Abscisic acid plays an important role in the regulation of strawberry fruit ripening

Autocatalytic biosynthesis of abscisic acid and its synergistic action with auxin to regulate strawberry fruit ripening

Abscisic acid (ABA) plays a major role in the regulation of strawberry fruit ripening; however, the origin of the ABA signal is largely unknown. Here, we report an autocatalytic mechanism for ABA biosynthesis and its synergistic interaction with the auxin to regulate strawberry fruit ripening. We demonstrate that ABA biosynthesis is self-induced in the achenes, but not in the receptacle, which results its substantial accumulation during ripening. ABA was found to regulate both IAA transport and biosynthesis, thereby modulating IAA content during both early fruit growth and later during ripening. Taken together, these results reveal the origins of the ABA signal and demonstrate the importance of its coordinated action with IAA in the regulation of strawberry fruit development and ripening.

Characterization and regulation mechanism analysis of ubiquitin-conjugating family genes in strawberry reveals a potential role in fruit ripening

BACKGROUND

E2 ubiquitin-conjugating (UBC) enzymes are an integral component of the ubiquitin proteasome system that play an important role in plant development, growth, and external stress responses. Several UBC genes have been identified in various plants. However, no studies exploring the functions of UBC genes in regulating fruit of strawberry have been reported. In the present study, a systematic analysis of the entire UBC family members were conducted in the genome of strawberry (Fragaria ×ananassa) based on bioinformatics method, and the gene functioning in strawberry ripening was explored.

RESULTS

A total of 191 UBC genes were identified in the genome of cultivated strawberry. These genes were unevenly distributed across the 28 chromosomes from the 4 subgenomes of cultivated strawberry, ranging from 3 to 11 genes per chromosome. Moreover, the expansion of FaUBC genes in strawberry was mainly driven by WGD. All the FaUBC genes were clarified into 13 groups and most of them were included in the group VI. The gene structure analysis showed that the number of exons varied from 1 to 23, and the structure of genes had few differences within the same groups but a distinction in different groups. Identification of the cis-acting elements of the promoter revealed multiple regulatory elements that responded to plant growth and development, phytohormone responsive, and abiotic and biotic stress. Data from functional annotation indicated that FaUBC genes play a role in a variety of biological processes. The RNA-seq data showed that FaUBC genes displayed different expression pattern during the fruit ripening process and clarified into 6 clusters. In particular, cluster 3 exhibiting a sudden expression increase in the turning red stage were speculated to be involved in fruit ripening. Hence, two FaUBC genes (FaUBC76 and FaUBC78) were selected for gene function analysis by transient over-expression method. The results indicated that FaUBC76 has a positive effect on the fruit development and ripening in strawberry by up-regulating accumulation of anthocyanins. Moreover, expression of some maturity-related genes were also significantly increased, further supporting a role for FaUBC76 in the regulation of fruit ripening or softening. On the contrary, the overexpression of FaUBC78 significantly increased the firmness of strawberry fruit, indicating that FaUBC78 had a positive role in inhibiting the decrease of strawberry fruit firmness.

CONCLUSION

Our study not only provide comprehensive information on system evolution and function on UBC genes, but also give a new insight into explore the roles of FaUBC genes in the regulation of strawberry ripening.

Horticultural innovation by viral-induced gene regulation of carotenogenesis

Multipartite viral vectors provide a simple, inexpensive and effective biotechnological tool to transiently manipulate (i.e. reduce or increase) gene expression in planta and characterise the function of genetic traits. The development of virus-induced gene regulation (VIGR) systems usually involve the targeted silencing or overexpression of genes involved in pigment biosynthesis or degradation in plastids, thereby providing rapid visual assessment of success in establishing RNA- or DNA-based VIGR systems in planta. Carotenoids pigments provide plant tissues with an array of yellow, orange, and pinkish-red colours. VIGR-induced transient manipulation of carotenoid-related gene expression has advanced our understanding of carotenoid biosynthesis, regulation, accumulation and degradation, as well as plastid signalling processes. In this review, we describe mechanisms of VIGR, the importance of carotenoids as visual markers of technology development, and knowledge gained through manipulating carotenogenesis in model plants as well as horticultural crops not always amenable to transgenic approaches. We outline how VIGR can be utilised in plants to fast-track the characterisation of gene function(s), accelerate fruit tree breeding programs, edit genomes, and biofortify plant products enriched in carotenoid micronutrients for horticultural innovation.

PuWRKY31 affects ethylene production in response to sucrose signal in pear fruit

The ripening of climacteric fruits is mainly controlled by the plant hormone ethylene. The regulatory effect of sucrose on ethylene biosynthesis in fruits remains unclear. Here we examined ethylene production in two Ussurian pear (Pyrus ussuriensis) varieties, 'Nanguo' (NG) pear and its bud sport variety (BNG), which has a higher sucrose content. We found that the peak of ethylene release occurred earlier in BNG fruit than in NG fruit during ripening. The expression of the transcription factor PuWRKY31 was higher in BNG fruit than in NG fruit, and was induced by sucrose treatment. Furthermore, PuWRKY31 bound to the promoters of ethylene biosynthetic genes and upregulated their transcription. Our findings suggest a mechanism by which sucrose regulates ethylene biosynthesis in pears.

Roles of abscisic acid in regulating ripening and quality of strawberry, a model non-climacteric fruit

Abscisic acid (ABA) is a dominant regulator of ripening and quality in non-climacteric fruits. Strawberry is regarded as a model non-climacteric fruit due to its extensive genetic studies and proven suitability for transgenic approaches to understanding gene function. Strawberry research has contributed to studies on color, flavor development, and fruit softening, and in recent years ABA has been established as a core regulator of strawberry fruit ripening, whereas ethylene plays this role in climacteric fruits. Despite this major difference, several components of the interacting genetic regulatory network in strawberry, such as MADS-box and NAC transcription factors, are similar to those that operate in climacteric fruit. In this review, we summarize recent advances in understanding the role of ABA biosynthesis and signaling and the regulatory network of transcription factors and other phytohormones in strawberry fruit ripening. In addition to providing an update on its ripening, we discuss how strawberry research has helped generate a broader and more comprehensive understanding of the mechanism of non-climacteric fruit ripening and focus attention on the use of strawberry as a model platform for ripening studies.

