Molecular Basis of Sugar Accumulation in Apple Fruits

doi:10.13560/j.cnki.biotech.bull.1985.2026-0069

Abstract

Abstract:

Apple is an important fruit widely cultivated worldwide. The soluble sugar content of the fruit is a core index determining its flavor quality and market competitiveness, and its accumulating process involves a complex genetic and molecular regulatory network. This paper systematically reviews the physiological basis, molecular mechanisms, and regulatory factors of sugar accumulation in apple fruits. At the physiological level, sugar accumulation relies on the coordinated action of multiple processes, including the synthesis, transport, unloading, metabolism, and vacuolar storage of photosynthates (mainly sorbitol and sucrose), which are mediated by key carriers such as sucrose transporters (SUTs), tonoplast sugar transporters (TSTs), and SWEET transporters. In terms of molecular mechanisms, sorbitol metabolism-related genes (MdS6PDH and MdSDH), sucrose metabolism-related genes (MdSuSy and MdSPS), and hexose metabolism-related genes (MdHXK and MdFRK) regulate the synthesis and conversion of different sugar components respectively; transcription factors such as MYB, bZIP, ERF, NAC, and WRKY accurately regulate the expression of sugar metabolism and transport-related genes by forming a complex regulatory network. In addition, plant hormones including auxin, abscisic acid, and ethylene participate in the regulation of sugar accumulation through cross-talk with sugar signals; environmental factors such as light, temperature, water, and nutrients affect the expression of related genes and enzyme activities through specific molecular pathways, thereby shaping the sugar characteristics of fruits. At present, issues such as the interaction of core regulatory factors, the mechanism of sugar component conversion, and the interaction between environmental factors and endogenous genes in apple sugar accumulation still need further exploration. This review provides a comprehensive reference for elucidating the genetic and molecular mechanisms of sugar accumulation in apple fruits, and is of great significance for promoting the molecular breeding process of apple quality improvement.

Key words: apple, fruit quality, sugar accumulation, molecular regulation, transporter protein, transcription factor

LUO Long-xin, LI Zhi, LI Tong, FENG Zi-quan, ZHAI Xin-yue, LIANG Cheng-lin, ZHANG Ya-li, WU Shang, LI Yuan-yuan, JIANG Han. Molecular Basis of Sugar Accumulation in Apple Fruits[J]. Biotechnology Bulletin, 2026, 42(3): 156-171.

Figures/Tables 3

Fig. 1 Sugar accumulation process in apple fruitSugars are synthesized as photosynthate in leaves, transported over long distances via the phloem and loaded into the fruit phloem, then undergo intracellular transport, conversion and formation within the flesh cells, and are finally stored in the vacuoles

Fig. 2 Molecular mechanism of sugar formation in apple fruitCore transcription factor families (MYB, bZIP, ERF, NAC, and WRKY) orchestrate the expression of sugar transport-related protein families (SUT, TST, and SWEET). These transporters mediate key processes including phloem transport, vacuolar sugar sequestration, and intercellular/intracellular sugar partitioning, ultimately governing the accumulation of fructose, glucose, sucrose, and sorbitol in the fruit

Fig. 3 Molecular network of environmental factors and plant hormones synergistically regulating apple fruit development via the sugar signaling pathwayEnvironmental factors (light, low temperature, nutrients, moderate drought, etc.) regulate the sugar signaling pathway by activating specific signaling molecules (e.g., MdCBF1/2, MdCIPK22, MdDREB2A). Plant hormones (CTK/GA, auxin, ethylene, ABA, etc.) form cross-response and signal synergy with the sugar signaling pathway by regulating photosynthetic area, sink strength, sugar metabolism, and drought response, thereby collectively influencing fruit development

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