Ethylene Biosynthesis Dynamics and Genome-Wide Characterization of the ACS and ACO Gene Families During Fruit Development in ‘Kuerle Xiangli’ (Pyrus sinkiangensis Yu)

Background: Fruit ripening in ‘Kuerle Xiangli’ (Pyrus sinkiangensis Yu) is a complex physiological process orchestrated by ethylene; however, the molecular mechanisms governing ethylene biosynthesis during fruit development to ripening remain incompletely understood. Methods: In this study, we systematically characterized ethylene production, fruit quality parameters, and the ACS (1-aminocyclopropane-1-carboxylic acid synthase) and ACO (ACC oxidase) gene families in ‘Kuerle Xiangli’ across nine developmental stages from 30 to 150 days after full bloom (DAFB). Results: Ethylene production remained suppressed during fruit growth and expansion but exhibited a massive climacteric burst at the ripening stage (150 DAFB). Soluble solids content peaked at 120 DAFB (16.2%), whereas fruit firmness declined significantly from 135 to 150 DAFB, indicating a temporal asynchrony between sugar accumulation and cell wall disassembly. A total of 12 PsinACS and 5 PsinACO genes were identified from the genome (Pyrus sinkiangensis Yu). Phylogenetic analysis classified the PsinACS proteins into four groups (I–IV) and the PsinACO proteins into two groups (B and C), with Group I ACS members clustering with well-characterized ripening-associated orthologs. Conserved motif and gene structure analyses revealed high conservation within each phylogenetic group. Gene duplication analysis indicated that most PsinACS and PsinACO genes originated from whole-genome duplication or segmental duplication events, with only a few arising from proximal or tandem duplications. Synteny analysis demonstrated strong evolutionary conservation across Rosaceae species, and Ka/Ks ratios (<1) indicated that both gene families have undergone strong purifying selection. Expression profiling by qRT-PCR revealed distinct patterns: PsinACS2, PsinACS3 and PsinACS4 were highly expressed in early fruitlets (30 DAFB), and eight ACS genes (PsinACS1, PsinACS5–9, PsinACS11–12) and PsinACO1 showed maximal expression at 150 DAFB, coinciding with the ethylene climacteric burst. Conclusions: Taken together, combined with phylogenetic tree analysis, indicates that PsinACS1 and PsinACO1 are crucial genes in ethylene biosynthesis that positively regulate fruit ripening. These results provide a comprehensive characterization of the ACS and ACO gene families in P. sinkiangensis and reveal a coordinated transcriptional network that governs ethylene biosynthesis during pear fruit ripening.