Why Black Flowers? An Extreme Environment and Molecular Perspective of Black Color Accumulation in the Ornamental and Food Crops

Pollinators are attracted to vibrant flower colors. That is why flower color is the key agent to allow successful fruit set in food or ornamental crops. However, black flower color is the least attractive to pollinators, although a number of plant species produce black flowers. Cyanidin-based anthocyanins are thought to be the key agents to induce black color in the ornamental and fruit crops. R2R3-MYB transcription factors (TFs) play key roles for the tissue-specific accumulation of anthocyanin. MYB1 and MYB11 are the key TFs regulating the expression of anthocyanin biosynthesis genes for black color accumulation. Post-transcriptional silencing of flavone synthase II (FNS) gene is the technological method to stimulate the accumulation of cyanidin-based anthocyanins in black cultivars. Type 1 promoter of DvIVS takes the advantage of FNS silencing to produce large amounts of black anthocyanins. Exogenous ethylene application triggers anthocyanin accumulation in the fruit skin at ripening. Environment cues have been the pivotal regulators to allow differential accumulation of anthocyanins to regulate black color. Heat stress is one of the most important environmental stimulus that regulates concentration gradient of anthocyanins in various plant parts, thereby affecting the color pattern of flowers. Stability of black anthocyanins in the extreme environments can save the damage, especially in fruits, caused by abiotic stress. White flowers without anthocyanin face more damages from abiotic stress than dark color flowers. The intensity and pattern of flower color accumulation determine the overall fruit set, thereby controlling crop yield and human food needs. This review paper presents comprehensive knowledge of black flower regulation as affected by high temperature stress, and the molecular regulators of anthocyanin for black color in ornamental and food crops. It also discusses the black color-pollination interaction pattern affected by heat stress for food and ornamental crops.

Abscisic Acid: Role in Fruit Development and Ripening

Abscisic acid (ABA) is a plant growth regulator known for its functions, especially in seed maturation, seed dormancy, adaptive responses to biotic and abiotic stresses, and leaf and bud abscission. ABA activity is governed by multiple regulatory pathways that control ABA biosynthesis, signal transduction, and transport. The transport of the ABA signaling molecule occurs from the shoot (site of synthesis) to the fruit (site of action), where ABA receptors decode information as fruit maturation begins and is significantly promoted. The maximum amount of ABA is exported by the phloem from developing fruits during seed formation and initiation of fruit expansion. In the later stages of fruit ripening, ABA export from the phloem decreases significantly, leading to an accumulation of ABA in ripening fruit. Fruit growth, ripening, and senescence are under the control of ABA, and the mechanisms governing these processes are still unfolding. During the fruit ripening phase, interactions between ABA and ethylene are found in both climacteric and non-climacteric fruits. It is clear that ABA regulates ethylene biosynthesis and signaling during fruit ripening, but the molecular mechanism controlling the interaction between ABA and ethylene has not yet been discovered. The effects of ABA and ethylene on fruit ripening are synergistic, and the interaction of ABA with other plant hormones is an essential determinant of fruit growth and ripening. Reaction and biosynthetic mechanisms, signal transduction, and recognition of ABA receptors in fruits need to be elucidated by a more thorough study to understand the role of ABA in fruit ripening. Genetic modifications of ABA signaling can be used in commercial applications to increase fruit yield and quality. This review discusses the mechanism of ABA biosynthesis, its translocation, and signaling pathways, as well as the recent findings on ABA function in fruit development and ripening.

Metabolomic and transcriptomic analyses reveal new insights into the role of abscisic acid in modulating mango fruit ripening

Mango (Mangifera indica L.) is a climacteric tropical fruit consumed around the world. Although ethylene and abscisic acid (ABA) have been considered to be stimulators that trigger mango fruit ripening, their regulation mechanisms in modulating mango fruit ripening remain uncertain. In this study, we performed integrative analyses of metabolome and transcriptome data combined with a series of physiological and experimental analyses in the 'Keitt' mango, and we characterized changes in accumulation of specific metabolites at different stages during fruit development and ripening, which were strongly correlated with transcriptional changes and embodied physiological changes as well as taste formation. Specifically, we found that ABA, rather than ethylene, was highly associated with mango ripening, and exogenous ABA application promoted mango fruit ripening. Transcriptomic analysis identified diverse ripening-related genes involved in sugar and carotenoid biosynthesis and softening-related metabolic processes. Furthermore, networks of ABA- and ripening-related genes (such as MiHY5, MiGBF4, MiABI5, and MibZIP9) were constructed, and the direct regulation by the key ABA-responsive transcription factor MiHY5 of ripening-related genes was experimentally confirmed by a range of evidence. Taken together, our results indicate that ABA plays a key role in directly modulating mango fruit ripening through MiHY5, suggesting the need to reconsider how we understand ABA function in modulating climacteric fruit ripening.

Flavonoid biosynthesis is differentially altered in detached and attached ripening bilberries in response to spectral light quality

Light spectral quality is known to affect flavonoid biosynthesis during fruit ripening. However, the response of fruits to different light conditions, when ripening autonomously from the parent plant (detached), has been less explored. In this study, we analyzed the effect of light quality on detached and naturally ripening (attached) non-climacteric wild bilberry (Vaccinium myrtillus L.) fruits accumulating high amounts of anthocyanins and flavonols. Our results indicated contrasting responses for the accumulation of phenolic compounds in the berries in response to red and blue light treatments. For detached berries, supplemental blue light resulted in the highest accumulation of anthocyanins, while naturally ripening berries had elevated accumulation under supplemental red light treatment. Both red and blue supplemental light increased the expression levels of all the major structural genes of the flavonoid pathway during ripening. Notably, the key regulatory gene of anthocyanin biosynthesis, VmMYBA1, was found to express fivefold higher under blue light treatment in the detached berries compared to the control. The red light treatment of naturally ripening berries selectively increased the delphinidin branch of anthocyanins, whereas in detached berries, blue light increased other anthocyanin classes along with delphinidins. In addition, red and far-red light had a positive influence on the accumulation of flavonols, especially quercetin and myricetin glycoside derivatives, in both ripening conditions. Our results of differential light effects on attached and detached berries, which lacks signaling from the mother plant, provide new insights in understanding the light-mediated regulatory mechanisms in non-climacteric fruit ripening.

