Transcriptome-based analysis of the role of endogenous hormones in regulating flower development in lemon (Citrus limon (L.) Burm.)

Gibberellin is the major flower formation inhibitor in most fruit trees, which is significantly different from the role of gibberellin in model plants. Inhibition of endogenous gibberellin formation in fruit trees is the most important mechanism of action of paclobutrazol (PBZ), which is then used to achieve the regulation of plant bud differentiation. In this study, we examined the impact of gibberellic acid and paclobutrazol on the differentiation of lemon buds. Our findings indicated that the elevated abscisic acid to zeatin riboside (ABA/ZR) ratio associated with paclobutrazol treatment markedly enhanced the number of flowering shoots. Conversely, the application of gibberellic acid significantly elevated the endogenous gibberellic acid content and demonstrated inhibitory effects on bud differentiation. Notably, genes associated with flowering within the circadian rhythm pathway exhibited reduced expression levels following gibberellic acid treatment. Through integrated metabolomic and transcriptomic analyses, it was demonstrated that exogenous application of gibberellic acid led to differential gene expression and a significant increase in gibberellin content. Moreover, these genes were associated with a reduction in the levels of gibberellin A4, gibberellin A7, and isopentenyl adenosine. Our findings provide molecular insights into how gibberellic acid and paclobutrazol modulate lemon bud differentiation through hormone signal transduction and circadian rhythm pathways, involving differential expression of flowering-related genes and metabolite shifts (GA 4 , GA 7 , isopentenyl adenosine, GA 3 , and trans-zeatin riboside), thereby offering a regulatory framework for flowering control in lemon.