Mn3O4nanozymes boost endogenous antioxidant metabolites in cucumber (Cucumis sativus) plant and enhance resistance to salinity stress

Nanomaterials with enzyme-like activities (nanozyme) have shown potential to augment the inherent functions of plants,e.g.photosynthesis and stress resistance. Mechanistically understanding the interaction between plants and nanozymes is important. In this study, Mn(3)O(4)NPs (1 and 5 mg per plant) with excellent antioxidant enzyme mimicking activities were synthesized and were foliarly applied to 3 week-old cucumber plants for 7 days. The response of plants to Mn(3)O(4)NPs was determined at physiological and metabolic levels. Results showed that Mn(3)O(4)NPs at 1 mg per plant significantly increased leaf photosynthetic pigment content, net photosynthesis, and biomass. Gas chromatography-mass spectrometry based metabolomics revealed that Mn(3)O(4)NPs at 1 mg per plant altered the metabolomes in the cucumber leaves. A notable change was that both precursors (phenylalanine and tyrosine) and down-stream products (resveratrol, chlorogenic acid, dihydroxycinnamic acid, benzenetriol, hydroxybenzoic acid, trihydroxybenzene, quinic acid, and catechin) in shikimate and phenylpropanoid pathways were up-regulated in response to the Mn(3)O(4)NPs, indicating the boosting of endogenous antioxidants. Mn(3)O(4)NPs were then applied to 3 week-old cucumber plants subjected to salinity stress to test their performance to stress tolerance. Mn(3)O(4)NPs at 1 mg per plant levels were found to successfully alleviate oxidative stress in cucumber plants allowing them to maintain their biomass. These results indicate that Mn(3)O(4)NPs can boost endogenous antioxidant defenses in the plant, which may enable Mn(3)O(4)NPs to be a promising nano-regulator helping to counter the undesirable effects of oxidative stress.