Integration of physiological, transcriptomic, and metabolomic approaches reveals the responses of sweet orange (Citrus sinensis) seedlings exposed to polystyrene microplastics

Polystyrene microplastics (PS-MPs) are ubiquitous in agricultural soils and can be captured by plants; nevertheless, the molecular mechanisms underlying responses of fruit trees to microplastic exposure remain largely unknown. Here, an integration of physiological, transcriptomic, and metabolomic strategies was employed to assess the effects of PS-MPs on sweet orange seedlings in pot experiments. PS-MP exposure increased H₂O₂ content by 40.33 % and reduced soluble protein levels by 38.56 %, while increasing superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities by 100.05 %, 200.36 %, and 300.47 %, respectively. PS-MPs significantly influenced the expression of genes involved in amino acid biosynthesis, elevating the levels of L-phenylalanine, L-proline, L-citrulline, L-arginine, L-tyrosine, and L-isoleucine by 181 %, 51.57 %, 486 %, 382 %, 154 %, and 123 %, respectively, while reducing the L-lysine content by 78 %. PS-MP also induced the production of a large quantity of amino acid derivatives, flavonoids, alkaloids, organic acids, and lipid accumulation. PS-MP also induced transcription of 9-cis-epoxycarotenoid dioxygenase (NCEDs), which elevated ABA content by 974 %. Moreover, PS-MP suppressed the expression of photosystem II light-harvesting complex protein genes (LHCIIs), impairing light energy absorption and transfer. PS-MPs induced the phenylpropanoid pathway by increasing the expression of phenylalanine ammonia-lyase genes (PALs), resulting in a 436 % increase in cinnamic acid. PS-MPs activated the serotonin pathway by up-regulation of TDCs expression, increasing tryptamine by 1570 % and serotonin by 1309 %. To the best of our knowledge, this is the first report of the serotonin pathway being activated by microplastic exposure. This study offers new insights into the mechanisms underlying sweet orange responses to PS-MP exposure.