Nitrate nitrogen uptake and metabolism in Mikania micrantha stem: insights into enhanced growth and invasiveness

The increasing atmospheric nitrogen deposition, characterized by a rising proportion of nitrate nitrogen (NO₃⁻-N), is exacerbating the spread of invasive plant species. Despite this trend, the response mechanisms of <i>Mikania micrantha</i>, a highly invasive plant, to NO₃⁻-N remain poorly understood. This study investigates the unique adaptation strategies of <i>M. micrantha</i> to elevated NO₃⁻-N levels, providing novel insights into its invasive success under changing nitrogen deposition patterns. Field experiments showed that <i>M. micrantha</i> rhizosphere soil contained higher NO₃ ^-N content and protease activity compared to companion plants (<i>Paederia scandens</i>, <i>Ipomoea nil</i>, and <i>Ipomoea cairica</i>). Both roots and stems of <i>M. micrantha</i> had higher NO₃ ^-N content and demonstrated stronger nitrogen metabolism capabilities. Pot experiments further showed that increasing NO₃⁻-N concentrations (0 mM-40 mM) significantly promoted <i>M. micrantha</i> growth, with optimal phenotypic responses (main stem length, leaf number, branch number, and biomass) observed at 5 mM NO₃⁻-N. Nitrogen metabolism enzyme assays revealed that nitrate reductase (NR), nitrite reductase (NiR), glutamate dehydrogenase (GDH), and free amino acid content increased progressively with NO₃⁻-N concentration. Transcriptome sequencing and qPCR analyses identified upregulation of key genes related to transcription factors, nitrate transporter-related, nitrogen metabolism enzyme, and amino acid synthesis pathway. These findings demonstrate that <i>M. micrantha</i> employs a multifaceted strategy to exploit elevated NO₃⁻-N conditions: enhanced NO₃⁻-N uptake from soil, efficient transport to stems, and robust nitrogen metabolism facilitated by coordinated gene expression. This study reveals the adaptation mechanisms of <i>M. micrantha</i> to NO₃⁻-N enrichment, offering critical insights for predicting and managing invasive species responses to global atmospheric nitrogen deposition changes. The results highlight the importance of considering nitrogen composition, rather than just quantity, in invasive species management strategies.