Genome-Wide Identification and Expression Profile Analysis of Citrus Sucrose Synthase Genes: Investigation of Possible Roles in the Regulation of Sugar Accumulation

Abstract

<div><p>Sucrose synthase (Sus) (EC 2.4.1.13) is a key enzyme for the sugar accumulation that is critical to form fruit quality. In this study, extensive data-mining and PCR amplification confirmed that there are at least six Sus genes (<i>CitSus</i>1-6) in the citrus genome. Gene structure and phylogeny analysis showed an evolutionary consistency with other plant species. The six Sus genes contain 12–15 exons and 11–14 introns and were evenly distributed into the three plant Sus groups (<i>CitSus</i>1 and <i>CitSus</i>2 in the Sus I group, <i>CitSus</i>3 and <i>CitSus</i>6 in the Sus II group, and <i>CitSus</i>4 and <i>CitSus</i>5 in the Sus III group). Transcripts of these six CitSus genes were subsequently examined. For tissues and organs, <i>CitSus</i>1 and 2 were predominantly expressed in fruit juice sacs (JS) whereas <i>CitSus</i>3 and 4 were predominantly expressed in early leaves (immature leaves), and <i>CitSus</i>5 and 6 were predominantly expressed in fruit JS and in mature leaves. During fruit development, <i>CitSus</i>5 transcript increased significantly and <i>CitSus</i>6 transcript decreased significantly in fruit JS. In the fruit segment membrane (SM), the transcript levels of <i>CitSus2</i> and 5 were markedly higher and the abundant levels of <i>CitSus</i>3 and 6 gradually decreased. Moreover, transcript levels of <i>CitSus</i>1-4 examined were higher and the <i>CitSus</i>5 transcript level was lower in the fruit SM than in fruit JS, while <i>CitSus</i>6 had a similar transcript level in fruit JS and SM. In addition, transcripts of <i>CitSus</i>1-6 responded differently to dehydration in mature leaves or to mild drought stress in fruit JS and SM. Finally, the possible roles of Sus genes in the regulation of sugar accumulation are discussed; however, further study is required.</p></div