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

Sugars Will Eventually be Exported Transporters (SWEETs) play important roles during plant growth and development. Bioinformatics revealed 27 <i>SWEET</i> genes in the apple (<i>Malus × domestica</i> 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 <i>MdSWEET</i> family. PlantCARE-based prediction of cis-acting elements of the <i>MdSWEET</i> promoter suggested that most <i>MdSWEETs</i> may be expressed specifically in the phloem and respond to hormones and stresses. qRT-PCR results showed that expression patterns of <i>MdSWEET</i>s displayed pronounced differences in different tissues/organs and different stages of apple fruit development. <i>MdSWEET5</i>, <i>MdSWEET7</i>, and <i>MdSWEET21</i> were strongly expressed in mature leaves, <i>MdSWEET2</i>, <i>MdSWEET12</i>, <i>MdSWEET13</i>, <i>MdSWEET14</i>, <i>MdSWEET17</i>, and <i>MdSWEET</i><i>22</i> were highly expressed in peduncles, <i>MdSWEET4</i> was highly expressed in young leaves, <i>MdSWEET3</i>, <i>MdSWEET</i><i>11</i>, <i>MdSWEET</i><i>15</i>, <i>MdSWEET</i><i>16</i>, <i>MdSWEET19</i>, <i>MdSWEET</i><i>24</i>, and <i>MdSWEET2</i><i>5</i> were highly expressed in different parts of flowers, and <i>MdSWEET1</i>, <i>MdSWEET6</i>, <i>MdSWEET8</i>, <i>MdSWEET9</i>, <i>MdSWEET10</i>, <i>MdSWEET18</i>, <i>MdSWEET20</i>, <i>MdSWEET23</i>, and <i>MdSWEET26</i> were strongly expressed in fruits. <i>MdSWEET8</i> showed higher expression in the early stage of fruit development, <i>MdSWEET1</i>, <i>MdSWEET</i><i>10</i>, and <i>MdSWEET</i>27 were highly expressed in the middle stage of fruit development, and <i>MdSWEET</i><i>6</i>, <i>MdSWEET</i><i>9</i>, <i>MdSWEET</i><i>18</i>, <i>MdSWEET</i><i>20</i>, <i>MdSWEET</i><i>23</i>, and <i>MdSWEET2</i><i>6</i> were sharply upregulated in the late developmental period. Our study could facilitate <i>SWEET</i> 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.