Systematic Characterization of the Monosaccharide Transporter (MST) Gene Family in Citrus and Identification of Candidate Members Associated with Sugar Accumulation

Abstract

The monosaccharide transporter (MST) family mediates soluble sugar transport and distribution, playing critical roles in plant growth, development, and fruit sugar accumulation. Citrus is a globally important fruit crop whose organoleptic quality depends heavily on sugar content. However, a systematic genome-wide analysis of MST genes in citrus is lacking. In this study, we identified 68 MST genes in the Citrus sinensis genome and classified them into seven subfamilies based on phylogenetic analysis. Chromosomal localization revealed uneven distribution across all nine chromosomes. Conserved motif, domain, and gene structure analyses further supported the evolutionary conservation and functional divergence. Syntenic analysis identified segmental duplication as the main driver for family expansion, and interspecific comparisons with Arabidopsis thaliana, Solanum lycopersicum, and Malus domestica provided evolutionary insights. Promoter cis-regulatory element analysis indicated that CsMSTs may respond to light, phytohormones, and stress signals, with several members carrying sugar-responsive elements potentially involved in sugar–acid metabolism. Moreover, by integrating soluble sugar content and transcriptome data across three fruit developmental stages, we performed a correlation analysis and identified 6 CsMST members showing high correlation with sucrose, glucose, and fructose simultaneously based on the Mantel test. qRT-PCR validation and linear correlation analysis confirmed that three of these members were significantly negatively correlated with sugar levels, whereas one was significantly positively correlated. This work provides a comprehensive characterization of the MST family in citrus and highlights candidate genes for future functional dissection of sugar transport and fruit quality improvement.