## 5 Conclusions and Future Prospects  
ROS are natural byproducts of many metabolic pathways or their respective electron transport activities in different cellular compartments. Thanks to cellular auto adjustment mechanisms, there is always a homeostatic balance between ROS production and removal machinery which make ROS less harmful to plants in normal environments. However, prolonged environmental stresses such as salinity, chilling, drought, water deficit, and UV radiation can severely exacerbate the production level of ROS by disrupting the natural cellular homeostasis, changing the ROS role as a signalling molecule to a damaging oxidant capable of harming lipids, proteins and DNA. Plants have developed different enzymatic and non-enzymatic antioxidative pathways to alleviate the adverse effects of oxidative damages. The term Induced Systemic Tolerance (IST) has been coined in this review to capture the concept of microbial modulation of enhanced plant tolerance against abiotic stresses through inducing physical and chemical alterations. The beneficial microbiome related to roots and plant tissues suppresses plant stress by a variety of processes. As an example among them, PGPRs are capable of enhancing plant micronutrient uptake, regulating phytohormones homeostasis, and stimulating the plant immune system against biotic and abiotic stresses.  
Insofar as the results of ROS experiments under a variety of imposed stresses are concerned, it is not entirely clear how plants deal with stress-induced ROS homeostasis disruption and the consequent cellular degradation. There are still many unsolved questions regarding the simultaneous antioxidative activities of enzymatic antioxidants and their non-enzymatic counterparts. These uncertainties can be considered as hints for further studies in ROS formation and ROS removal machinery. In light of ROS studies, the use of state of art analytical and/or imaging techniques might provide a broader insight into understanding the complex antioxidant networks involved in plant responses to elevated ROS levels. Further, the combined application of advanced functional genomics, proteomics and metabolomics might be helpful in ROS network elaboration. Transgenic techniques can also be used for producing plants with high tolerance to multiple stresses.  
Gathering the required information and drafting the manuscript: P.Z. and E.S.; Revising and editing the manuscript: E.S. and P.Z. All authors have read and agreed to the published version of the manuscript.  
This research did not receive any funding from authors or their corresponding Universities.  
Not applicable.  
No associated data marked.  
The authors declared that there is no conflict of interest with reference to this review.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Plant Species** & **Stress Type** & \begin{tabular}{c} **Antioxidant Reported** \\ **+ (Increase)**/**– (Decrease)** \\ \end{tabular} & **Reference** \\ \hline _Lavendula apusustiglia C._ & & \begin{tabular}{c} +Proline, +CAT, +APX, +POX, +SOD, \\ +MDHA, +Flavonoids, and +Phenols in \\ leaves \\ \end{tabular} & [351] \\ \hline _Rosmarinus officinalis_ L. & Drought & \
\begin{tabular}{c} +Carotenoids, and +\(\alpha\)-tocopherol in \\ leaves \\ \end{tabular} & [352] \\ \hline _Cucumis sativus_ L. & Heat stress & +SOD, +CAT, and +POX in leaves & [353] \\ \hline _Glycine max_ L. cv. Clark & UV-B radiation &
\begin{tabular}{c} -APX, -CAT, +SOD, +DHA, and \\ +Flavonoid in leaves \\ \end{tabular} & [354] \\ \hline \hline \end{tabular}
\end{table}
Table 2: _Cont._