Genome-Wide Identification and Expression Profiling Reveal the Potential Functions of the SWEET Gene Family during the Sink Organ Development Period in Apple (Malus × domestica Borkh.)

Sugars Will Eventually be Exported Transporters (SWEETs) play important roles during plant growth and development. Bioinformatics revealed 27 SWEET genes in the apple (Malus × domestica Borkh.) genome and classified them into four clades. These genes are unevenly located on 12 chromosomes, and 8 tandem-repeat genes and 18 fragment-repeat genes are present in the MdSWEET family. PlantCARE-based prediction of cis-acting elements of the MdSWEET promoter suggested that most MdSWEETs may be expressed specifically in the phloem and respond to hormones and stresses. qRT-PCR results showed that expression patterns of MdSWEETs displayed pronounced differences in different tissues/organs and different stages of apple fruit development. MdSWEET5, MdSWEET7, and MdSWEET21 were strongly expressed in mature leaves, MdSWEET2, MdSWEET12, MdSWEET13, MdSWEET14, MdSWEET17, and MdSWEET22 were highly expressed in peduncles, MdSWEET4 was highly expressed in young leaves, MdSWEET3, MdSWEET11, MdSWEET15, MdSWEET16, MdSWEET19, MdSWEET24, and MdSWEET25 were highly expressed in different parts of flowers, and MdSWEET1, MdSWEET6, MdSWEET8, MdSWEET9, MdSWEET10, MdSWEET18, MdSWEET20, MdSWEET23, and MdSWEET26 were strongly expressed in fruits. MdSWEET8 showed higher expression in the early stage of fruit development, MdSWEET1, MdSWEET10, and MdSWEET27 were highly expressed in the middle stage of fruit development, and MdSWEET6, MdSWEET9, MdSWEET18, MdSWEET20, MdSWEET23, and MdSWEET26 were sharply upregulated in the late developmental period. Our study could facilitate SWEET functional analysis in different tissue/organs and in sugar accumulation throughout the development and ripening of apple fruits. These findings provide potential opportunities to increase sugar accumulation in fruit, thereby improving fruit quality and yield.