Research progress in genetic diversity and related factors of plum peel color

Abstract: Plum is one of the important stone fruit crops, belonging to the Prunus of Rosaceae. China is the origin center of Chinese plum (Prunus salicina Lindl.) with rich germplasm resources. It is well renowned for its beautiful, fragrant, juicy and rich in essential nutrients, which naturally makes it a characteristic and popular fruit. Sufficient facts show that the slow breeding process, insufficient exploration and utilization of plum germplasm resources, lacking of independent intellectual property rights and unclear functionality have become a serious issue affecting the industrial development of Chinese plum. The peel color of the fruit is a remarkable indicator of maturity, and it is also an essential agronomic and economic trait for fruit quality. As the classic model fruit, plum has a behavior of diverse peel colors (for example, green, yellow, pink, red, purple, blue purple, purple black, etc.). Among Chinese plum germplasm resources, purple red is the main genetic type of peel color (accounting for 55.4%), with the second highest being red color, intermediate orange yellow color, and less being purple black and pink, whereas blue black was the least. The epidermis of plum fruit is rich in anthocyanins, which can significantly benefit the human body's anti-ageing ability and cardiovascular function, prevent high blood pressure and enhance the human body's anti-mutation response-ability and vision. In recent years, plum has become a favorite functional fruit. Anthocyanins are the largest group of water-soluble pigments in the plant and belong to the family of compounds known as flavonoids, which widely exist in plant roots, stems, leaves, flowers, fruits and seeds. They play an important key role in the period of coloration because of the impact on the formation of various color, quality and flavor of plants. The color of plum peel is closely related to the types, contents and components of anthocyanins. The main anthocyanins in plums are pelargonidin, cyanidin, delphinidin, paeoniflorin, petunidin and malvidin. The group of derivatives show different peel colors, in which the darker the color of the plum, the higher the anthocyanin contents. There is no anthocyanin in the yellow or green peel, which is determined by the content of carotenoids and chlorophyll. The results have showed that CHS, CHI, F3H, DFR, LDOX, UFGT, PsMYB1, LAR and MYB are involved in anthocyanin synthesis, in which PcMYB10.654 plays an important role in anthocyanin accumulation in Chinese plum fruit. PcMYB10.6 is a major gene affecting anthocyanin biosynthesis in the purple-leaf plum. At the same time, environmental factors affect anthocyanin synthesis pathway, which can regulate both structural and regulatory genes. Appropriate light, temperature and water promote the accumulation of fruit coloring. The anthocyanin content is correlated with plant endogenous hormones including ABA (abscisic acid), IAA (indoleacetic acid), GA3 (gibberellin) and ethylene, enzyme, sugar, acid and vitamin C. Mineral elements not only provide nutritional elements for fruit growth, but also are related to the anthocyanin content in peel. Low-nitrogen increases anthocyanin, whereas high nitrogen will promote the formation of carotene and chlorophyll. The concentrations of Na+ , Zn2+ , Mn2+ and Ca2+ , Cu2+ , Al3+ all have hyperchromic effects, the former of which is able to enhance the stability of anthocyanins, while the latter has no significant effect on the stability of anthocyanins; Fe2+ , Fe3+ and Pb2+ can destroy anthocyanins, which decreases their stability. The peel color of plum is one of the important characteristic indexes to represent the diversity of germplasm resources and evaluate the breeding of new plum varieties in China. Foreign breeders have tried to abandon the breeding goal of the weight and taste of fresh fruits and have bred some Chinese plum cultivars with rich anthocyanin as a main breeding goal, and these plums are being commercially grown for processing into functional products, achieving conversion of food pigments into quality health-care products. However, similar breeding behaviors are still rare in the processing of plum in China, and a large number of anthocyanin-rich germplasm are urgently studied and utilized. Here, we shall summarize the related research on plum peel color, provide testimony for the in- depth exploration of outstanding plum germplasm resources and fine evaluation of outstanding traits in China, speed up the process of plum breeding and improve the international competitiveness of this country’s plum fruit.