Integrated Transcriptomic and Metabolomic Analysis of Exogenous NAA Effects on Maize Seedling Root Systems under Potassium Deficiency

Auxin plays a crucial role in regulating root growth and development, and its distribution pattern under environmental stimuli significantly influences root plasticity. Under K deficiency, the interaction between K+ transporters and auxin can modulate root development. This study compared the differences in root morphology and physiological mechanisms of the low-K-tolerant maize inbred line 90-21-3 and K-sensitive maize inbred line D937 under K-deficiency (K+ = 0.2 mM) with exogenous NAA (1-naphthaleneacetic acid, NAA = 0.01 mM) treatment. Root systems of 90-21-3 exhibited higher K+ absorption efficiency. Conversely, D937 seedling roots demonstrated greater plasticity and higher K+ content. In-depth analysis through transcriptomics and metabolomics revealed that 90-21-3 and D937 seedling roots showed differential responses to exogenous NAA under K-deficiency. In 90-21-3, upregulation of the expression of K+ absorption and transport-related proteins (proton-exporting ATPase and potassium transporter) and the enrichment of antioxidant-related functional genes were observed. In D937, exogenous NAA promoted the responses of genes related to intercellular ethylene and cation transport to K-deficiency. Differential metabolite enrichment analysis primarily revealed significant enrichment in flavonoid biosynthesis, tryptophan metabolism, and hormone signaling pathways. Integrated transcriptomic and metabolomic analyses revealed that phenylpropanoid biosynthesis is a crucial pathway, with core genes (related to peroxidase enzyme) and core metabolites upregulated in 90-21-3. The findings suggest that under K-deficiency, exogenous NAA induces substantial changes in maize roots, with the phenylpropanoid biosynthesis pathway playing a crucial role in the maize root’s response to exogenous NAA regulation under K-deficiency.