Magnesium-Induced Transporters Remodel the Ionome of Mulberry Leaves by Promoting the Nitrate Reductase (NIA) Gene in Nitrogen Metabolism.

Magnesium (Mg) is fundamental for diverse physiological, biochemical, and molecular processes in higher plants because its deficiency or toxicity severely restricts development. The supply of Mg remodels the plant ionome and transporter profiles while simultaneously activating the nitrate reductase enzyme, which catalyzes nitrate reduction to nitrite and enhances nitrogen (N) assimilation. However, the specific mechanisms involving ion interactions and transporters in Mg metabolism that improve N metabolism in mulberry remain largely unexplored. To address this research gap, we employed physiological, transcriptomic, and functional characterizations of the nitrate reductase (NIA) gene in mulberry (Morus alba L. car. Yu-711) plants exposed to six Mg levels: deficiency at 0mM, low at 1mM, moderately low at 2mM, sufficiency (control, CK) at 3mM, toxicity at 6mM, and higher toxicity at 9mM for twenty days. Findings showed that Mg allocation follows an acropetal decrease, where roots retain the highest proportion followed by stems and leaves. Detailed correlation and regression analyses identified significant positive and negative interactions between Mg and other essential elements (N, K, P, S, Ca, Zn, Cu, and Fe), confirming that Mg supply alters the mulberry ionome. These alterations led to the induction of various transporters, coupled with upregulation of the NIA gene, which triggered activation of the N metabolism pathway. Functional characterization via virus-induced gene silencing (VIGS) of MaNIA resulted in a marked decrease in both Mg accumulation and NIA expression levels and subsequently reduced nitrate reductase (NR) enzyme activity. Subcellular localization studies further determined that NIA is localized in the cytoplasm. This research offers novel perspectives on the complex interplay between Mg and essential nutrients in the mulberry ionome and provides a critical framework for designing metabolic engineering strategies to optimize growth in plants suffering from Mg disorders.