Citation: Jan, R.; Khan, M.-A.; Asaf, S.; Lubna; Waqas, M.; Park, J.-R.; Asif, S.; Kim, N.; Lee, I.-J.; Kim, K.-M. Drought and UV Radiation Stress Tolerance in Rice Is Improved by Overaccumulation of Non-Enzymatic Antioxidant Flavonoids. Antioxidants 2022, 11, 917. https://doi.org/ 10.3390/antiox11050917 Academic Editor: Stanley Omaye Received: 11 April 2022 Accepted: 4 May 2022 Published: 6 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). antioxidants Article Drought and UV Radiation Stress Tolerance in Rice Is Improved by Overaccumulation of Non-Enzymatic Antioxidant Flavonoids Rahmatullah Jan 1,2 , Muhammad-Aaqil Khan 1 , Sajjad Asaf 3 , Lubna 4 , Muhammad Waqas 5 , Jae-Ryoung Park 1,6 , Saleem Asif 1 , Nari Kim 1 , In-Jung Lee 1 and Kyung-Min Kim 1,2, * 1 Division of Plant Biosciences, Department of Applied Biosciences, College of Agriculture and Life Science, Kyungpook National University, 80 Dahak-ro, Buk-gu, Daegu 41566, Korea; rahmat2021@knu.ac.kr (R.J.); aqil_bacha@knu.ac.kr (M.-A.K.); icd0192@korea.kr (J.-R.P.); saleemasif10@knu.ac.kr (S.A.); jennynari@knu.ac.kr (N.K.); ijlee@knu.ac.kr (I.-J.L.) 2 Costal Agriculture Research Institute, Kyungpook National University, 80 Dahak-ro, Buk-gu, Daegu 41566, Korea 3 Natural and Medical Science Research Center, University of Nizwa, Nizwa 616, Oman; sajjadasaf@unizwa.edu.om 4 Department of Botany, Garden Campus, Abdul Wali Khan University, Mardan 23200, Pakistan; lubnabilal68@gmail.com 5 Department of Agriculture Extension, Government of Khyber Pakhtunkhwa, Mardan 23200, Pakistan; agranomist89@yahoo.com 6 Crop Breeding Division, National Institute of Crop Science, Rural Development Administration, Wanju 55365, Korea * Correspondence: kkm@knu.ac.kr; Tel.: +82-53-950-5711; Fax: +82-539-586-880 Abstract: Drought and ultraviolet radiation (UV radiation) are the coexisting environmental factors that negatively affect plant growth and development via oxidative damage. Flavonoids are reactive, scavenging oxygen species (ROS) and UV radiation-absorbing compounds generated under stress conditions. We investigated the biosynthesis of kaempferol and quercetin in wild and flavanone 3-hydroxylase (F3H) overexpresser rice plants when drought and UV radiation stress were imposed individually and together. Phenotypic variation indicated that both kinds of stress highly reduced rice plant growth parameters in wild plants as compared to transgenic plants. When combined, the stressors adversely affected rice plant growth parameters more than when they were imposed individually. Overaccumulation of kaempferol and quercetin in transgenic plants demonstrated that both flavonoids were crucial for enhanced tolerance to such stresses. Oxidative activity assays showed that kaempferol and quercetin overaccumulation with strong non-enzymatic antioxidant activity mitigated the accumulation of ROS under drought and UV radiation stress. Lower contents of salicylic acid (SA) in transgenic plants indicated that flavonoid accumulation reduced stress, which led to the accumulation of low levels of SA. Transcriptional regulation of the dehydrin (DHN) and ultraviolet-B resistance 8 (UVR8) genes showed significant increases in transgenic plants compared to wild plants under stress. Taken together, these results confirm the usefulness of kaempferol and quercetin in enhancing tolerance to both drought and UV radiation stress. Keywords: drought and UV radiation stress; flavonoids content; antioxidant content; salicylic acid; genes expression 1. Introduction Over the last few decades, anthropogenic pollutants have significantly reduced strato- spheric ozone, which could lead to a considerable increase in UV radiation [1]. This is a distressing situation from the standpoint of environmental safety. Living organisms are naked to UV radiation and absorb it to different degrees depending on the angle of the Antioxidants 2022, 11, 917. https://doi.org/10.3390/antiox11050917 https://www.mdpi.com/journal/antioxidants