Roles of abscisic acid in regulating ripening and quality of strawberry, a model non-climacteric fruit

Abscisic acid (ABA) is a dominant regulator of ripening and quality in non-climacteric fruits. Strawberry is regarded as a model non-climacteric fruit due to its extensive genetic studies and proven suitability for transgenic approaches to understanding gene function. Strawberry research has contributed to studies on color, flavor development, and fruit softening, and in recent years ABA has been established as a core regulator of strawberry fruit ripening, whereas ethylene plays this role in climacteric fruits. Despite this major difference, several components of the interacting genetic regulatory network in strawberry, such as MADS-box and NAC transcription factors, are similar to those that operate in climacteric fruit. In this review, we summarize recent advances in understanding the role of ABA biosynthesis and signaling and the regulatory network of transcription factors and other phytohormones in strawberry fruit ripening. In addition to providing an update on its ripening, we discuss how strawberry research has helped generate a broader and more comprehensive understanding of the mechanism of non-climacteric fruit ripening and focus attention on the use of strawberry as a model platform for ripening studies.

FvMYB79 Positively Regulates Strawberry Fruit Softening via Transcriptional Activation of FvPME38

Strawberry is a soft fruit with short postharvest life, due to a rapid loss of firmness. Pectin methylesterase (PME)-mediated cell wall remodeling is important to determine fruit firmness and softening. Previously, we have verified the essential role of FvPME38 in regulation of PME-mediated strawberry fruit softening. However, the regulatory network involved in PME-mediated fruit softening is still largely unknown. Here, we identified an R2R3-type MYB transcription factor FvMYB79, which activates the expression level of FvPME38, thereby accelerating fruit softening. During fruit development, FvMYB79 co-expressed with FvPME38, and this co-expression pattern was opposite to the change of fruit firmness in the fruit of 'Ruegen' which significantly decreased during fruit developmental stages and suddenly became very low after the color turning stage. Via transient transformation, FvMYB79 could significantly increase the transcriptional level of FvPME38, leading to a decrease of firmness and acceleration of fruit ripening. In addition, silencing of FvMYB79 showed an insensitivity to ABA-induced fruit ripening, suggesting a possible involvement of FvMYB79 in the ABA-dependent fruit softening process. Our findings suggest FvMYB79 acts as a novel regulator during strawberry ripening via transcriptional activation of FvPME38, which provides a novel mechanism for improvement of strawberry fruit firmness.

MAPK5 and MAPK10 overexpression influences strawberry fruit ripening, antioxidant capacity and resistance to Botrytis cinerea

FaMAPK5 and FaMAPK10 genes were involved in ABA-mediated strawberry fruit ripening and could enhance the antioxidant capacity by increasing non-enzymatic components and enzymatic antioxidants. Mitogen-activated protein kinases (MAPKs) are the key proteins involved in plant stress response by activating an antioxidant defense system, which cooperates with plant hormones. However, the involvement of MAPKs in the regulation of strawberry fruit ripening and resistance is unclear. In this study, two genes, FaMAPK5 and FaMAPK10, were isolated, and their expression pattern and function analysis were conducted. The results showed FaMAPK5 and FaMAPK10 were expressed in all tested tissue/organ types and reached the highest expression level at the white stage during strawberry fruit development and ripening. Transient overexpression of FaMAPK5 and FaMAPK10 increased the fruit anthocyanin, abscisic acid (ABA), total sugar, and glucose contents. ABA and especially hydrogen peroxide (H₂O₂) treatment induced the production of large amounts of H₂O₂ and noticeably increased the expression levels of FaMAPK5 and FaMAPK10 in strawberry fruit, while the reduced glutathione (GSH) had the opposite effect. The level of total phenol and activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) significantly increased in FaMAPK5 overexpression fruit, and increased activities of SOD and CAT were observed in FaMAPK10 overexpression fruit. In addition, Botrytis cinerea treatment showed that overexpression of FaMAPK5 conferred retarded disease symptom development and enhanced fruit disease resistance. Our research revealed that FaMAPK5 and FaMAPK10 might participate in ABA-mediated H₂O₂ signaling in regulating strawberry fruit ripening and resistance.

Impact of Chitosan, Sucrose, Glucose, and Fructose on the Postharvest Decay, Quality, Enzyme Activity, and Defense-Related Gene Expression of Strawberries

Strawberry is one of the most highly consumed fruits worldwide, but is extremely perishable. This study systematically compared the effects of chitosan, sucrose, glucose, and fructose immersion on the physiology and disease development in harvested strawberries. After storage at 15 °C for 9 days, all sugar treatment groups had significantly higher total soluble solids and total anthocyanin content than those of the control group. All sugar treatment groups inhibited malondialdehyde accumulation. At the end of the storage, chitosan, glucose, and fructose maintained higher superoxide dismutase activity and chitosan maintained higher catalase activity. The chitosan and glucose groups had lowest fruit decay index, followed by sucrose and fructose groups. The fruit firmness and luster were better maintained in the glucose group. Furthermore, genes related to sucrose metabolism (e.g., FaSUS1 and FaSUS2), titratable acidity accumulation (e.g., FaMDH1, FaMDH2, FaCS1, and FaCS2), disease resistance (e.g., FaPGIP1, FaWRKY1, and FaWRKY33) and to anabolic jasmonic acid and abscisic acid pathways (e.g., FaJAZ1, FaJAZ2, FaOPR3, FaNCED1, and FaNCED2) were regulated to varying degrees, suggesting that chitosan and glucose participate in plants’ immune signaling networks and regulate disease resistance in fruit through hormone pathways. The findings provide new insights into the physiological regulation of harvested strawberries.

