT3 interacts with SoNRT2a to enhance low-nitrogen tolerance in spinach.
- Transcriptome analysis suggests SoNRT3 promotes root development and nitrate assimilation.

## Abstract

High nitrogen use efficiency is crucial for enhancing spinach’s tolerance to low nitrogen stress and minimizing nitrate accumulation. Here, we report that SoNRT3, a NAR2 family protein, modulates nitrate uptake and plant growth under low-nitrate conditions. SoNRT3 expression was induced by low nitrate availability in roots and prolonged nitrogen deficiency in shoots. Compared to wild-type Arabidopsis thaliana, lines overexpressing SoNRT3 exhibited higher root fresh weight, activities of nitrogen reduction/assimilation-related enzymes, tap root length, and total root diameter under low-nitrate (0.25 mM) conditions. SoNRT3 silencing reduced taproot length, lateral root number, shoot/root biomass, and 15NO3− uptake in spinach grown under low-nitrate conditions. SoNRT3 partially compensated for 15NO3− uptake in atnrt2.1 and atnrt3.1 mutants. Transcriptome analysis showed that SoNRT3 may enhance nitrate uptake and root development by promoting the expression of high-affinity nitrate transporters, nitrogen assimilation, auxin signaling, and cell differentiation. Additionally, SoNRT3 can interact with a spinach NRT2 family protein (SoNRT2a), whose transcription level was also induced by low N and N deficiency. Together, this study clarifies the key roles and regulatory network of SoNRT3 in low-nitrate tolerance, which contributes to a novel understanding of nitrate utilization in spinach.