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.

The microRNA319 (miR319) family is conserved among diverse plant species. In rice (Oryza sativa L.), the miR319 gene family is comprised of two members, Osa-miR319a and Osa-miR319b. We found that overexpressing Osa-miR319b in rice resulted in wider leaf blades and delayed development. Here, we focused on the biological function and potential molecular mechanism of the Osa-miR319b gene in response to cold stress in rice. The expression of Osa-miR319b was down-regulated by cold stress, and the overexpression of Osa-miR319b led to an enhanced tolerance to cold stress, as evidenced by higher survival rates and proline content. Also, the expression of a handful of cold stress responsive genes, such as DREB1A/B/C, DREB2A, TPP1/2, was increased in Osa-miR319b transgenic lines. Furthermore, we demonstrated the nuclear localization of the transcription factors, OsPCF6 and OsTCP21, which may be Osa-miR319b-targeted genes. We also showed that OsPCF6 and OsTCP21 expression was largely induced by cold stress, and the degree of induction was obviously repressed in plants overexpressing Osa-miR319b. As expected, the down-regulation of OsPCF6 and OsTCP21 resulted in enhanced tolerance to cold stress, partially by modifying active oxygen scavenging. Taken together, our findings suggest that Osa-miR319b plays an important role in plant response to cold stress, maybe by targeting OsPCF6 and OsTCP21.

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MicroRNA319 Positively Regulates Cold Tolerance by Targeting OsPCF6 and OsTCP21 in Rice (Oryza sativa L.)

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.)

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.

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

DREB2 (dehydration-responsive element-binding factor 2)-type transcription factors play a critical role in the stress-related regulation network in plants. In this study, we isolated and characterized a DREB2 homolog from Malus sieversii Roem., designated MsDREB2C (GenBank accession No. JQ790526). MsDREB2C localized to the nucleus and transactivated reporter genes in yeast strain YGR-2. Quantitative real-time PCR analysis demonstrated that MsDREB2C was constitutively expressed and significantly induced by drought, salt, cold, heat and ABA. Transgenic Arabidopsis plants overexpressing MsDREB2C exhibited increased root and leaf growth and proline levels, and reduced water loss and stomatal aperture. The transcriptional level of genes that function downstream of dehydration-responsive elements was greater in the transgenic Arabidopsis plants than in wild-type plants under control and abiotic stress conditions. Furthermore, constitutive expression of MsDREB2C repressed the expression of pathogenesis-related (PR) genes and the activity of peroxidase in transgenic plants under control and pathogenic conditions. As a result, transgenic plants were more tolerant to drought, heat and cold, but more sensitive to Pst DC3000 (Pseudomonas syringae pv . tomato DC3000) infection than control plants. β-Glucuronidase expression analysis of the MsDREB2C promoter in transgenic tobacco plants showed that MsDREB2C was mainly expressed in the vascular tissues and seeds. Deletion analysis identified the regulatory regions responsible for the plant's response to drought (-831 to -680), ABA (-831 to -680 and -335 to -148), salt (-831 to -335), cold (-1,317 to -831 and -335 to -148) and heat (-335 to -148).

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Isolation and Characterization of Dehydration-Responsive Element-Binding Factor 2C (MsDREB2C) from Malus sieversii Roem.

Identification and characterization of FaSOC1, a homolog of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 from strawberry

Dehydration responsive element binding (DREBs) play a critical role in improving the stress tolerance of plants by interacting with a DRE/CRT cis-element present in the promoter region of various abiotic stress-responsive genes. In this study, a DREB2 homologous gene from Malus sieversii Roem., designated MsDREB2C (GenBank accession No. JQ790526), was isolated and characterized. MsDREB2C was localized to the nucleus of Arabidopsis protoplasts. qRT-PCR analysis demonstrated that MsDREB2C was induced by salicylic acid (SA) and jasmonic acid (JA). Furthermore, transgenic Arabidopsis plants overexpressing MsDREB2C exhibited increased sensitivity to Alternaria mali infection, and the constitutive expression of MsDREB2C in transgenic plants repressed the expression of PR genes and the activity of peroxidase under normal and pathogenic conditions. We then functionally characterized the MsDREB2C promoter region. Deletion analysis identified the regulatory regions responsible for the gene’s response to SA (−335 to −148 bp) and JA (−831 to −680 bp and −335 to −148 bp). These results suggested that overexpression of MsDREB2C decreased resistance to A. mali.

Qi Wang

Papers

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

Isolation and Characterization of Dehydration-Responsive Element-Binding Factor 2C (MsDREB2C) from Malus sieversii Roem.

Identification and characterization of FaSOC1, a homolog of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 from strawberry

Arabidopsis Plants Overexpressing the MsDREB2C Exhibit Increased Susceptibility to Alternaria mali Infection

Malus sieversii (Ledeb.) Roem-derived plant growth-related protein, and coding gene and application thereof