Phenolic profiles and their responses to pre- and post-harvest factors in small fruits: a review

The consumption of small fruits has increased in recent years. Besides their appealing flavor, the commercial success of small fruits has been partially attributed to their high contents of phenolic compounds with multiple health benefits. The phenolic profiles and contents in small fruits vary based on the genetic background, climate, growing conditions, and post-harvest handling techniques. In this review, we critically compare the profiles and contents of phenolics such as anthocyanins, flavonols, flavan-3-ols, and phenolic acids that have been reported in bilberries, blackberries, blueberries, cranberries, black and red currants, raspberries, and strawberries during fruit development and post-harvest storage. This review offers researchers and breeders a general guideline for the improvement of phenolic composition in small fruits while considering the critical factors that affect berry phenolics from cultivation to harvest and to final consumption.

Transcriptome Analysis Revealed the Mechanism by Which Exogenous ABA Increases Anthocyanins in Blueberry Fruit During Veraison

Blueberry (Vaccinium spp.) is a popular healthy fruit worldwide. The health value of blueberry is mainly because the fruit is rich in anthocyanins, which have a strong antioxidant capacity. However, because blueberry is a non-model plant, little is known about the structural and regulatory genes involved in anthocyanin synthesis in blueberries. Previous studies have found that spraying 1,000 mg/L abscisic acid at the late green stage of "Jersey" highbush blueberry fruits can increase the content of anthocyanins. In this experiment, the previous results were verified in "Brightwell" rabbiteye blueberry fruits. Based on the previous results, the anthocyanin accumulation process in blueberry can be divided into six stages from the late green stage to the mature stage, and the transcriptome was used to systematically analyze the blueberry anthocyanin synthesis process. Combined with data from previous studies on important transcription factors regulating anthocyanin synthesis in plants, phylogenetic trees were constructed to explore the key transcription factors during blueberry fruit ripening. The results showed that ABA increased the anthocyanin content of blueberry fruits during veraison. All structural genes and transcription factors (MYB, bHLH, and WD40) involved in the anthocyanin pathway were identified, and their spatiotemporal expression patterns were analyzed. The expression of CHS, CHI, DFR, and LDOX/ANS in ABA-treated fruits was higher in the last two stages of maturity, which was consistent with the change in the anthocyanin contents in fruits. In general, six MYB transcription factors, one bHLH transcription factor and four WD40 transcription factors were found to change significantly under treatment during fruit ripening. Among them, VcMYBA plays a major role in the regulation of anthocyanin synthesis in ABA signaling. This result preliminarily explained the mechanism by which ABA increases the anthocyanin content and improves the efficiency of the industrial use of blueberry anthocyanins.

Citrus transcription factor CsHB5 regulates abscisic acid biosynthetic genes and promotes senescence

Senescence is a gradual physiological process involving the integration of numerous internal and environmental signals. Abscisic acid (ABA) is a well-known inducer of senescence. However, the regulatory mechanisms underlying ABA-mediated senescence remain largely unknown. Here, we report that the citrus homeodomain leucine zipper I (HD-ZIP I) transcription factor CsHB5 functions as a regulator of ABA-triggered senescence. CsHB5 acts as a nucleus-localized transcriptional activator, the expression of which appeared to be closely associated with citrus senescence. Overexpression of CsHB5 in citrus calli upregulated the expression of ABA- and reactive oxygen species (ROS)-related genes, and significantly increased the content of ABA and hydrogen peroxide (H₂ O₂ ), whereas silencing CsHB5 in citrus calli downregulated the expression of ABA-related genes. Additionally, heterogenous overexpression of CsHB5 in Solanum lycopersicum (tomato) and Arabidopsis thaliana (Arabidopsis) leads to early leaf yellowing under dark-induced senescence conditions. Meanwhile, the levels of ABA and H₂ O₂ in transgenic tomatoes increased significantly and the lycopene content decreased. Transcriptome analysis of CsHB5-overexpressing citrus calli and tomato showed that CsHB5 was involved in multiple senescence-associated processes, including chlorophyll degradation, nutrient compound biosynthesis and transport, as well as ABA and ROS signal transduction. The results of yeast one-hybrid assays, electrophoretic mobility shift assays and dual luciferase assays indicated that CsHB5 directly binds to the promoters of ABA biosynthetic genes, including β-carotene hydroxylase 1 (BCH1) and 9-cis-epoxycarotenoid dioxygenase 2 (NCED2), thereby activating their transcription. Our findings revealed that CsHB5 participates in senescence, at least partly, by directly controlling ABA accumulation. Our work provides insight into the regulatory mechanisms underlying ABA-mediated senescence.

Red and blue light treatments of ripening bilberry fruits reveal differences in signalling through abscisic acid-regulated anthocyanin biosynthesis

The biosynthesis of anthocyanins has been shown to be influenced by light quality. However, the molecular mechanisms underlying the light-mediated regulation of fruit anthocyanin biosynthesis are not well understood. In this study, we analysed the effects of supplemental red and blue light on the anthocyanin biosynthesis in non-climacteric bilberry (Vaccinium myrtillus L.). After 6 days of continuous irradiation during ripening, both red and blue light elevated concentration of anthocyanins, up to 12- and 4-folds, respectively, compared to the control. Transcriptomic analysis of ripening berries showed that both light treatments up-regulated all the major anthocyanin structural genes, the key regulatory MYB transcription factors and abscisic acid (ABA) biosynthetic genes. However, higher induction of specific genes of anthocyanin and delphinidin biosynthesis alongside ABA signal perception and metabolism were found in red light. The difference in red and blue light signalling was found in 9-cis-epoxycarotenoid dioxygenase (NCED), ABA receptor pyrabactin resistance-like (PYL) and catabolic ABA-8'hydroxylase gene expression. Red light also up-regulated expression of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) domain transporters, which may indicate involvement of these proteins in vesicular trafficking of anthocyanins during fruit ripening. Our results suggest differential signal transduction and transport mechanisms between red and blue light in ABA-regulated anthocyanin and delphinidin biosynthesis during bilberry fruit ripening.

Recent Advances in Phytohormone Regulation of Apple-Fruit Ripening

Apple (Malus domestica) is, globally, one of the largest fruits in terms of cultivated area and yield. Apple fruit is generally marketed after storage, which is of great significance for regulating the market supply in the off-season of fruit production. Apple-fruit ripening, which culminates in desirable changes in structural and textural properties, is governed by a complex regulatory network. Much is known about ethylene as one of the most important factors promoting apple-fruit ripening. However, the dynamic interplay between phytohormones also plays an important part in apple-fruit ripening. Here, we review and evaluate the complex regulatory network concerning the action of phytohormones during apple-fruit ripening. Interesting future research areas are discussed.

