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. 2020 Dec 23;10(1):11. doi: 10.3390/plants10010011

Figure 7.

Figure 7

The probable mechanistic roles of allantoin (AT) in alleviating the adverse effects of UVC stress in tomato plants. Foliar application of AT positively altered several physio-biochemical processes in tomato plants. (A) UVC stress induces higher reactive oxygen species (ROS, O2•−, H2O2, and •OH) accumulation by lowering several non-enzymatic and enzymatic antioxidants, and ROS induces higher lipoxygenase (LOX) activity and causes lipid peroxidation; as a result, tomato plants accumulate higher amounts of malondialdehyde (MDA) and methylglyoxal (MG). However, exogenous AT treatment enhances non-enzymatic and enzymatic antioxidants and lowers LOX activity and ultimately reduced MDA and MG accumulation. (B) Exposure to UVC stress causes chlorophyll (Chl) degradation by enhancing ROS that deactivates Chl synthesizing enzymes or directly oxidize Chl pigments. UVC stress also lowers the anthocyanin content and accelerates photo-oxidation-induced Chl degradation. The efficiency of photosynthesis is reduced in the UVC-stressed tomato plants due to the Chl degradation and decreased water content and primary metabolites in plant leaves, leading to growth retardation. On the other hand, exogenous AT enhances the endogenous allantoin content and upregulates anthocyanin content, primary metabolites, and water content. Moreover, AT-mediated higher antioxidants inhibits ROS accumulation and protects Chl pigments. Thus, exogenous AT enhances the growth of tomato plants under UVC stress. (C) UVC stress lowers H2S and Cys contents and impairs plant signaling networks. The decreased phenylalanine ammonia-lyase (PAL) and increased polyphenol oxidase (PPO) activity induced by UVC cause less secondary metabolite contents. Enhanced ROS and NO simultaneously cause nitrosative stress, and reduced epicuticular wax causes osmotic stress in UVC-stressed tomato plants. Application of exogenous AT improves the signaling system, secondary metabolism, osmotic balance, and mitigates nitrosative stress by altering the aforementioned processes. SC, soluble carbohydrate content; FAA, free amino acid content; TPC, total phenolic compound content.