Combined transcriptome and metabolome analysis revealed the toxicity mechanism of individual or combined of microplastic and salt stress on maize

In saline alkaline soils, microplastics inevitably form a combined stress with NaCl to limit crop growth, but the molecular mechanisms of their toxic effects remain vague and inadequate. We analyzed the molecular mechanisms underlying the response of maize seedlings to single or combined stresses of MPs and NaCl by means of combined metabolomic and transcriptomic analyses. MPs and NaCl single or combined stresses reduced plant fresh weight by 36.78 %, 50.65 % and 73.97 %, respectively. Analyses showed 2476 differentially expressed genes (DEGs) and 809 differential metabolites (DMs) for MPs, 2306 DEGs and 901 DMs for NaCl, and 2706 DEGs and 938 DMs for the combined stresses, compared to CK. Single or combined stresses mainly altered amino acid synthesis and phenylpropane biosynthetic metabolic pathways. Stress up-regulated glutamine synthetase (glnA), alanine transaminase (ALT), aspartate aminotransferase (ASP), ornithine carbamoyl transferase (argF), and glycine hydroxymethyl transferase (SHM) genes expression and promotes glutamine, 2-oxoglutarate, glutamate, fumarate, arginine, aspartate, L-isoleucine, L-valine, and serine synthesis. NaCl stimulated phenylpropanoid biosynthesis (tyrosine, 4-coumarate, and ferulate), whereas MPs decreased it. In addition, both individual or combined NaCl and MPs stress increased the expression of cinnamyl-alcohol dehydrogenase (CAD) and cinnamoyl-CoA reductase (CCR) to promote sinapaldehyde synthesis. Our study provides a molecular perspective on the response of crops, such as maize, to individual or combined NaCl and MPs stress.