Apple (Malus x domestica Borkh.), a member of the family Rosaceae, is an economically important fruit crop worldwide. Sugar content is an essential component of fruit organoleptic quality, especially fruit sweetness, and thus is given a high priority in apple breeding programs. However, the details on the complex molecular mechanism underlying fruit sugar accumulation remain scarce. 

The molecular breeding of fruit crops group led by Prof. HAN Yuepeng from the Key Laboratory of Plant Germplasm Enhancement and Specialty Agriculture at the Wuhan Botanical Garden reported the role of sucrose transporter (SUT) family in regulating fruit sugar accumulation in apple. 

Gene-tagged markers were developed to conduct candidate gene-based association study, and an SUT4 member MdSUT4.1 was found to be significantly associated with fruit sugar accumulation. MdSUT4.1 encoded a tonoplast localized protein and its expression level was negatively correlated with fruit sugar content.  

Overexpression of MdSUT4.1 in strawberry and apple callus had an overall negative impact on sugar accumulation, suggesting that it functioned to remobilize sugar out of the vacuole.

In addition, MdSUT4.1 is located on chromosomal region harboring a previously reported quantitative trait loci (QTL) for sugar content, suggesting that it is a candidate gene for fruit sugar accumulation in apple.    

Results demonstrate the tonoplast-localized sucrose transporter, MdSUT4.1, participates in regulating fruit sugar accumulation in apple.

This study is useful for better understanding of the impact of SUT4 proteins on fruit sugar accumulation. Moreover, the molecular marker of MdSUT4.1 may serve as a tool for genetic improvement of fruit sweetness in breeding programs of apple.   

This work was supported by grants from National Key R&D Program of China, National Natural Science Foundation of China, and the Special Fund for Strategic Pilot Technology of Chinese Academy of Sciences.

Results were published on BMC Plant Biology entitled "The sucrose transporter MdSUT4.1 participates in the regulation of fruit sugar accumulation in apple".   

Overexpression of MdSUT4.1 in strawberry fruit (Image by PENG Qian)