Genome-wide analysis of SWEET and TST sugar transporters in sugarbeet: Structural characterization and expression during development and postharvest storage

SWEET (Sugars Will Eventually be Exported Transporter) and TST (Tonoplast Sugar Transporter) genes are implicated in sucrose accumulation and sucrose loss during sugarbeet root production and postharvest storage, although information regarding their identities, structures and expression is limited. Research, therefore, was conducted to identify and structurally characterize sugarbeet SWEET and TST gene families and determine their organ-specific, developmental, and storage-related expression. Sixteen SWEET genes and four TST genes were found in the sugarbeet genome. SWEET genes were notably diverse in genomic and transcriptomic structure, yet shared similarities in protein motifs and structure, whilst TST genes were generally uniform in genomic, transcriptomic, and protein size and structure. Expression analysis revealed high expression of two SWEET genes (SWEET 1, SWEET12) in leaves, relatively low levels of SWEET gene expression in roots throughout all but the earliest stages of development, and a dramatic upregulation of four SWEET genes (SWEET1, SWEET12, SWEET14/N3, SWEET17b) during storage, including a greater than 17,000-fold increase in SWEET14/N3 expression. TST genes were expressed in all tissue types, with most TST expression derived from a single gene (TST2.1) which was highly expressed in roots, minimally expressed in leaves, and elevated in expression during root storage. Overall, these results point to the likely importance of SWEET1 and SWEET12 in leaves and TST2.1 in roots during development when sucrose is actively being accumulated and the likely importance of SWEET14/N3, and possibly SWEET1, SWEET12, SWEET17b, and TST2.1 during storage when vacuolar sequestered sucrose is remobilized and catabolized.