biology Review The Role of Plant Growth-Promoting Bacteria in Alleviating the Adverse Effects of Drought on Plants Khaled Abdelaal 1, * , Muneera AlKahtani 2 , Kotb Attia 3 , Yaser Hafez 1 ,Lóránt Király 4 and András Künstler 4   Citation: Abdelaal, K.; AlKahtani, M.; Attia, K.; Hafez, Y.; Király,L.; Künstler, A. The Role of Plant Growth-Promoting Bacteria in Alleviating the Adverse Effects of Drought on Plants. Biology 2021, 10, 520. https://doi.org/10.3390/ biology10060520 Academic Editors: Ana Alexandre and Kathrin Wippel Received: 8 May 2021 Accepted: 9 June 2021 Published: 11 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Excellence Center (EPCRS), Plant Pathology and Biotechnology Laboratory, Faculty of Agriculture, Kafrelsheikh University, Kafr Elsheikh 33516, Egypt; yasser.abdelgwad@agr.kfs.edu.eg 2 Biology Department, College of Science, Princess Nourah Bint Abdulrahman University, Riyadh 11564, Saudi Arabia; mdfkahtani@gmail.com 3 Center of Excellence in Biotechnology Research, King Saud University, Riyadh 11451, Saudi Arabia; kattia1.c@ksu.edu.sa 4 Centre for Agricultural Research, Plant Protection Institute, ELKH, 15 Herman Ottó Str., H-1022 Budapest, Hungary; kiraly.lorant@atk.hu (L.K.); kunstler.andras@atk.hu (A.K.) * Correspondence: khaled.elhaies@gmail.com Simple Summary: Among abiotic stress factors, drought is one of the most detrimental factors in arid and semiarid regions, causing a significant decrease in plant growth and yield in most species, including crops. Under drought conditions, morphological, physiological and biochemical characteristics such as plant height and enzymatic activities are negatively affected. This negative effect may be alleviated with the aid of plant growth-promoting bacteria. Application of plant growth- promoting bacteria such as Paenibacillus, Azospirillum, Rhizobium, Bacillus, Azotobacter, Klebsiella, Pseudomonas and Serratia can enhance hormonal balance, maintain nutrient status and improve plant growth characters as well as increase yield. This review discusses the pivotal role of plant growth-promoting bacteria in mitigating drought stress by improving plant growth characters and yield. Abstract: Plant growth-promoting bacteria play an essential role in enhancing the physical, chemical and biological characters of soils by facilitating nutrient uptake and water flow, especially under abiotic stress conditions, which are major constrains to agricultural development and production. Drought is one of the most harmful abiotic stress and perhaps the most severe problem facing agricultural sustainability, leading to a severe shortage in crop productivity. Drought affects plant growth by causing hormonal and membrane stability perturbations, nutrient imbalance and phys- iological disorders. Furthermore, drought causes a remarkable decrease in leaf numbers, relative water content, sugar yield, root yield, chlorophyll a and b and ascorbic acid concentrations. However, the concentrations of total phenolic compounds, electrolyte leakage, lipid peroxidation, amounts of proline, and reactive oxygen species are considerably increased because of drought stress. This nega- tive impact of drought can be eliminated by using plant growth-promoting bacteria (PGPB). Under drought conditions, application of PGPB can improve plant growth by adjusting hormonal balance, maintaining nutrient status and producing plant growth regulators. This role of PGPB positively affects physiological and biochemical characteristics, resulting in increased leaf numbers, sugar yield, relative water content, amounts of photosynthetic pigments and ascorbic acid. Conversely, lipid peroxidation, electrolyte leakage and amounts of proline, total phenolic compounds and reactive oxygen species are decreased under drought in the presence of PGPB. The current review gives an overview on the impact of drought on plants and the pivotal role of PGPB in mitigating the negative effects of drought by enhancing antioxidant defense systems and increasing plant growth and yield to improve sustainable agriculture. Keywords: plant growth-promoting bacteria; drought; antioxidant enzymes; chlorophylls; phenols Biology 2021, 10, 520. https://doi.org/10.3390/biology10060520 https://www.mdpi.com/journal/biology