Integrated Transcriptomic and Metabolomic Analysis Reveals the Molecular Regulatory Mechanism of Flavonoid Biosynthesis in Maize Roots under Lead Stress

Flavonoids are secondary metabolites that play important roles in the resistance of plants to abiotic stress. Despite the widely reported adverse effects of lead (Pb) contamination on maize, the effects of Pb on the biosynthetic processes of flavonoids in maize roots are still unknown. In the present work, we employed a combination of multi-omics and conventional assay methods to investigate the effects of two concentrations of Pb (40 and 250 mg/kg) on flavonoid biosynthesis in maize roots and the associated molecular regulatory mechanisms. Analysis using conventional assays revealed that 40 and 250 mg/kg Pb exposure increased the lead content of maize root to 0.67 ± 0.18 mg/kg and 3.09 ± 0.02 mg/kg, respectively, but they did not result in significant changes in maize root length. The multi-omics results suggested that exposure to 40 mg/kg of Pb caused differential expression of 33 genes and 34 metabolites related to flavonoids in the maize root system, while 250 mg/kg of Pb caused differential expression of 34 genes and 31 metabolites. Not only did these differentially expressed genes and metabolites participate in transferase activity, anthocyanin-containing compound biosynthetic processes, metal ion binding, hydroxyl group binding, cinnamoyl transferase activity, hydroxycinnamoyl transferase activity, and flavanone 4-reductase activity but they were also significantly enriched in the flavonoid, isoflavonoid, flavone, and flavonol biosynthesis pathways. These results show that Pb is involved in the regulation of maize root growth by interfering with the biosynthesis of flavonoids in the maize root system. The results of this study will enable the elucidation of the mechanisms of the effects of lead on maize root systems.