The phytohormone abscisic acid (ABA) plays a crucial role not only in fruit development and ripening, but also in adaptive responses to biotic and abiotic stresses. In these processes, the actions of ABA are under the control of complex regulatory mechanisms involving ABA metabolism, signal transduction, and transport. The endogenous ABA content is determined by the dynamic balance between biosynthesis and catabolism, processes which are regulated by 9-cis-epoxycarotenoid dioxygenase (NCED) and ABA 8'-hydroxylase (CYP707A), respectively. ABA conjugation by cytosolic UDP-glucosyltransferases, or release by β-glucosidases, is also important for maintaining ABA homeostasis. Recently, multiple putative ABA receptors localized at different subcellular sites have been reported. Among these is a major breakthrough in the field of ABA signalling-the identification of a signalling cascade involving the PYR/PYL/RCAR protein family, the type 2C protein phosphatases (PP2Cs), and subfamily 2 of the SNF1-related kinases (SnRK2s). With regard to transport, two ATP-binding cassette (ABC) proteins and two ABA transporters in the nitrate transporter 1/peptide transporter (NRT1/PTR) family have been identified. In this review, we summarize recent research progress on the role of ABA in fruit ripening, stress response, and transcriptional regulation, and also the functional verification of both ABA-responsive and ripening-related genes. In addition, we suggest possible commercial applications of genetic manipulation of ABA signalling to improve fruit quality and yields.

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The role of abscisic acid in fruit ripening and responses to abiotic stress

Phytohormone abscisic acid (ABA) induces anthocyanin biosynthesis; however, the underlying molecular mechanism is less known. In this study, we found that the apple MYB transcription factor MdMYB1 activated anthocyanin biosynthesis in response to ABA. Using a yeast screening technique, we isolated MdbZIP44, an ABA-induced bZIP transcription factor in apple, as a co-partner with MdMYB1. MdbZIP44 promoted anthocyanin accumulation in response to ABA by enhancing the binding of MdMYB1 to the promoters of downstream target genes. Furthermore, we identified MdBT2, a BTB protein, as an MdbZIP44-interacting protein. A series of molecular, biochemical, and genetic analysis suggested that MdBT2 degraded MdbZIP44 protein through the Ubiquitin-26S proteasome system, thus inhibiting MdbZIP44-modulated anthocyanin biosynthesis. Taken together, we reveal a novel working mechanism of MdbZIP44-mediated anthocyanin biosynthesis in response to ABA.

Apple bZIP transcription factor MdbZIP44 regulates abscisic acid-promoted anthocyanin accumulation.

This review contains functional roles of MYB transcription factors in the transcriptional regulation of anthocyanin biosynthesis in horticultural plants This review describes potential uses of MYB TFs as tools for metabolic engineering for anthocyanin production Anthocyanins (ranging from red to blue) are controlled by specific branches of the anthocyanin biosynthetic pathway and are mostly visible in ornamentals, fruits, and vegetables In the present review, we describe which R2R3-MYB transcription factors (TFs) control the transcriptional regulation of anthocyanin structural genes involved in the specific branches of the anthocyanin biosynthetic pathway in various horticultural plants (eg, ornamentals, fruits, and vegetables) In addition, some MYBs responsible for anthocyanin accumulation in specific tissues are described Moreover, we highlight the phylogenetic relationships of the MYBs that suppress or promote anthocyanin synthesis in horticultural crops Enhancement of anthocyanin synthesis via metabolic genetic engineering of anthocyanin MYBs, which is described in the review, is indicative of the potential use of the mentioned anthocyanin-related MYBs as tools for anthocyanin production Therefore, the MYBs would be suitable for metabolic genetic engineering for improvement of flower colors, fruit quality, and vegetable nutrients

Roles of R2R3-MYB transcription factors in transcriptional regulation of anthocyanin biosynthesis in horticultural plants

Sweet cherry is a diploid tree species and its fruit skin has rich colours from yellow to blush to dark red. The colour is closely related to anthocyanin biosynthesis and is mainly regulated at the transcriptional level by transcription factors that regulate the expression of multiple structural genes. However, the genetic and molecular bases of how these genes ultimately determine the fruit skin colour traits remain poorly understood. Here, our genetic and molecular evidences identified the R2R3 MYB transcription factor PavMYB10.1 that is involved in anthocyanin biosynthesis pathway and determines fruit skin colour in sweet cherry. Interestingly, we identified three functional alleles of the gene causally leading to the different colours at mature stage. Meanwhile, our experimental results of yeast two-hybrid assays and chromatin immunoprecipitation assays revealed that PavMYB10.1 might interact with proteins PavbHLH and PavWD40, and bind to the promoter regions of the anthocyanin biosynthesis genes PavANS and PavUFGT; these findings provided to a certain extent mechanistic insight into the gene's functions. Additionally, genetic and molecular evidences confirmed that PavMYB10.1 is a reliable DNA molecular marker to select fruit skin colour in sweet cherry.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/pbi.12568

The R2R3 MYB transcription factor PavMYB10.1 involves in anthocyanin biosynthesis and determines fruit skin colour in sweet cherry (Prunus avium L.)

Understanding the genetic regulation of anthocyanin biosynthesis in plants - Tools for breeding purple varieties of fruits and vegetables.

The MYB transcription factors and plant hormone ABA have been suggested to play a role in fruit anthocyanin biosynthesis, but supporting genetic evidence has been lacking in sweet cherry. The present study describes the first functional characterization of an R2R3-MYB transcription factor, PacMYBA, from red-colored sweet cherry cv. Hong Deng (Prunus avium L.). Transient promoter assays demonstrated that PacMYBA physically interacted with several anthocyanin-related basic helix-loop-helix (bHLH) transcription factors to activate the promoters of PacDFR, PacANS and PacUFGT, which are thought to be involved in anthocyanin biosynthesis. Furthermore, the immature seeds of transgenic Arabidopsis plants overexpressing PacMYBA exhibited ectopic pigmentation. Silencing of PacMYBA, using a Tobacco rattle virus (TRV)-induced gene silencing technique, resulted in sweet cherry fruit that lacked red pigment. ABA treatment significantly induced anthocyanin accumulation, while treatment with the ABA biosynthesis inhibitor nordihydroguaiaretic acid (NDGA) blocked anthocyanin production. PacMYBA expression peaked after 2 h of pre-incubation in ABA and was 15.2-fold higher than that of sweet cherries treated with NDGA. The colorless phenotype was also observed in the fruits silenced in PacNCED1, which encodes a key enzyme in the ABA biosynthesis pathway. The endogenous ABA content as well as the transcript levels of six structural genes and PacMYBA in PacNCED1-RNAi (RNA interference) fruit were significantly lower than in the TRV vector control fruit. These results suggest that PacMYBA plays an important role in ABA-regulated anthocyanin biosynthesis and ABA is a signal molecule that promotes red-colored sweet cherry fruit accumulating anthocyanin.

Kai Zhao

Papers

A role for PacMYBA in ABA-regulated anthocyanin biosynthesis in red-colored sweet cherry cv. Hong Deng (Prunus avium L.).