Ameliorative Effects of Paclobutrazol via Physio-Biochemical and Molecular Manifestation in Rice under Water Deficit Stress

We investigated the effects of an optimized dose (100 mg/kg) of paclobutrazol (PBZ) on various physio-biochemical and molecular parameters in three rice varieties (N22, IR64, and IR64 DTY1.1) under water deficit stress (WDS). Concurrently, a dose of 100 mg/kg of PBZ was applied through drenching. Samples were collected at both mild stress [75%‒80% relative water content (RWC)] and severe stress (60%‒65% RWC) conditions. Physio-biochemical parameters, including dry matter, RWC, membrane stability index (MSI), grain yield, total chlorophyll content, chlorophyll stability index, drought susceptibility index, drought tolerance efficiency, total antioxidant activity, abscisic acid (ABA) content, and gibberellic acid (GA) content, were measured. The results showed that PBZ treatment positively influenced the physio-biochemical parameters, significantly increasing dry matter (16.27%‒ 61.91%), RWC (6.48%‒16.34%), MSI (4.37%‒10.35%), and total chlorophyll content (8.97%‒29.09%) in the rice varieties under both mild and severe WDS. Moreover, PBZ treatment reduced drought susceptibility (0.83‒0.95) and enhanced drought tolerance efficiency (60.92%‒86.78%), indicating its potential as a stress-mitigating agent. This study also explored the molecular responses of the rice varieties to PBZ treatment under WDS. Global methylation analysis revealed changes in DNA methylation patterns, indicating the regulatory influence of PBZ on gene expression. The expression analysis of genes involved in the diversification of geranylgeranyl pyrophosphate towards the biosynthesis of ABA, GA, and chlorophyll showed alterations in their expression levels, suggesting the involvement of PBZ in the isoprenoid pathway. Overall, this study provides valuable insights into the potential mechanisms by which PBZ modulates physiological and molecular responses in rice plants under WDS. The findings highlight the importance of PBZ as a promising tool for enhancing drought tolerance in rice and offer valuable information for future research in crop stress management.