Molecular mechanism of abscisic acid in regulating fruit ripening

Abstract: Abscisic acid (ABA), as an important plant hormone, not only participates in a variety of plant developmental processes and stresses, but also plays a major role in fruit ripening, especially in non-climacteric fruit ripening. With the continuous study of ABA biosynthesis, metabolism and physiological effect in plants, especially the establishment of models for ABA perception and signaling transduction, the research on ABA in fruit development and ripening has been greatly promoted. De-greening and coloration are common process during fruit ripening, which involves the synergistic role of ABA and the early signals as well as multiple hormones, constituting a complex regulatory network. This paper reviewed the molecular mechanism of ABA biosynthesis, metabolism and physiological roles in plants. Finally, a model of ABA in regulating fruit ripening was proposed to lay a theoretical foundation for improving fruit quality and shelf-life. Hormones are important factors in regulating fruit ripening. According to whether the fruit ripening depends on ethylene or not, it can be divided into ethylene dependent (climacteric) and independent (non-climacteric) types. For the climacteric fruits (such as tomatoes and bananas), both respiratory intensity and the release rate of ethylene reach to peak at ripening. However, there is no such peak in non-climacteric fruits (such as grapes and strawberries), whose ripening is mainly controlled by ABA. ABA plays an important role in regulating fruit ripening, especially in non- climacteric fruits. For example, ABA content in strawberry receptacle increases during strawberry fruit ripening and exogenous ABA treatment can accelerate strawberry ripening, indicating that ABA may participate in the regulation of strawberry ripening. In addition, exogenous ABA can also promote the ripening of grapes, avocados, tomatoes, cucumbers, oranges, figs and so on. In a word, ABA is a core hormonein regulating fruits ripening. The synergetic regulation of ABA from biosynthe-sis to signaling transduction is the key to plant growth and fruit ripening. In higher plants,“biosynthesisdegradation”and“binding-dissociation”pathways synergistically regulate the level of ABA, and 9-cisepoxycarotenoiddioxygenase (NCED) is the first step specific to ABA biosynthesis. In the whole process of strawberry fruit development, the level of ABA and the expression of the FaNCED1 show the same tendency, suggesting that NCED is the key enzyme determining ABA level in strawberry fruit, especially exogenous ABA can restore the coloration of the FaNCED1-RNAi, but cannot restore the coloration of the FaCHLH/ABAR-RNAi fruit. 8’-hydroxylation is the main pathway of ABA oxidative catabolism. The UDP-glycosyltransferases (UGTs) can catalyze ABA to ABA glucose ester (ABA-GE). The β- Glucosidase (BGs) can catalyze the dissociation of ABA glucose esters into free ABA. Comparing with de novo synthesis, the rapid metabolic mode make plant quickly change the level of endogenous ABA to adapt to changes of the environment. In conclusion, NCED, UGT71, CYP707A and BG play important roles in strawberry ripening, revealing that ABA is the key plant hormone in regulating fruit ripening. The ABA receptors play a critical role in ABA perception, and protein reversible phosphorylation plays a key role in ABA signaling transduction pathway. The core signal components of“PP2CSnRK2”are the key to strawberry fruit ripening. FaPYR1 is closely related to initiate and quality format in fruit ripening.“PYR1-PP2C-SnRK2”is a conservative mechanism for regulating fruit ripening. CHLH/ABAR has multiple biological functions, including chlorophyll biosynthesis, plastid-nuclear retrograde signaling and ABA signaling transduction. As a co-receptor of FaABAR, FaRIPK1 participates in the initiation of strawberry fruit ripening, and FaABAR regulates fruit ripening, confirming that FaABAR/CHLH is a positive regulator for fruit ripening. In a word, the core signaling component“ABAPYR1-PP2C-SnRK2”is the conservative mechanismin regulating fruit ripening, while“ABA-ABARRIPK1-ABI4”is a new mechanism for regulating strawberry fruit ripening, suggesting that ABA regulation network is complex, conservative and diverse. The development of fruit includes early cell division and expansion, followed by chlorophyll degradation, cell wall softening, and metabolism changes including phenylpropionic acid, flavonoids, starch/sucrose, and carotenoid during ripening. These processes are strictly controlled by plant hormones, emphasizing on the role of ethylene in climacteric and ABA in non- climacteric fruit ripening, as well as two interactions. In the study of fig fruit, a special type of ripening, ABA can promote the accumulation of ethylene and initiate fruit ripening, and the regulation of fruit ripening by ethylene depends on the perception of ABA receptors. Therefore, the role of ABA is closely related to ethylene system Ⅰ/Ⅱ. In conclusion, this paper tryed to propose the molecular mechanism of ABA regulating fruit ripening. With the initiation of fruit ripening, the development signals such as sugar, NO, Ca2+ and environmental signals such as light would lead to the accumulation of ROS, triggering the biosynthesis and accumulation of ABA. At the same time, they would synergistically inhibit the biosynthesis and effect of GA, IAA and CTK, and synergistically promote the biosynthesis and effect of ethylene, JA, PA and BR. All these hormones could constitute a complex regulatory network, in which ABA is the core hormone in regulating fruit ripening. In the past decade, the research on plant molecular biology in China has made rapid progress, and has begun to transform from model plants to fruit and woody plants. However, the long time of cultivation of fruit trees and the bottleneck of genetic transformation system have restricted the development of fruit tree molecular biology. In the future, the combination of the genomics, transcriptomics, proteomics, metabonomics and epigenetics, as well as the gene knockout technology CRISPR/Cas9, in-depth analysis of the commonness and specificity of the hormone regulation mechanism in fruit ripening will be an important direction, so does the molecular mechanism of seed and pulp synergetic regulation network.