Abstract
In plants, sugar transporters play an important role in the allocation of sugars from cells in source organs to cells in sink organs. Hence, an understanding of the molecular basis and regulation of assimilate partitioning by sugar transporters is essential. Leaves are the main source of photosynthetic products. In jujube (Ziziphus jujuba Mill.), the mechanisms regulating initial sugar unloading in leaves are still unclear. In this study, an expression profiling analysis showed that ZjSWEET2.2, encoding a sugar transporter in the SWEET family, is highly expressed in leaves. Over-expression of ZjSWEET2.2 increased carbon fixation in photosynthetic organs. Our analyses showed that ZjSWEET2.2 encodes a plasma membrane-localized sugar transporter protein. Its expression levels were found to be suppressed under drought stress and by high concentrations of exogenous sugars, but increased by low concentrations of exogenous sugars. Finally, DNA sequence analyses revealed several cis-elements related to sugar signaling in the promoter of ZjSWEET2.2. Together, these results suggest that ZjSWEET2.2 functions to mediate photosynthesis by exporting sugars from photosynthetic cells in the leaves, and its gene expression is regulated by sugar signals.
Highlights
In higher plants, photosynthates produced at the photosynthetic “source” are essential for growth and development, and are major nutritional components of fruits (Zhu et al, 2010)
Our analyses showed that ZjSWEET2.2 is in the clade I SWEET gene family, and is phylogenetically closest to AtSWEET2
We found that ZjSWEET2.2 could be regulated by exogenous glucose, but its expression was relatively insensitive to exogenous sucrose
Summary
Photosynthates produced at the photosynthetic “source” (mainly mesophyll cells) are essential for growth and development, and are major nutritional components of fruits (Zhu et al, 2010). Adequate photosynthate production in the leaves can substantially increase the number of flowers and fruit weight. Improvements in allocation efficiency can increase the proportion of total biomass allocated to harvestable organs (Zhu et al, 2010; Paterson and Li, 2011). Sugar transporters play crucial roles in mediating carbohydrate fluxes (Patrick et al, 2001) and are linked with biomass gain. Sugar transporters found in plants mainly include three types: Abbreviations: SWEET2a, sugars will eventually be exported transporter 2a; RBCS, ribulose bisphosphate carboxylase/ oxygenase; PGK, phosphoglycerate kinase; RPI, ribose-5-phosphate isomerase; PRK, phosphoribulokinase
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