Citrus sucrose transporter genes: genome-wide identification and transcript analysis in ripening and ABA-injected fruits

The sucrose transporter (SUT) plays a major role in the transport of sucrose apoplastically. An extensive mining of the data confirmed that at least three SUT genes (CitSUT1, CitSUT2, and CitSUT3) were found in the databases of the three currently available citrus genomes. The exon and intron sizes varied among the three CitSUT genes, and the similarities among the genes were 40–50 %. The phylogenetic analysis revealed that the CitSUT1, CitSUT2, and CitSUT3 were distributed into groups SUT I, SUT II, and SUT III, respectively. The analysis of the spatiotemporal expression of the genes showed that the CitSUT1, CitSUT2, and CitSUT3 were predominantly expressed in the stamens, young leaves, and fruits [juice sacs (JS) and/or segment membrane (SM)], respectively. Moreover, in addition to the significant increase in sucrose levels from 114 days after anthesis (DAA) to 160 DAA, the transcript levels of CitSUT1 and CitSUT3 increased significantly in the JS and decreased significantly in the SM, whereas the CitSUT2 transcript levels decreased significantly either in the JS or in the SM. Additionally, treatment with ABA increased the accumulations of sucrose and fructose in fruits, which were accompanied by the induction of the CitSUT2 transcripts in fruits (JS and SM) and the CitSUT3 transcripts in the SM and the reduction of the CitSUT1 and CitSUT3 transcript levels in the JS. Thus, the CitSUT1 and CitSUT3 might play important roles in the transport of sucrose into the fruit JS during normal fruit development; the transcript alterations of the CitSUT2 and CitSUT3 under the ABA treatment might contribute to the increased accumulation of sucrose.