Integrated physiological and transcriptomic analysis reveals the effects of abscisic acid and ethylene on cell wall metabolism during fruit ripening of Citrus sinensis ‘Newhall’

Phytohormones regulate fruit quality formation, including peel coloration, texture softening, and sugar–acid metabolism. However, the regulatory roles of abscisic acid (ABA) and ethylene (ETH) during citrus fruit physiological maturation, especially in modulating texture properties, remain fragmented. Integrated physiological, biochemical, and transcriptomic analyses were conducted to examine the effects of exogenous ABA and ethephon (ethylene-releasing agent) on cell wall metabolism during ‘Newhall’ navel orange ripening. Both treatments accelerated fruit ripening, as evidenced by enhanced peel coloration, sugar–acid metabolism, and softening. They promoted the degradation of cellulose, hemicellulose, pectin and lignin, accompanied by increased activities of cell wall degrading enzymes. Transcriptomic analysis indicated that numerous cell wall metabolism related genes, especially CesA / Csl superfamily members, were downregulated upon hormone treatment, with abundant ABA responsive elements in their promoters. Weighted gene co-expression network analysis (WGCNA) identified key hormone responsive modules and candidate transcription factors potentially mediating ABA/ETH signaling to cell wall metabolic genes. Collectively, ABA and ETH promote citrus fruit softening inhibiting cell wall biosynthesis and promoting cell wall degradation. The stronger efficacy of ETH may reflect both direct activation of catabolic pathways and indirect amplification via endogenous ABA accumulation. These findings reveal citrus-specific features of hormone-mediated cell wall remodeling, including tissue-differential ACC induction and temporally attenuated ABA efficacy at late ripening stages, and provide a transcriptional framework and gene resources for understanding hormone-regulated fruit softening in non-climacteric fruits.