A Method for Assaying of Protein Kinase Activity In Vivo and Its Use in Studies of Signal Transduction in Strawberry Fruit Ripening

Strawberry (Fragaria × ananassa) fruit ripening is regulated by a complex of cellular signal transduction networks, in which protein kinases are key components. Here, we report a relatively simple method for assaying protein kinase activity in vivo and specifically its application to study the kinase, FaMPK6, signaling in strawberry fruit. Green fluorescent protein (GFP)-tagged FaMPK6 was transiently expressed in strawberry fruit and after stimuli were applied to the fruit it was precipitated using an anti-GFP antibody. The precipitated kinase activity was measured in vitro using ³²P-ATP and myelin basic protein (MBP) as substrates. We also report that FaMPK6 is not involved in the abscisic acid (ABA) signaling cascade, which is closely associated with FaMPK6 signaling in other plant species. However, methyl jasmonate (MeJA), low temperature, and high salt treatments were all found to activate FaMPK6. Transient manipulation of FaMPK6 expression was observed to cause significant changes in the expression patterns of 2749 genes, of which 264 were associated with MeJA signaling. The data also suggest a role for FaMPK6 in modulating cell wall metabolism during fruit ripening. Taken together, the presented method is powerful and its use will contribute to a profound exploration to the signaling mechanism of strawberry fruit ripening.

SlNAC6, A NAC transcription factor, is involved in drought stress response and reproductive process in tomato

Tomato plants are susceptible to drought stress, but the mechanism involved in this process still remains poorly understood. In the present study, we demonstrated that SlNAC6, a nuclear-localized protein induced by exogenous abscisic acid (ABA) or polyethylene glycol (PEG) stress treatment, plays a positive role in tomato plant response to PEG stress. Down-regulation of SlNAC6 (SlNAC6-RNAi) resulted in a semidwarf phenotype, and the SlNAC6-RNAi lines showed reduced tolerance to PEG stress, exhibiting a higher water loss rate and degree of oxidative damage, as well as lower values of proline content and antioxidant enzyme activity, when compared with those in wild type (WT). In contrast, overexpression of SlNAC6 (SlNAC6-OE) leads to a significant delay of growth, and the SlNAC6-OE lines showed greatly enhanced tolerance to PEG stress concomitant with a lower water loss rate and degree of oxidative damage, as well as higher values of proline content and antioxidant enzyme activity. Further study showed that the transcription level of ABA signaling-related genes and the ABA content are respectively decreased or increased in SlNAC6-RNAi and SlNAC6-OE seedlings, as verified by multiple physiological parameters, such as stomatal conductance, water loss rate, seed germination, and root length. Moreover, overexpression of SlNAC6 can accelerate tomato fruit ripening. Collectively, this study demonstrates SlNAC6 exerts important roles in tomato development, drought stress response, and fruit ripening processes, some of them perhaps partly through modulating an ABA-mediated pathway, which implies SlNAC6 may hold the potential applications in improving agronomic traits of tomato or other Solanaceae crops.

Molecular basis for optimizing sugar metabolism and transport during fruit development

Sugars are fundamental metabolites synthesized in leaves and further delivered to fruit in fruit crops. They not only provide "sweetness" as fruit quality traits, but also function as signaling molecules to modulate the responses of fruit to environmental stimuli. Therefore, the understanding to the molecular basis for sugar metabolism and transport is crucial for improving fruit quality and dissecting responses to abiotic/biotic factors. Here, we provide a review for molecular components involved in sugar metabolism and transport, crosstalk with hormone signaling, and the roles of sugars in responses to abiotic and biotic stresses. Moreover, we also envisage the strategies for optimizing sugar metabolism during fruit quality maintenance.

Profiling Analysis of Volatile and Non-volatile Compounds in Vitis Vinifera Berries (cv. Chardonnay) and Spontaneous Bud Mutation

A novel clonal variety of Vitis vinifera was identified from “Chardonnay” using inter-simple sequence repeat (ISSR) markers and called “bud mutation. ” The metabolomic profiles in Chardonnay and bud mutation berries indicated essential differences in the expression of key genes in the pathways of 2-C-methyl-D-erythritol-4-phosphate (MEP) and lipoxygenase-hydroperoxide lyase (LOX-HPL). Bud mutation fruits also matured 10 days earlier than Chardonnay and have higher carotenoid, sugar, and acidic compound contents. Furthermore, the gene expression was examined in the biosynthetic pathways of two ripening-associated hormones, abscisic acid (ABA) and jasmonic acid (JA), which significantly increased in bud mutation compared with the Chardonnay fruit. The synthesis and metabolism of amino acids, terpenes, fatty acids, volatile components, and specialized metabolites significantly increased in bud mutation. Therefore, in comparison with Chardonnay, bud mutation is considered a highly aroma-producing grape variety for an improvement in the beverage industry.

Differential Phenolic Compounds and Hormone Accumulation Patterns between Early- and Mid-Maturing Sweet Cherry (Prunus avium L.) Cultivars during Fruit Development and Ripening

Color acquisition is one of the most distinctive features of fruit development and ripening processes. The color red is closely related to the accumulation of polyphenolic compounds, mainly anthocyanins, during sweet cherry fruit maturity. In non-climacteric fruit species like sweet cherry, the maturity process is mainly controlled by the phytohormone abscisic acid (ABA), though other hormones may also play a role. However, the coordinated stage-specific production of polyphenolic compounds and their relation with hormone content variations have not been studied in depth in sweet cherry fruits. To further understand the accumulation dynamics of these compounds (hormones and metabolites) during fruit development, two sweet cherry cultivars ("Lapins" and "Glenred") with contrasting maturity timing phenotypes were analyzed using targeted metabolic analysis. The ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) approach revealed that phenolic acids, flavonols, and flavan-3-ols accumulated mainly until the straw-yellow stage in the early-maturing cultivar, while accumulation was mainly at the green stage in the mid-maturing cultivar, suggesting a cultivar-dependent stage-specific production of secondary metabolites. In the mid-maturing cultivar, anthocyanins were detected only from the red stage onward, whereas detection began at the pink stage in the early-maturing cultivar. ABA negatively correlated (p-value < 0.05) with the flavonols and flavan-3-ols in both cultivars. ABA and anthocyanin content increased at the same time in the early-season cultivar. In contrast, anthocyanins accumulated and the pink color initiation started several days after the ABA increase in the mid-maturing cultivar. Differential accumulation patterns of GA₄, a ripening antagonizing hormone, between the cultivars could explain this difference. These results showed that both red-colored cultivars presented different accumulation dynamics of phenolic compounds and plant hormones during fruit development, suggesting underlying differences in the sweet cherry fruit color evolution.

