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NLP7 (NIN-LIKE-PROTEIN 7) is the major transcriptional factor responsible for the primary nitrate response (PNR), but the role of its homolog, NLP6, in nitrogen signaling and the interplay between NLP6 and NLP7 remain to be elucidated. In this study, we show that, like NLP7, nuclear localization of NLP6 via a nuclear retention mechanism is nitrate dependent, but nucleocytosolic shuttling of both NLP6 and NLP7 is independent of each other. Compared with single mutants, the nlp6nlp7 double mutant displays a synergistic growth retardation phenotype in response to nitrate. The transcriptome analysis of the PNR showed that NLP6 and NLP7 govern ∼50% of nitrate-induced genes, with cluster analysis highlighting 2 distinct patterns. In the A1 cluster, NLP7 plays the major role, whereas in the A2 cluster, NLP6 and NLP7 are partially functionally redundant. Interestingly, comparing the growth phenotype and PNR under high- and low-nitrate conditions demonstrated that NLP6 and NLP7 exert a more dominant role in the response to high nitrate. Apart from nitrate signaling, NLP6 and NLP7 also participated in high ammonium conditions. Growth phenotypes and transcriptome data revealed that NLP6 and NLP7 are completely functionally redundant and may act as repressors in response to ammonium. Other NLP family members also participated in the PNR, with NLP2 and NLP7 acting as broader regulators and NLP4, -5, -6, and -8 regulating PNR in a gene-dependent manner. Thus, our findings indicate that multiple modes of interplay exist between NLP6 and NLP7 that differ depending on nitrogen sources and gene clusters., (© American Society of Plant Biologists 2023. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Nitrate is an essential nutrient, and is involved in many adaptive responses of plants, such as localized proliferation of roots, flowering or stomatal movements. How such nitrate-specific mechanisms are regulated at the molecular level is poorly understood. Although the Arabidopsis ANR1 transcription factor appears to control stimulation of lateral root elongation in response to nitrate, no regulators of nitrate assimilation have so far been identified in higher plants. Legume-specific symbiotic nitrogen fixation is under the control of the putative transcription factor, NIN, in Lotus japonicus. Recently, the algal homologue NIT2 was found to regulate nitrate assimilation. Here we report that Arabidopsis thaliana NIN-like protein 7 (NLP7) knockout mutants constitutively show several features of nitrogen-starved plants, and that they are tolerant to drought stress. We show that nlp7 mutants are impaired in transduction of the nitrate signal, and that the NLP7 expression pattern is consistent with a function of NLP7 in the sensing of nitrogen. Translational fusions with GFP showed a nuclear localization for the NLP7 putative transcription factor. We propose NLP7 as an important element of the nitrate signal transduction pathway and as a new regulatory protein specific for nitrogen assimilation in non-nodulating plants.