SWEET Transporters and the Potential Functions of These Sequences in Tea (Camellia sinensis).

Lan Jiang,Jianke Yang,Cheng Song,Xi Zhu

doi:10.3389/fgene.2021.655843

Abstract

Tea (Camellia sinensis) is an important economic beverage crop. Its flowers and leaves could be used as healthcare tea for its medicinal value. SWEET proteins were recently identified in plants as sugar transporters, which participate in diverse physiological processes, including pathogen nutrition, seed filling, nectar secretion, and phloem loading. Although SWEET genes have been characterized and identified in model plants, such as Arabidopsis thaliana and Oryza sativa, there is very little knowledge of these genes in C. sinensis. In this study, 28 CsSWEETs were identified in C. sinensis and further phylogenetically divided into four subfamilies with A. thaliana. These identified CsSWEETs contained seven transmembrane helixes (TMHs) which were generated by an ancestral three-TMH unit with an internal duplication experience. Microsynteny analysis revealed that the large-scale duplication events were the main driving forces for members from CsSWEET family expansion in C. sinensis. The expression profiles of the 28 CsSWEETs revealed that some genes were highly expressed in reproductive tissues. Among them, CsSWEET1a might play crucial roles in the efflux of sucrose, and CsSWEET17b could control fructose content as a hexose transporter in C. sinensis. Remarkably, CsSWEET12 and CsSWEET17c were specifically expressed in flowers, indicating that these two genes might be involved in sugar transport during flower development. The expression patterns of all CsSWEETs were differentially regulated under cold and drought treatments. This work provided a systematic understanding of the members from the CsSWEET gene family, which would be helpful for further functional studies of CsSWEETs in C. sinensis.

Highlights

SWEET is a novel kind of low-affinity sugar transporter, which does not depend on the pH value of the environment and transports in two directions along the concentration gradient (Yuan and Wang, 2013)
To gain insight into the evolutionary relationship of CsSWEETs in C. sinensis, the SWEETs from C. sinensis and A. thaliana were aligned by MAFFT software
The SWEETs from C. sinensis and A. thaliana were clustered into four subfamilies, including subfamily I, subfamily II, subfamily III, and subfamily IV (Figure 1)

Summary

Introduction

SWEET is a novel kind of low-affinity sugar transporter, which does not depend on the pH value of the environment and transports in two directions along the concentration gradient (Yuan and Wang, 2013). The membrane proteins encoded by the SWEET have a certain number of conserved transmembrane domains, which are named MtN3/saliva (Hamada et al, 2005). This domain was first found in nodulin from the roots of Medicago sativa (Gamas et al, 1996). RPG1 (ATSWEET8) plays a vital role in microspore outer wall formation in Arabidopsis (Guan et al, 2008); in Oryza sativa, the SWEET family members are involved in reproductive development (Wang et al, 2010); SAG29 (AtSWEETl5), located on the plasma membrane in Arabidopsis, can regulate cell activity in a hypersaline environment (Seo et al, 2011). The members of the CsSWEET gene family have not yet been identified, and the roles of CsSWEETs remain unclear in tea plant (Camellia sinensis)

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Conclusion