Unraveling the Complex Hybrid Ancestry and Domestication History of Cultivated Strawberry

Cultivated strawberry (Fragaria × ananassa) is one of our youngest domesticates, originating in early eighteenth-century Europe from spontaneous hybrids between wild allo-octoploid species (Fragaria chiloensis and Fragaria virginiana). The improvement of horticultural traits by 300 years of breeding has enabled the global expansion of strawberry production. Here, we describe the genomic history of strawberry domestication from the earliest hybrids to modern cultivars. We observed a significant increase in heterozygosity among interspecific hybrids and a decrease in heterozygosity among domesticated descendants of those hybrids. Selective sweeps were found across the genome in early and modern phases of domestication—59–76% of the selectively swept genes originated in the three less dominant ancestral subgenomes. Contrary to the tenet that genetic diversity is limited in cultivated strawberry, we found that the octoploid species harbor massive allelic diversity and that F. × ananassa harbors as much allelic diversity as either wild founder. We identified 41.8 M subgenome-specific DNA variants among resequenced wild and domesticated individuals. Strikingly, 98% of common alleles and 73% of total alleles were shared between wild and domesticated populations. Moreover, genome-wide estimates of nucleotide diversity were virtually identical in F. chiloensis,F. virginiana, and F. × ananassa (π = 0.0059–0.0060). We found, however, that nucleotide diversity and heterozygosity were significantly lower in modern F. × ananassa populations that have experienced significant genetic gains and have produced numerous agriculturally important cultivars.

The NAC transcription factor FaRIF controls fruit ripening in strawberry

In contrast to climacteric fruits such as tomato, the knowledge on key regulatory genes controlling the ripening of strawberry, a nonclimacteric fruit, is still limited. NAC transcription factors (TFs) mediate different developmental processes in plants. Here, we identified and characterized Ripening Inducing Factor (FaRIF), a NAC TF that is highly expressed and induced in strawberry receptacles during ripening. Functional analyses based on stable transgenic lines aimed at silencing FaRIF by RNA interference, either from a constitutive promoter or the ripe receptacle-specific EXP2 promoter, as well as overexpression lines showed that FaRIF controls critical ripening-related processes such as fruit softening and pigment and sugar accumulation. Physiological, metabolome, and transcriptome analyses of receptacles of FaRIF-silenced and overexpression lines point to FaRIF as a key regulator of strawberry fruit ripening from early developmental stages, controlling abscisic acid biosynthesis and signaling, cell-wall degradation, and modification, the phenylpropanoid pathway, volatiles production, and the balance of the aerobic/anaerobic metabolism. FaRIF is therefore a target to be modified/edited to control the quality of strawberry fruits.

Expression profiling of endo-xylanases during ripening of strawberry cultivars with contrasting softening rates. Influence of postharvest and hormonal treatments

BACKGROUND

Softening is one of the main features that determine fruit quality during strawberry (Fragaria x ananassa, Duch.) ripening and storage. Being closely related to textural changes, the molecular and biochemical bases underlying strawberry cell-wall metabolism is a matter of interest. Here we investigated the abundance of transcripts encoding putative strawberry endo-xylanases in plant tissues, during fruit ripening and under postharvest and hormonal treatments. Total xylanase activity and expression of related genes in strawberry varieties with contrasting firmness were analyzed.

RESULTS

FaXynA and FaXynC mRNA abundance was significantly higher than FaXynB in each plant tissue studied. Higher total xylanase activity was detected at the end of the ripening of the softer cultivar ('Toyonoka') in comparison with the firmer one ('Camarosa'), correlating with the abundance of FaXynA and FaXynC transcripts. Postharvest 1-methylcyclopropene treatment up-regulated FaXynA and FaXynC expressions. FaXynC mRNA abundance decreased with heat treatment but the opposite was observed for FaXynA. Calcium chloride treatment down-regulated FaXynA and FaXynC expression. Both genes responded differently to plant growth regulators' exposure. FaXynC expression was down-regulated by auxins and gibberellins treatment and up-regulated by abscisic acid. FaXynA was up-regulated by auxins, while no changes in mRNA levels were evident by abscisic acid and gibberellins treatment. Ethephon exposure did not change FaXynA and FaXynC expressions.

CONCLUSION

New knowledge about the presence of xylanases in ripening strawberry fruit and their response to postharvest and hormonal treatments is provided. Our findings suggest a role for endo-xylanases in hemicelluloses depolymerization and possibly in strawberry fruit softening. © 2020 Society of Chemical Industry.

High-spatiotemporal-resolution transcriptomes provide insights into fruit development and ripening in Citrus sinensis

Citrus fruit has a unique structure with soft leathery peel and pulp containing vascular bundles and several segments with many juice sacs. The function and morphology of each fruit tissue are different. Therefore, analysis at the organ-wide or mixed-tissue level inevitably obscures many tissue-specific phenomena. High-throughput RNA sequencing was used to profile Citrus sinensis fruit development based on four fruit tissue types and six development stages from young fruits to ripe fruits. Using a coexpression network analysis, modules of coexpressed genes and hub genes of tissue-specific networks were identified. Of particular, importance is the discovery of the regulatory network of phytohormones during citrus fruit development and ripening. A model was proposed to illustrate how ABF2 mediates the ABA signalling involved in sucrose transport, chlorophyll degradation, auxin homoeostasis, carotenoid and ABA biosynthesis, and cell wall metabolism during citrus fruit development. Moreover, we depicted the detailed spatiotemporal expression patterns of the genes involved in sucrose and citric acid metabolism in citrus fruit and identified several key genes that may play crucial roles in sucrose and citric acid accumulation in the juice sac, such as SWEET15 and CsPH8. The high spatial and temporal resolution of our data provides important insights into the molecular networks underlying citrus fruit development and ripening.

