Application of 2,4-Epibrassinolide Improves Drought Tolerance in Tobacco through Physiological and Biochemical Mechanisms

SIMPLE SUMMARY: Drought stress is one of the most serious abiotic stresses and negatively affects crop growth and development. Given global climate change, it is important to identify effective methods of alleviating drought stress effects. Brassinosteroids (2,4-epibrassinolide-EBR) play an important role in mitigating the negative effects of drought stress on plants. Therefore, this study evaluated the role of EBR in improving drought tolerance. The results demonstrated that EBR application improved drought tolerance by strengthening the enzymatic antioxidant defense system and osmoregulation to scavenge reactive oxygen species. EBR application improved BR and IAA phytohormone content and improved drought tolerance by upregulating genes related to their respective signaling pathways. Therefore, EBR application is an effective strategy for improving drought tolerance in crop plants. ABSTRACT: Drought stress is a major abiotic stress that hinders plant growth and development. Brassinosteroids (BR), including 2,4-epibrassinolide (EBR), play important roles in plant growth, development, and responses to abiotic stresses, including drought stress. This work investigates exogenous EBR application roles in improving drought tolerance in tobacco. Tobacco plants were divided into three groups: WW (well-watered), DS (drought stress), and DSB (drought stress + 0.05 mM EBR). The results revealed that DS decreased the leaf thickness (LT), whereas EBR application upregulated genes related to cell expansion, which were induced by the BR (DWF4, HERK2, and BZR1) and IAA (ARF9, ARF6, PIN1, SAUR19, and ABP1) signaling pathway. This promoted LT by 28%, increasing plant adaptation. Furthermore, EBR application improved SOD (22%), POD (11%), and CAT (5%) enzyme activities and their related genes expression (FeSOD, POD, and CAT) along with a higher accumulation of osmoregulatory substances such as proline (29%) and soluble sugars (14%) under DS and conferred drought tolerance. Finally, EBR application augmented the auxin (IAA) (21%) and brassinolide (131%) contents and upregulated genes related to drought tolerance induced by the BR (BRL3 and BZR2) and IAA (YUCCA6, SAUR32, and IAA26) signaling pathways. These results suggest that it could play an important role in improving mechanisms of drought tolerance in tobacco.