Involvement of ascorbate oxidase in the regulation of the leaf development mediated by mepiquat chloride in cotton seedlings

Mepiquat chloride (MC), a plant growth regulator, is extensively used in agricultural practices. However, the mechanism of action of MC is complex and remains largely unclear. In this study, the effects of MC on the leaf development of cotton seedlings and the underlying regulatory mechanism were investigated by integrating morphological, physiological, transcriptomic analysis and virus-induced gene silencing technology. The results showed that MC treatment caused a reduction in leaf area and an increase in chlorophyll content and photosynthetic efficiency. The ascorbic acid content initially increased and then declined after MC treatment. Transcriptomic data demonstrated that the expression of the ascorbate oxidase (AO) gene was initially upregulated and subsequently downregulated by MC. The top 20 KEGG pathways identified from the differentially expressed genes were mainly enriched in plant–pathogen interaction, the MAPK signaling pathway, ascorbate and aldarate metabolism, plant hormone signal transduction, amino acid metabolism, and secondary metabolism. Silencing GhAO1 enhanced the ascorbic acid concentration, which was accompanied by increased plant height, internode length, and leaf area in cotton seedlings. Silencing GhAO1 reduced the sensitivity of cotton seedlings to MC. This study reveals that MC may primarily serve as an abiotic stressor that regulates the stress response during the early stage of MC treatment and may further enhance chlorophyll content by promoting amino acid and secondary metabolism. AO plays an important role in MC-mediated growth regulation in cotton seedlings.