Multi‐Omics Insights Into Phenylpropanoid and Lipid Barrier Biosynthesis in Maize Roots Under Salt and Microplastic Stresses

Microplastics accumulate during saline-alkali land improvement from agricultural inputs such as fertilizers and plastic films, forming a composite stress on crop growth. This study investigated the response mechanisms of maize (Zea mays L.) seedling roots to individual and combined stresses from microplastics and salt. Maize seedlings were exposed to 100 mM NaCl and 50 mg l^-1 Polystyrene microplastics. Transcriptomic and metabolomic analyses were conducted to elucidate molecular responses. Salt (NaCl) and microplastics (MPs), both individually and in combination (PNaCl), significantly inhibited root growth and water metabolism. NaCl and PNaCl stresses upregulated phenylalanine ammonia-lyase and 4-coumaroyl-CoA ligase in the phenylpropanoid pathway, promoting the accumulation of phenolic metabolites such as ferulic acid, while MPs stress had the opposite effect. Under NaCl stress, catechol-O-methyltransferase expression increased, enhancing caffeic acid conversion and antioxidant capacity. Additionally, NaCl and PNaCl stresses also promoted the expression of genes involved in the biosynthesis of cutin, suberin, and wax, whereas MPs stress suppressed these genes. Overall, maize roots respond to NaCl and PNaCl stresses by activating phenylpropanoid metabolism, accumulating phenolic compounds, and enhancing the biosynthesis of cutin, suberin, and wax, thereby improving stress resistance and providing insights into plant adaptation to complex environmental conditions.