摘要
甜度作为水果风味的重要组成部分,在果实品质形成中扮演重要角色。糖含量和比例是影响果实风味与甜度的重要因素,在果实发育进程中受到糖转运和代谢的高度调控。因此,了解糖代谢与含量之间的关系,是提高果实品质的关键。植物中的碳水化合物不仅为体内代谢提供能量,而且可以作为响应环境的信号分子起关键作用。重点综述了糖代谢关键酶和相关基因在果实糖含量调控中的作用及糖信号响应不同环境变化的生理和生化过程,旨在为改良果实品质提供理论参考。
Soluble sugars, including sucrose, fructose and glucose, are not only essential to fleshy fruit growth and development but also central to fruit quality. Fruit taste and flavor are positively related to the composition and concentration of sugars. As the composition and concentration of sugars during fruit maturation are determined by metabolic and transport processes during fruit development, understanding these processes and their regulation is significant for fruit quality improvement. This article focuses on the key role of sugar metabolism-related enzymes and key genes in the regulation of sugar content and the physiological and biochemical processes of sugar signals in response to different environmental changes. It aims to provide a theoretical reference for improving fruit quality and increasing yield. At the center of sugar metabolism in sink cells is the sucrose cycle, previously called futile recycle, which includes the breakdown of sucrose by invertase and sucrose synthase(SUSY, EC 2.4.1.13),the phosphorylation of the resulting hexoses and the interconversion between hexose phosphates and UDP-glucose(UDPG), and the re-synthesis of sucrose via sucrose-6-phosphate synthase(SPS, EC2.4.1.14) and sucrose-6-phosphate phosphatase(EC 3.1.3.24). This metabolic system is associated with many other metabolic pathways, such as glycolysis and tricarboxylic acid(TCA) cycle, starch synthesis and cellulose synthesis, and its coordination with the sugar transport system on the tonoplast is expected to determine the distribution of sugars between metabolism in the cytosol and accumulation in the vacuole. In apple and other Rosaceae species, sorbitol is an important carbohydrate transported from source tissues through the phloem to the sink, accounting for 60-80% of the total carbon in leaves. In source leaves, sorbitol is synthesized from glucose-6-phosphate(G6 P) in a two-step process: G6 P is first converted to sorbitol-6-phosphate(S6 P) via aldose-6-phosphate reductase(A6 PR;EC 1.1.1.200), then followed by dephosphorylation of S6 P to sorbitol via S6 P phosphatase. The loading of both sorbitol and sucrose into the companion cell-sieve element(SE-CC) complex through a symplastic pathway. After being unloaded from SE-CC complexes into the cell wall space in fruit, sorbitol is taken up into the cytosol of parenchyma cells by sorbitol transporter(SOT), and then converted to fructose by sorbitol dehydrogenase(SDH, EC1.1.1.14). Sucrose is directly transported into parenchyma cells by sucrose transporters(SUT), or converted to glucose and fructose by cell wall invertase(CWINV) first, and then transported into parenchyma cells by hexose transporters. Once taken up into parenchyma cells of fruit,both sorbitol and sucrose feed into the sucrose cycle to meet the carbon requirement for fruit growth and development while excess carbon is converted to starch for storage in plastids or transported into vacuole by sugar transporters for accumulation. Transporter-mediated sugar accumulation in the vacuole is related to the regulation of sugars available in the cytosol. However, the availability and composition of sugars are highly regulated by sugar metabolism. Photosynthetic products in leaves are transported through protein transmembrane and a series of enzyme reactions and finally disperse in different parts of the fruit in the form of sucrose, fructose and glucose, which endows the fruit with a unique flavor quality. Sorbitol metabolism, sucrose metabolism, and hexose metabolism pathway are three major sugar metabolism pathways, in which different types of metabolic enzymes are involved. Currently, major metabolic enzymes include suc-phosphate synthase(SPS), sucrose synthase(SUSY), neutral invertase(NINV) and fructokinase(FRK). In terms of sugar-related metabolic enzyme genes, the expression differences in related metabolic genes in different fruit growth and development periods will lead to differences in the amount of sugar accumulation. Many sugar-related genes have been cloned through biotechnology from different species, such as SDH1 and FRK2. In fleshy fruits, the concentration and distribution of sugars in parenchyma cells are affected via this cycle by environmental factors, such as water, light and temperature, respectively. The dynamic changes of sugar components are strictly regulated by key metabolic enzymes and genes, including SUSY, FRK, and some transporters. Thus, more research and techniques are needed to understand how to control metabolic enzymes to provide sufficient nutrients for plant growth and development in appropriate time, space and quantity, such as dynamic radioactive tracer imaging technology for phloem transport, dynamic interpretation of sugar transport, metabolism, and accumulation under different environmental conditions. Plants are equally faced with challenges brought by fungal diseases, such as powdery mildew and brown spot, especially in fruit trees. After plants are infected with the pathogen, increasing invertase activity could change the proportion of extracellular hexose/sucrose and cause hexose-mediated defense response. In addition, genome engineering methods can change transcription regulation or protein modification affecting pathogen-targeted invertase activity(CWINV) or sugar transporters(SWEET4), and produce resistance to pathogens without affecting fruit quality, thus providing essential strategies for plant defense against diseases. Therefore, a comprehensive study on the relationship between environmental factors and fruit sugar content can provide a theoretical basis for the ecological regulation of fruit sugar metabolism and achieve the goal of improving fruit quality through effective environmental regulation measures. Further study on the regulation mechanism of sugar metabolism-related enzymes and key gene expression on fruit sugar content has theoretical and practical significance for fundamentally improving fruit quality.
作者
苏静
祝令成
刘茜
彭云静
马百全
马锋旺
李明军
SU Jing;ZHU Lingcheng;LIU Xi;PENG Yunjing;MA Baiquan;MA Fengwang;LI Mingjun(College of Horticulture,Northwest A&F University/State Key Laboratory of Crop Stress Biology in Arid Areas/Shaanxi Key Laboratory of Apple,Yangling 712100,Shaanxi,China)
出处
《果树学报》
CAS
CSCD
北大核心
2022年第2期266-279,共14页
Journal of Fruit Science
基金
国家自然科学基金(31872043)。
关键词
果实
糖代谢
糖含量
环境
调控
Fruit
Sugar metabolism
Sugar concentration
Environment
Regulation
作者简介
苏静,男,在读博士研究生,研究方向为苹果糖酸品质的形成与调控。Tel:18119446137,E-mail:sujing@nwafu.edu.cn;通信作者:李明军,Tel:029-87082613,E-mail:limingjun@nwsuaf.edu.cn。
