**5. Conclusions**

The present study explored the collective decline in drought stress resilience in plants due to impaired photosynthetic activities, severe growth retardation, excess ROS accumulation, and damaged root morphology under drought stress (Figure 11). Per contra, the stress induced by drought was effectively mitigated by exogenous application of melatonin, as oxidative damage is reduced (with refining in antioxidant defense system and inhibiting ROS production), root architecture is enhanced, photosynthesis efficiency is strengthened and thus growth attributes are recovered. In addition, the soluble sugars and proline content noticeably decline, and the formation of AsA-DHA and GSH-GSSG arere prominently improved in ME-pretreated tomato plants. This suggests that exogenous ME is an effective protectant that improves DR tolerance in tomato seedlings by enhancing antioxidant enzymes and reducing oxidative damages. Nevertheless, under drought stress and melatonin-mediated resilience in plants, and its mechanism should be investigated in-depth, along with the explicit study of molecular approaches.

**Figure 11.** A proposed model showing how melatonin mitigates drought stress tolerance in tomato seedlings.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/antiox11020309/s1, Table S1: The detail of primers.

**Author Contributions:** Conceptualization, M.A.A. and M.-X.R.; methodology, M.A.A.; software, R.S. and B.L.J.; validation, S.N., M.-X.R. and M.A.A.; formal analysis, L.U.K.; investigation, M.A.A.; resources, M.-X.R.; data curation, M.M.A.; writing—original draft preparation, M.A.A.; review and editing, R.K.T., M.K.L., M.A.S., R.K., M.A.N., M.S.J. and P.A.; visualization, M.A.A.; supervision, M.-X.R.; project administration, M.-X.R.; funding acquisition, M.-X.R., B.L.J. and P.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This program was financially supported by the Hainan Key Science and Technology Program (Grant No. ZDKJ202008-1-2), National Natural Science Foundation of China (41871041).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to thank Wang Zhiwei from the School of Horticulture, Hainan University, Haikou for his kind guidance and laboratory equipment. The authors would also like to extend their sincere appreciation to the Researchers Supporting Project Number (RSP-2021/168), King Saud University, Riyadh, Saudi Arabia.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

