Sucrose synthase gene family analysis in loquat reveals EjSUS4 as a key regulator of sugar accumulation

Introduction Soluble sugar composition critically determines the commercial quality of loquat ( Eriobotrya japonica ), a prominent Rosaceae fruit. Sucrose synthase (SUS) plays a central role in carbohydrate partitioning and fruit sink strength, yet the SUS gene family in loquat remains uncharacterized. Methods Seven EjSUS genes were identified from the loquat genome and analyzed via phylogenetics, chromosomal mapping, synteny, and structural characterization. Spatiotemporal expression profiling was performed during fruit development in two cultivars (‘YingShuang’ and ‘ZheHong 16’). Soluble sugar contents were measured, and Pearson correlation analysis was conducted. Recombinant EjSUS4 protein was subjected to in vitro enzymatic assays. Promoter cis-element analysis was also performed. Results The seven EjSUS genes were classified into three subfamilies. Segmental and tandem duplications drove family expansion. EjSUS4 (Group I) was specifically and highly upregulated during the terminal ripening stage, coinciding with peak sucrose accumulation, whereas other EjSUS members showed very low or undetectable expression. Loquat was confirmed as a hexose-accumulating fruit. EjSUS4 transcript levels showed a highly significant positive correlation with sucrose content. Recombinant EjSUS4 exhibited robust sucrose synthesis activity, and its promoter contained ABRE cis-elements potentially linking it to ABA signaling. Discussion Our findings demonstrate that EjSUS4 is the predominant SUS member driving late-stage sugar accumulation in loquat. This work provides valuable genetic resources and a theoretical basis for molecular breeding to improve loquat fruit quality.