## 4 Antioxidant Machinery in Plant Systems and Microbial Mediation in Promoting Plant Tolerance
[45] have shown that the inoculation of _Cucumis sativus_ (cucumber) with a consortium of three PGPR strains could induce systemic tolerance in drought-imposed plants by maintaining root vigour, photosynthetic performance, and increased generation and activity of SOD, CAT and prolines in the leaves. In _Lycopersicon esculent_ (tomato) plants treated by a _Bacillus cereus_ AR156 supernatant, an appreciable enhancement to drought stress was reported by [PERSON] et al. [306]. The induced tolerance was associated with the increased chlorophyll a and b contents, as well as the enhanced activities of CAT, SOD, and peroxidase (POX). Enhanced salinity tolerance in _Panicum turgidum_ Forssk plants has been attributed to the _Arbuscular mycorrhizal_ fungi contribution by modifyingphotosynthetic and antioxidant pathways [307]. Similarly, PGPR alleviated drought stress in potato (_Solanum tuberosum_ L.) plants treated with _Bacillus subtilis_ HAS31 by maintaining higher photosynthetic processes, total soluble sugars, proteins and prolines with elevated activity of POX, CAT, and SOD [281]. In another study by [PERSON] et al. [308], it was revealed that PGPR treatment significantly augmented drought tolerance in _Agrostis palustris_ by lowering MDA accumulation and developing osmotic adjustments associated with higher synthesis and accumulation of compatible solutes, including soluble sugars, free amino acids, proteins, and not least the non-enzymatic prolines. Plant inoculation with PGPRs adds up to the available quantity of proline in plant systems under stress conditions [309]. As such, a sizable quantity of prolines was increased in _Zea mays_ L. after inoculation with _P. fluorescens_ under drought stress [310]. A noticeable enhancement in drought tolerance of _Lavandula dentate_ plants treated with PGPR _B. thuringiensis_ was recorded under drought conditions, which was attributed to the increased shoot proline contents [311]. Also, there was an excess secretion of proline in the root of tomato (_Lycopersicon esculent_ Mill cv. [PERSON]) plants after exposition to _Bacillus polypnxa_[312]. Similar to plants, PGPRs secrete osmolytes (e.g., proline) at the time of water scarcity, which cumulatively helps in stimulating plant growth [313]. The application of _Trichoderma afroharzianum_ strain T22 has been shown to enhance tomato (_Solanum lycopersicum_ L.) seed germination under biotic stress conditions by alleviating oxidative damage in salt-stressed seedlings [314]. Similarly, [PERSON] et al. [282] demonstrated that the PGPF _Trichoderma lugibraciatum_ strain T6 induced the tolerance of wheat seedlings to salt stress through upregulation of SOD, CAT and POX genes, improving the antioxidant defence machinery. Compared to uninoculated rice genotypes Swama and Swama sub-1 under severe drought conditions, the PCPFs (_Fusarium pallidoroseum_ strain-10 and _Trichoderma harzianum_ strain-35) inoculated rice genotypes showed higher activity of SOD, POX, and CAT [315]. Table 2 briefly presents the antioxidative responses in selected plant species after exposure to various environmental stresses.