## 5 Conclusions  
The versatility of oxidative stress occurrence in response to environmental conditions, like extreme temperatures, drought, soil salinity, pests, and diseases, highlights the importance for developing crops which are able to withstand various environmental challenges simultaneously. In the present review we have provided evidence that enhancing the content of various low molecular weight antioxidants, isoprenoids, flavonoids, ascorbate, or glutathione increases the sustainability of higher plants under those factors, which are accompanied by the elevated ROS level. The major knowledge presented in the literature about the role of enzymes in the synthesis of non-enzymatic antioxidants, as well as the characteristics of plant transformantsants with increased content of these antioxidants, was obtained using classical transgenes. The main aim of the present review was to summarize the information on existing techniques of plant engineering that have led to successful increases in non-enzymatic antioxidant production in order to identify the most promising future strategies. This review describes plenty of the examples of experimental works dedicated to overexpression of genes of the antioxidant's biosynthesis. The review highlights that the most relevant strategy is to create overexpression of the genes, encoding the enzymes of the final stages of antioxidant biosynthesis. The attempts for overexpression of the genes of initial stages led to a rapid depletion of the common precursor for the biosynthesis of these antioxidants and to a decrease in their content and, as a consequence, to creation of stress-sensitive plants. The described strategies for creating mutants by regulating the intensity of TF functioning are of importance, since the creation of such mutants can lead to intensified expression of a number of antioxidants. In this case, both overexpression of TF-activators and Crispr/Cas9 knockout of TF-repressors were successfully used [163; 164]. When choosing tools for editing genes involved in the biosynthesis of antioxidants, it is worth paying attention to the characteristics of plant transformation and the target organs for increased antioxidant production. The widely used 35S promoter of cauliflower mosaic virus does not always lead to the desired effect, for example, when increasing the level of antioxidants in seeds. In such cases it is more relevant to use a seed-specific promoter. In addition to increasing the synthesis of the antioxidants itself, the strategy of parallel intensification of the production of corresponding chaperone proteins and storage sites for these substances is preferable, such as chromoplasts for carotenoids or plastoglobules for PQ and plastochromanol storage. The current actively developing techniques of creating mutants using various genome editing methods significantly expands the capabilities of researchers. These techniques allow both the gene sequences and regulatory elements to be inserted into a certain given region of the plant genome. In addition, it makes it possible to introduce mutations into one or several target genes with an accuracy of one nucleotide. The genome editing methods are based on the manipulations of the nucleotide sequences in a strictly specified location and with a minimum number of off-target changes in the genome, and are supposed to be environmentally friendly. Nevertheless, transgenesis and genome editing are not mutually exclusive, but can complement each other. However, considering the above information concerning gene editing, it seems likely that the developing dCAs9 technique will displace the classical transgenesis for the creation of overexpressing mutants. Overall, we have considered that the most prospective approach is the complex multi-strategy engineering, which, in addition to all of the above, also takes into account the fact that antioxidants are the precursors of some hormones and other metabolites.