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Despite the wide utilization of chitosan nanoparticles (CSNPs) as a promising approach for sustainable agriculture, their efficiency under elevated CO 2 (eCO 2), has not been evaluated. The interactive effects of CSNPs and eCO 2 were evaluated on the growth and C and N metabolism of soybean plants. Plants were treated with CSNPs and grown under ambient CO 2 (410 ppm, aCO 2) or eCO 2 (645 ppm). Regardless of CO 2 level, CSNPs improved the net photosynthetic rate. CSNPs aggravated the effect of eCO 2 treatment on the levels of non-structural carbohydrates (i.e., glucose, fructose, sucrose, and starch), especially in shoots, which was inconsistence with the upregulation of carbohydrates metabolizing enzymes. Being the most pivotal energetic and signaling organic compounds in higher plants, the synergistic action of CSNPs and eCO 2 on the accumulation of soluble sugars upregulated the N metabolism as indicated by induced activities of nitrate reductase, arginase, glutamate dehydrogenase, glutamine synthetase, and glutamine oxoglutarate aminotransferase which was manifested finally as increased shoot and root total nitrogen content as well as proline and aspartate in roots. At the hormonal level, the coexistence of eCO 2 with CSNPs further supports their positive impact on the contents of IAA and, to a lesser extent, GAs. The present data prove that the biofertilization capacity of CSNPs is even more potent under futuristic eCO 2 levels and could even further improve the growth and resilience of plants. [ABSTRACT FROM AUTHOR]

Future climate CO 2 (eCO 2) and contamination with nano-sized heavy metals (HM-NPs) represent concurrent challenges threatening plants. The interaction between eCO 2 and HM-NPs is rarely investigated, and no study has addressed their synchronous impact on the metabolism of the multifunctional stress-related metabolites, such as sugars and amino acids. Moreover, the characteristic responses of C3 and C4 plant systems to the concurrent impact of eCO 2 and HM-NPs are poorly understood. Herein, we have assessed the impact of eCO 2 (620 ppm) and/or HgO-NPs (100 mg/Kg soil) on growth, physiology and metabolism of sugars and amino acids, particularly proline, in C3 (wheat) and C4 (maize) plant systems. Under Hg-free conditions, eCO 2 treatment markedly improved the growth and photosynthesis and induced sugars levels and metabolism (glucose, fructose, sucrose, starch, sucrose P synthase and starch synthase) in wheat (C3) only. In contrast, HgO-NPs induced the uptake, accumulation and translocation of Hg in wheat and to less extend in maize plants. Particularly in wheat, this induced significant decreases in growth and photosynthesis and increases in photorespiration, dark respiration and levels of tricarboxylic acid cycle organic acids. Interestingly, the co-application of eCO 2 reduced the accumulation of Hg and recovered the HgO-NPs-induced effects on growth and metabolism in both plants. At stress defense level, HgO-NPs induced the accumulation of sucrose and proline, more in maize, via upregulation of sucrose P synthase, ornithine amino transferase, ∆1-pyrroline-5-carboxylate (P5C) synthetase and P5C reductase. The co-existence of eCO 2 favored reduced sucrose biosynthesis and induced proline catabolism, which provide high energy to resume plant growth. Overall, despite the difference in their response to eCO 2 under normal conditions, eCO 2 induced similar metabolic events in C3 and C4 plants under stressful conditions, which trigger stress recovery. Unlabelled Image • Under HgO-NPs free conditions, the biofertilization impact of eCO 2 is only evident in wheat (C3). • HgO-NPs increased Hg uptake, accumulation and translocation in the tested plants. • Maize plants (C4) were more tolerant to HgO-NPs toxicity than wheat. • HgO-NPs induced the accumulation of sucrose and proline, more in maize. • Coexistence of eCO 2 with HgO-NPs reduced sucrose biosynthesis and induced proline catabolism in both plants. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) are beneficial for the plant growth under heavy metal stress. Such beneficial effect is improved by elevated CO 2 (eCO 2). However, the mechanisms by which eCO 2 improves AMF symbiotic associations under arsenite (AsIII) toxicity are hardly studied. Herein, we compared these regulatory mechanisms in species from two agronomical important plant families – grasses (wheat) and legumes (soybean). AsIII decreased plant growth (i.e., 53.75 and 60.29% of wheat and soybean, respectively) and photosynthesis. It also increased photorespiration and oxidative injury in both species, but soybean was more sensitive to oxidative stress as indicated by higher H 2 O 2 accumulation and oxidation of protein and lipid. eCO 2 significantly improved AMF colonization by increasing auxin levels, which induced high carotenoid cleavage dioxygenase (CCDs) activity, particularly in soybean roots. The improved sugar metabolism in plant shoots by co-application of eCO 2 and AsIII allocated more sugars to roots sequentially. Sugar accumulation in plant roots is further induced by AMF, resulting in more C skeletons to produce organic acids, which are effectively exudated into the soil to reduce AsIII uptake. Exposure to eCO 2 reduced oxidative damage and this mitigation was stronger in soybean. This could be attributed to a greater reduction in photorespiration as well as a stronger antioxidant and detoxification defence systems. The grass/legume-specificity was supported by principal component analysis, which revealed that soybean was more affected by AsIII stress and more responsive to AMF and eCO 2. This study provided a mechanistic understanding of the impact of AMF, eCO 2 and their interaction on As-stressed grass and legume plants, allowing better practical strategies to mitigate AsIII phytotoxicity. [Display omitted] • Soybean was more sensitive to As stress but more reponsive to AMF and eCO 2 impact. • High strigolactone biosynthesis by eCO 2 induced AMF colonization, mainly in soybean. • Increased photosynthesis by eCO 2 allocated more sugars to AMF roots of soybean and wheat. • eCO 2 increased soil retention resulting in less As accumulation in AMF treated plants. • eCO 2 strengthen AMF impact on decreasing ROS production and increasing their detoxification. [ABSTRACT FROM AUTHOR]