Advances in the metabolism and regulation of astringent substances in fruits

Abstract: In recent years, with the rapid development of China’s fruit tree industry and the improvement of people’s consumption level, the demand for high-quality fruit is increasing day by day. Fruit quality is mainly evaluated by two aspects: physicochemical and sensory measurements, the former mainly includes the measurement of types and contents of the nutrients, the latter mainly refelects the flavor substances, pigment substances and fruit flesh quality. Among them, fruit flavor, as an intrinsic index to evaluate fruit quality, is also an important factor to determine the market share and planting area, so it has become the hot issue of scientific research. Astringency is one of the basic flavors in fruits, which usually exists in unripe fruits and gradually decreases as the fruits mature. However, some wild resources and cultivars are still relatively astringent after fruit ripening, which has a negative impact on the utilization of wild germplasm and selection of good varieties. The astringent substances in fruits mainly include tannins, catechins, epicatechin, chlorogenic acid, neochlorogenic acid and other polyphenolic secondary metabolites, and the strength of astringency is closely related to the content of condensed tannins, i.e. proanthocyanidins. The biosynthetic pathways of astringent substances in plants have been studied clearly, and they are synthesized in plants mainly through three pathways: phenylpropane, flavonoids and phenolic acids. Among them, PAL, LAR and ANR, HCT and C3H are the key enzymes in the astringent synthesis pathways, respectively. In this paper, we summarized the progress of previous studies on the metabolism of astringent substances in persimmon, grape, apple and peach fruits, and found that transcription factors of MYB, bHLH, WD40, NAC, WRKY and bZIP families are involved in the metabolism of astringent substances in fruits by positively or negatively regulating theexpression levels of structural genes. It was also found that the mechanisms of action of homologous genes were not identical in different fruit trees. It is worth mentioning that the environmental factors such as light, temperature, water and hormones also affect the synthesis and accumulation of the astringent substances in fruits. In addition, this paper also briefly introduced the mechanism and common methods of fruit astringency removal. The previous research results on the classification of the fruit astringent substances, biosynthetic pathways, metabolic regulation and fruit deacidification had laid the foundation for improving the formation mechanism of fruit astringency, and also provided the basis for selecting new varieties of fruit trees with low or no astringency. However, there were some shortcomings at the same time, so we also put forward future research suggestions: (1) Establishing precise fruit astringency evaluation standards. At present, fruit astringency is classified into four grades: none, slight, medium and much, but the boundaries between different astringency grades are rather vague, and the astringency sensitivity varies from person to person, and subjective judgments can make the astringency evaluation results biased. Therefore, scientific sensory evaluation methods need to adopt to establish accurate fruit astringency evaluation standards. (2) Improving the formation mechanism of fruit astringency, and exploring the genetic law of astringent substances in different fruits. There are many types of astringent substances in fruits, but the strength of astringency caused by different astringent substances is not clear. The route by which catechins and epicatechins polymerize to form tannins, the main astringent substance, is not clear, and chlorogenic acid and neochlorogenic acid are not fully studied. At the same time, due to the difference in the content and species of astringent substances in different fruit trees, and the specificity of the regulatory pathways of homologous genes in different species, it is of great guiding significance to meet different breeding needs by making full use of a variety of experimental methods to improve the formation mechanism of astringent taste in fruits, to explore the genetic law of astringent substances in different fruits, and to explore the key genes. (3) Strengthening the research on fruit astringency removal. At present, the research on deastringency technology focuses on persimmon fruits, but in production practice, it has been found that some germplasms in other fruits also have obvious astringency, but there is a lack of corresponding research. Therefore, it is necessary to explore different fruit deastringency determination methods in order to provide technical support for improving fruit quality and germplasm innovation. (4) Seeking the balance between the flavor and the antioxidant capacity for germplasm innovation. The tannins, chlorogenic acid and neochlorogenic acid all have strong antioxidant properties and play a positive regulatory role in fruit tree growth and development, resistance to biotic stress and abiotic stress, and promotion of human health. However, the high content of these substances can affect fruit flavor and reduce fruit quality. Therefore, it is necessary to select and breed new varieties with strong antioxidant capacity and low astringency through germplasm innovation. In summary, this paper reviewed the progress made in the classification, synthesis and accumulation of the fruit astringent substances, metabolic regulation, as well as the mechanism and technology of fruit astringency removal, and proposed some suggestions on fruit astringency research in order to provide ideas for using wild resources and selecting and breeding new varieties with low or no astringency.