Effect of Exogenous Auxin Treatment on Cell Wall Polymers of Strawberry Fruit

The role of auxin in the fruit-ripening process during the early developmental stages of commercial strawberry fruits (Fragaria x ananassa) has been previously described, with auxin production occurring in achenes and moving to the receptacle. Additionally, fruit softening is a consequence of the depolymerization and solubilization of cell wall components produced by the action of a group of proteins and enzymes. The aim of this study was to compare the effect of exogenous auxin treatment on the physiological properties of the cell wall-associated polysaccharide contents of strawberry fruits. We combined thermogravimetric (TG) analysis with analyses of the mRNA abundance, enzymatic activity, and physiological characteristics related to the cell wall. The samples did not show a change in fruit firmness at 48 h post-treatment; by contrast, we showed changes in the cell wall stability based on TG and differential thermogravimetric (DTG) analysis curves. Less degradation of the cell wall polymers was observed after auxin treatment at 48 h post-treatment. The results of our study indicate that auxin treatment delays the cell wall disassembly process in strawberries.

N6-methyladenosine RNA modification regulates strawberry fruit ripening in an ABA-dependent manner

BACKGROUND

Epigenetic mark such as DNA methylation plays pivotal roles in regulating ripening of both climacteric and non-climacteric fruits. However, it remains unclear whether mRNA m6A methylation, which has been shown to regulate ripening of the tomato, a typical climacteric fruit, is functionally conserved for ripening control among different types of fruits.

RESULTS

Here we show that m6A methylation displays a dramatic change at ripening onset of strawberry, a classical non-climacteric fruit. The m6A modification in coding sequence (CDS) regions appears to be ripening-specific and tends to stabilize the mRNAs, whereas m6A around the stop codons and within the 3' untranslated regions is generally negatively correlated with the abundance of associated mRNAs. We identified thousands of transcripts with m6A hypermethylation in the CDS regions, including those of NCED5, ABAR, and AREB1 in the abscisic acid (ABA) biosynthesis and signaling pathway. We demonstrate that the methyltransferases MTA and MTB are indispensable for normal ripening of strawberry fruit, and MTA-mediated m6A modification promotes mRNA stability of NCED5 and AREB1, while facilitating translation of ABAR.

CONCLUSION

Our findings uncover that m6A methylation regulates ripening of the non-climacteric strawberry fruit by targeting the ABA pathway, which is distinct from that in the climacteric tomato fruit.

Comparative transcriptome analysis reveals key genes associated with pigmentation in radish (Raphanus sativus L.) skin and flesh

Radish (Raphanus sativus) is an important vegetable worldwide that exhibits different flesh and skin colors. The anthocyanins responsible for the red and purple coloring in radishes possess nutritional value and pharmaceutical potential. To explore the structural and regulatory networks related to anthocyanin biosynthesis and identify key genes, we performed comparative transcriptome analyses of the skin and flesh of six colored radish accessions. The transcript profiles showed that each accession had a species-specific transcript profile. For radish pigmentation accumulation, the expression levels of anthocyanin biosynthetic genes (RsTT4, RsC4H, RsTT7, RsCCOAMT, RsDFR, and RsLDOX) were significantly upregulated in the red- and purple-colored accessions, but were downregulated or absent in the white and black accessions. The correlation test, combined with metabolome (PCC > 0.95), revealed five structural genes (RsTT4, RsDFR, RsCCOAMT, RsF3H, and RsBG8L) and three transcription factors (RsTT8-1, RsTT8-2, and RsPAR1) to be significantly correlated with flavonoids in the skin of the taproot. Four structural genes (RsBG8L, RsDFR, RsCCOAMT, and RsLDOX) and nine transcription factors (RsTT8-1, RsTT8-2, RsMYB24L, RsbHLH57, RsPAR2L, RsbHLH113L, RsOGR3L, RsMYB24, and RsMYB34L) were found to be significantly correlated with metabolites in the flesh of the taproot. This study provides a foundation for future studies on the gene functions and genetic diversity of radish pigmentation and should aid in the cultivation of new valuable radish varieties.

Genome-Wide and Comprehensive Analysis of the Multiple Stress-Related CAF1 (CCR4-Associated Factor 1) Family and Its Expression in Poplar

Poplar is one of the most widely used tree in afforestation projects. However, it is susceptible to abiotic and biotic stress. CCR4-associated factor 1 (CAF1) is a major member of CCR4-NOT, and it is mainly involved in transcriptional regulation and mRNA degradation in eukaryotes. However, there are no studies on the molecular phylogeny and expression of the CAF1 gene in poplar. In this study, a total of 19 PtCAF1 genes were identified in the Populus trichocarpa genome. Phylogenetic analysis of the PtCAF1 gene family was performed with two closely related species (Arabidopsis thaliana and Oryza sativa) to investigate the evolution of the PtCAF1 gene. The tissue expression of the PtCAF1 gene showed that 19 PtCAF1 genes were present in different tissues of poplar. Additionally, the analysis of the expression of the PtCAF1 gene showed that the CAF1 family was up-regulated to various degrees under biotic and abiotic stresses and participated in the poplar stress response. The results of our study provide a deeper understanding of the structure and function of the PtCAF1 gene and may contribute to our understanding of the molecular basis of stress tolerance in poplar.

Molecular and Hormonal Mechanisms Regulating Fleshy Fruit Ripening

This article focuses on the molecular and hormonal mechanisms underlying the control of fleshy fruit ripening and quality. Recent research on tomato shows that ethylene, acting through transcription factors, is responsible for the initiation of tomato ripening. Several other hormones, including abscisic acid (ABA), jasmonic acid (JA) and brassinosteroids (BR), promote ripening by upregulating ethylene biosynthesis genes in different fruits. Changes to histone marks and DNA methylation are associated with the activation of ripening genes and are necessary for ripening initiation. Light, detected by different photoreceptors and operating through ELONGATED HYPOCOTYL 5(HY5), also modulates ripening. Re-evaluation of the roles of 'master regulators' indicates that MADS-RIN, NAC-NOR, Nor-like1 and other MADS and NAC genes, together with ethylene, promote the full expression of genes required for further ethylene synthesis and change in colour, flavour, texture and progression of ripening. Several different types of non-coding RNAs are involved in regulating expression of ripening genes, but further clarification of their diverse mechanisms of action is required. We discuss a model that integrates the main hormonal and genetic regulatory interactions governing the ripening of tomato fruit and consider variations in ripening regulatory circuits that operate in other fruits.

