Genome-wide identification of SWEET gene family and the sugar transport function of three candidate genes during female flower bud induction stage of Juglans sigillata Dode.

Sugar Will Eventually be Exported Transporter (SWEET) transports sugar to sink organs and regulates intercellular sugar transport to provide energy for plant growth and development. In this study, twenty-two SWEET genes were identified and distributed on eleven chromosomes. Phylogenetic tree analysis showed that these genes could be divided into four subfamilies. Metabolism and transcriptome analysis showed that sucrose and fructose were accumulated in female flower buds at physiological differentiation stage (PDS). The third branch of JsSWEET1 and the fourth branch of JsSWEET9 and JsSWEET17 were highly expressed in female flower buds at undifferentiated stage (UDS) and PDS, which promoted sugar accumulation in female flower bud differentiation, so these three candidate SWEET genes were considered to be involved in the accumulation of sugar in the flower bud differentiation of Juglans sigillata. The subcellular localization showed that all three candidate genes were located on the cell membrane, and JsSWEET17 was also expressed and functioned in the vacuolar membrane. Through overexpression in callus and silencing in female flower buds at UDS and PDS, it was found that JsSWEET1 positively regulated the accumulation of sucrose in female flower buds, and JsSWEET9 and JsSWEET17 are involved in the transport and accumulation of fructose during flower bud differentiation. These results could provide a comprehensive understanding of the JsSWEETs gene family and provide theoretical guidance for further study of the function of SWEET-induced sugar accumulation in plant flower development.