Nitrate-induced NLP1 SUMOylation regulates nitrate signaling and root nodulation.

Nitrate serves both as an essential nutrient and a key signaling molecule that shapes plant growth. In legumes, high nitrate concentrations suppress symbiotic nitrogen fixation, a process mediated by MtNLP1 (NIN-like protein1). Although nitrate minimally affects NLP transcript levels, it strongly controls their nuclear localization. How posttranslational modifications regulate MtNLP1 function, however, has remained unclear. Here, we show that nitrate induces SUMOylation of MtNLP1 at lysine 589 and 795 and that this modification is essential for its biological activity. Loss of these SUMO sites compromises nitrate-mediated inhibition of nodulation and weakens MtNLP1 interactions with MtNIN and itself. Components of the SUMOylation machinery in Medicago truncatula physically interact and are essential for both nodulation and nitrate responsiveness, indicating broader roles for SUMOylation in symbiosis. A SUMO-deficient Arabidopsis thaliana AtNLP7(3KR) mutant fails to complement the Atnlp7-1 phenotype, demonstrating that SUMOylation is a conserved regulatory mechanism among NLPs. Together, our findings reveal SUMOylation as a previously unrecognized layer of regulation that integrates nutrient signaling with root nodule symbiosis.