Effects of polystyrene microplastics on the phenylpropane metabolic pathway in cucumber plants

Currently, microplastic pollution represents a major problem and the mechanisms of resistance to microplastic pollution in plants have not been investigated extensively. Therefore, in this study, cucumber plants were treated with polystyrene (PS) of two different particle sizes (5 and 0.1 µm) to explore the effect of PS on the phenylpropane metabolic pathway. The results indicated that the 5 µm PS treatment significantly promoted the increase in cucumber plant height, while 0.1 µm PS treatment significantly promoted the achievement of maximum root length. Both 5 and 0.1 µm PS treatments significantly promoted the increase in fresh and dry weight of the aboveground and underground parts of the cucumber plants. In addition, the metabolites of the phenylpropane metabolic pathway, including naringenin, pinobanksin, caffeoylquinic acid, and sinapyl alcohol, were significantly accumulated in cucumber leaves treated with PS (5 and 0.1 µm). This indicates that cucumber plants can resist the stress caused by microplastic pollution by increasing the accumulation of metabolites such as flavonoids and phenolic acids. Treatment with both particle sizes of PS also significantly downregulated the expression of phenylalanine ammonia-lyase, trans-cinnamate 4-monooxygenase, hydroxycinnamoyl transferase, cinnamoyl-CoA reductase, peroxidase, and other enzyme-related genes, indicating that PS treatment decreased the stability of the cell wall and membrane by decreasing the rate of lignin synthesis and significantly increasing the relative permeability of the plasma membrane. Changes in β-glucosidase activity and increase in levels of soluble protein and soluble sugar content represent strategies adopted by plants to maintain the structure of cell walls and cell membranes. The results of the present study provide new insights into the mechanisms underlying plant resistance to microplastic pollution.