Dose-dependent gibberellic acid and benzylaminopurine priming coordinates transcriptional and metabolic reprogramming, enhancing photochemical efficiency to confer drought tolerance in pigeonpea
Drought stress adversely affects photosynthesis, redox homeostasis and metabolic signalling in plants. Exogenous application of hormonal and nonhormonal elicitors, have emerged as promising strategies to improve plant resilience to changing climatic conditions. In the current study, the effect of foliar application of gibberellic acid (GA 3 ) and 6–benzylaminopurine (6–BAP) individually and in combination was assessed at different doses (0.2 and 0.4 mM) in pigeonpea ( Cajanus cajan ) under drought stress through integrated physiological, metabolomic and transcriptomic approaches. Growth, photosynthetic performance and metabolic homeostasis were severely affected by drought stress. The combined application of 0.4 mM GA3+6-BAP resulted in 20.6%, 29.7% and 33.2% increase in the maximum quantum efficiency of PSII (Fv/Fm), ΔpH, and ΔΨ, respectively, compared to untreated drought-stressed plants, which shows that the stability of PSII and the energy regulation of the thylakoids improved with combined phytohormone application. Untargeted metabolite profiling revealed that drought reduced the abundance of metabolites belonging to carbohydrates, lipids and organoheterocyclic compounds respectively. While GA3+6-BAP led to the restoration of metabolites involved in carbon metabolism and antioxidative metabolism wherein highest accumulation of flavonoids, phenolics and terpenoids was noticed. Hormonal application induced 24 unique metabolic signatures including quercetin 3-(2-glucosylrhamnoside), camellianin A, 3-O-p-coumaroylquinic acid, byakangelicin. Besides, transcriptomic profiling demonstrated treatment-associated changes in the abundance of transcripts involved in photosynthesis, redox regulation and calcium-based signaling, suggesting coordinated stress signaling. Overall, 0.4 mM GA3+ 6-BAP treatment proved to be the most effective among other treatments for the induction of drought stress tolerance in pigeonpea by dynamic transcriptional and metabolic regulatory mechanisms.