Advances in ABA promoting fruit coloration

Abstract: Fruit color is a basic evaluation index of maturity and quality. Chlorophyll, carotenoids and flavonoids are the three major coloring pigments. Most immature fruits contain large amounts of chlorophyll. For ripened fruits, the color is primarily composed of flavonoids and carotenoids. Fruits showing either red or purple color are primarily determined by the anthocyanins which are a group of important flavonoids and synthesized by a series of intermediate products. Anthocyanins occur abundantly as glycosides. Different anthocyanin glycoside types determine different fruit colors. With the fruit ripening, the chlorophyll content decreases and the anthocyanin content gradually increases. The fruit coloring process is not only controlled by genetic factors but also regulated by light, temperature, plant growth regulators and other external factors. Abrupt environmental condition changes can cause poor color, and reduce the value of the product. It is therefore necessary to take effective measures to promote the coloring for some fruits. Applying growth regulators is a convenient and quick method. Abscisic acid (ABA) , as an important plant hormone, which plays an important regulatory role in the maturation of climacteric and non-climacteric fruits, and can effectively promote fruit coloring and improve fruit quality. Currently, ABA is widely used to improve grape fruit quality and nutrition in the United States and Japan. It is being used to improve the coloring of grapes, peaches, litchis, oranges, and strawberries in China. According to a research report, the concentration of endogenous ABA significantly increases after ripening. Spraying exogenous ABA during the color-change period increases the accumulation of the anthocyanins and antioxidant ability of the fruit. Exogenous application of ABA promotes fruit coloration and maturation by affecting the balance and endogenous levels of ethylene, ABA, IAA, GA and ZR. The biosynthesis of anthocyanins is regulated by a variety of enzymes. Exogenous ABA can promote thesynthesis of anthocyanins by affecting activities of anthocyanin related enzymes. ABA treatment was found to enhance the activities of PAL and UFGT in pericarp which promoted the synthesis of anthocyanin and the accumulation of soluble solids in the fruit peel. During fruit coloration, related genes of the anthocyanin pathway were significantly increased in the transcriptional level that primarily related to the anthocyanin contents. The biosynthesis of anthocyanins is controlled by the expression of structural genes and transcription factors. The structural genes including chalcone synthases (CHS) , chalcone isomerase (CHI) , flavonoid 3-hydroxylase (F3 H) , flavonoid 3'-hydroxylase (F3'H) , flavonoid 3', 5'-hydroxylase (F3'5'H) , dihydroflavonol 4-reductase (DFR) , leucoanthocyanidin dioxygenase (LDOX) and UDP glucose-flavonoid 3-O-glucosyl-tranferase (UFGT) . The anthocyanin biosynthesis pathway was regulated by the highly conserved MYB-b HLH-WD repeat (MBW) transcriptional complex model.In the dicot Arabidopsis, anthocyanin biosynthesis genes can be divided in two subgroups: early biosynthesis genes (EBGs) reactivated by co-activator independent R2 R3-MYB transcription factors, whereas late biosynthesis genes (LBGs) require an MBW complex. It has a MBW complex and anthocyanin regulatory system in grapes that are similar to Arabidopsis. Many researches have broadened our understanding of the regulation of anthocyanin synthesis in fruits, indicating that a regulatory system based on the cooperation of MYB, b HLH and WD40 proteins that control fruit pigmentation is common to many species. MYB is a family of important regulator genes, which can regulate the expression of the UFGT gene. UFGT is a key enzyme in the anthocyanin biosynthesis pathway, and it is also one of the most affected enzymes by transcription factors in the biosynthesis pathway. The expression of structural genes determines the content and species of anthocyanin. The type and number of MYB genes plays an important role in anthocyanin synthesis. Exogenous ABA can promote fruit coloring by regulating the expression of structural and MYB regulatory genes in the pathway of anthocyanin biosynthesis. The fruit coloring problem has been the hotspot of agricultural research. Exogenous application of growth regulators is a simple, effective and inexpensive method. Many researches have confirmed that ABA can promote fruit color, and improve fruit quality. However, the required time and concentration of ABA in different fruit trees still needs further study. In order to provide a definite theoretical foundation for improving fruit color, many experiments have been performed using exogenous ABA to treat different fruit trees. The mechanism as to how exogenous ABA treatment improves fruit color is still debated. One approach is that ABA treatment can increase the level of endogenous ABA, so as to promote fruit ripening and coloring. The other is that ABA can change the dynamic balance of the endogenous hormone in fruit to promote fruit coloration and maturity. It is not absolutely clear which process ABA affects. However, to date, the mechanism of how ABA is promoting fruit coloring is elusive, which needs further study in order to reach the best conclusion for efficient cultivation. In this review, we summarize recent research progress on the role of ABA in fruit coloring and transcriptional regulation, and also the functional verification of both ABA-responsive and coloring-related genes. In addition, we suggest possible commercial applications of genetic manipulation of ABA signaling to improve fruit quality and yields.