CO(2) Enrichment Differentially Upregulated Sugar, Proline, and Polyamine Metabolism in Young and Old Leaves of Wheat and Sorghum to Mitigate Indium Oxide Nanoparticles Toxicity

Soil contamination with indium oxide nanoparticles (In(2)O(3)-NPs) is a challenge for plant growth and productivity. Despite In(2)O(3)-NPs toxicity, their effects on plant growth and metabolism are largely unknown, particularly under future climate CO(2) (eCO(2)). Therefore, the In(2)O(3)-NPs toxicity and stress mitigating impact of eCO(2) in the young and old leaves of C3 (wheat) and C4 (sorghum) plants were investigated. Overall, In(2)O(3)-NPs significantly retard the biomass and photosynthetic machinery of all tested crops, particularly the young leaves of C3 plants. Consequently, In(2)O(3)-NPs altered C and N metabolism in C3 and C4 plants. On the other hand, eCO(2) contrarily alleviated the hazardous effects of In(2)O(3)-NPs on growth and photosynthesis, especially in the young leaves of C4 plants. Increased photosynthesis consequently enhanced the soluble sugars’ accumulation and metabolism (e.g., sucrose P synthase, cytosolic, and vacuolar invertase) in all stressed plants, but to a greater extent in C4 young leaves. High sugar availability also induced TCA organic and fatty acids’ accumulation. This also provided a route for amino acids and polyamines biosynthesis, where a clear increase in proline biosynthetic enzymes [e.g., pyrroline-5-carboxylate synthetase (P5CS), ornithine aminotransferase (OAT), Pyrroline-5-carboxylate reductase (P5CR), pyrroline-5-carboxylate dehydrogenase (P5CDH), and proline dehydrogenase (PRODH)] and polyamine metabolic enzymes (e.g., spermine and spermidine synthases, ornithine decarboxylase, and adenosyl methionine decarboxylase) were mainly recorded in C4 young leaves. The observed increases in these metabolites involved in osmo- and redox-regulation to reduce In(2)O(3)-NPs induced oxidative damage. Overall, our study, for the first time, shed light on how eCO(2) differentially mitigated In(2)O(3)-NPs stress in old and young leaves of different species groups under the threat of In(2)O(3)-NPs contamination.