Genome-wide identification of sucrose transporter genes in Camellia oleifera and characterization of CoSUT4

Sucrose transporters (SUTs) play a crucial role in carbon allocation from the source leaf to the sink end, and the function of SUTs varies among family members. However, the genome-wide identification of the SUT superfamily in Camellia oleifera is lacking, and their biological function remains elusive. In this study, four SUT genes - designated CoSUT1-4 - were identified in C. oleifera through a genome-wide analysis and classified into three subfamilies. We used a combination of cis-acting elements analysis, mRNA quantification, histochemical analysis, and heterologous transformation to evaluate the expression profiles and functions of these SUTs. A key finding is that CoSUT4, localized on the plasma membrane, is highly expressed in mature leaves and the early stage of seed development in C. oleifera. In vitro culture of C. oleifera seed revealed the responsiveness of CoSUT4 to various exogenous hormones such as ABA and GA. CoSUT4 was able to restore the growth of the yeast strain SUSY7/ura3 (a sucrose transport-deficient mutant) on sucrose-containing media and specifically contributed to sucrose translocation and tissue growth in CoSUT4-overexpressed apple calli. In situ hybridization identified chalazal nucellus and transfer cells as the action sites of CoSUT4 at the maternal-filial interface mediating sucrose transportation in oil tea seeds. CoSUT4 overexpression in Arabidopsis thaliana atsuc4 mutant restored the growth and seed yield deficiencies of the mutant, leading to an increase in filled seeds and oil content. Additionally, CoSUT4 overexpression enhanced the drought and salt stress tolerance by augmenting sugar content. Overall, these findings provide valuable insights into the function of SUTs and present promising candidates for the genetic enhancement of seed production in C. oleifera.