An effector-reporter system to study cellular signal transduction in strawberry fruit (Fragaria ananassa)

An effector-reporter system is a powerful tool used to study cellular signal transduction, but this technique has been traditionally used in protoplasts. A similar system to study cellular signal transduction in fruits has not yet been established. In this study, we aimed to establish an effector-reporter system for strawberry fruit, a model nonclimacteric fruit. We first investigated the characteristics of transient gene expression in strawberry fruits and found marked variation in gene expression levels among individual fruits, and this variation has complicated the establishment of a technical system. To overcome this difficulty, we investigated a sampling strategy based on a statistical analysis of the activity pattern of four different reporters (GUS, GFP, FLuc, and RLuc) among individual fruits and combinations of pairs of reporters (GUS/GFP and RLuc/FLuc). Based on an optimized sampling strategy, we finally established a step-by step protocol for the effector/reporter assay. Using FaMYB10 and FaWRKY71 as the effectors and GUS driven by the FaCHS promoter as the reporter, we demonstrated that this effector/reporter system was practical and reliable. This effector/reporter technique will contribute to an in-depth exploration of the signaling mechanism for the regulation of strawberry fruit ripening.

The long noncoding RNA FRILAIR regulates strawberry fruit ripening by functioning as a noncanonical target mimic

Long noncoding RNAs (lncRNAs) are emerging as important regulators in plant development, but few of them have been functionally characterized in fruit ripening. Here, we have identified 25,613 lncRNAs from strawberry ripening fruits based on RNA-seq data from poly(A)-depleted libraries and rRNA-depleted libraries, most of which exhibited distinct temporal expression patterns. A novel lncRNA, FRILAIR harbours the miR397 binding site that is highly conserved in diverse strawberry species. FRILAIR overexpression promoted fruit maturation in the Falandi strawberry, which was consistent with the finding from knocking down miR397, which can guide the mRNA cleavage of both FRILAIR and LAC11a (encoding a putative laccase-11-like protein). Moreover, LAC11a mRNA levels were increased in both FRILAIR overexpressing and miR397 knockdown fruits, and accelerated fruit maturation was also found in LAC11a overexpressing fruits. Overall, our study demonstrates that FRILAIR can act as a noncanonical target mimic of miR397 to modulate the expression of LAC11a in the strawberry fruit ripening process.

Fig fruit ripening is regulated by the interaction between ethylene and abscisic acid

Fleshy fruit ripening is typically regulated by ethylene in climacteric fruits and abscisic acid (ABA) in non-climacteric fruits. Common fig (Ficus carica) shows a dual-ripening mechanism, which is not fully understood. Here, we detected separate peaks of ethylene and ABA in fig fruits at the onset- and on-ripening stages, in conjunction with a sharp rise in glucose and fructose contents. In a newly-designed split-fruit system, exogenous ethylene failed to rescue fluridone-inhibited fruit ripening, whereas exogenous ABA rescued 2-amino-ethoxy-vinyl glycine (AVG)-inhibited fruit ripening. Transcriptome analysis revealed changes in the expression of genes key to both ABA and ethylene biosynthesis and perception during fig fruit ripening. At the de-greening stage, downregulation of FcACO2 or FcPYL8 retarded ripening, but downregulation of FcETR1/2 did not; unexpectedly, downregulation of FcAAO3 promoted ripening, but it inhibited ripening only before the de-greening stage. Furthermore, we detected an increase in ethylene emissions in the FcAAO3-RNAi ripening fruit and a decrease in ABA levels in the FcACO2-RNAi unripening fruit. Importantly, FcPYL8 can bind to ABA, suggesting that it functions as an ABA receptor. Our findings support the hypothesis that ethylene regulates the fig fruit ripening in an ABA-dependent manner. We propose a model for the role of the ABA-ethylene interaction in climacteric/non-climacteric processes.

Double NCED isozymes control ABA biosynthesis for ripening and senescent regulation in peach fruits

During ripening, peach fruits (Prunus persica L. Batsch) rapidly progress to the senescent stage, resulting in a brief shelf life. Abscisic acid (ABA) plays an important role in regulating the ripening process, both in climacteric and non-climacteric fruits. A key enzyme for ABA biosynthesis in higher plants is 9-cis-epoxycarotenoid dioxygenase (NCED). In this study, two NCED isozymes, PpNCED1 and PpNCED5, were identified in peach fruits. While both NCED genes had similar transcriptional patterns (up-regulation) at the beginning of peach ripening, PpNCED5 showed a consistently lower expression level than PpNCED1. During the post-harvest stage, gene expression of PpNCED1 declined, while PpNCED5 expression increased relative to PpNCED1 expression. Considering the dynamic process of ABA accumulation during fruit ripening and senescence in peach, this study indicates that both NCED genes cooperatively control ABA biosynthesis in peach fruits. Moreover, spatio-temporal expression and transcriptional response to hormone and abiotic stress suggested that there is functional divergence between PpNCED1 and PpNCED5 genes in peach. A carotenoid-rich callus system was used to verify the function of PpNCED1 and PpNCED5. In the transgenic callus system, both PpNCED1 and PpNCED5 isozymes promoted ABA biosynthesis, which likely accelerated cell senescence through activating ROS signals. The results from this study provide evidence supporting an ABA biosynthetic regulation process via the two NCED genes in peach fruit, and suggest a mechanism of ABA-induced fruit ripening and senescence.

Grafting Delays Watermel on Fruit Ripening by Altering Gene Expression of ABA Centric Phytohormone Signaling

Grafting cultivation is implemented worldwide mainly to resist abiotic and biotic stresses and is an effective method to improve watermelon production. However, grafting may affect fruit development and quality. In our experiment, pumpkin-grafted (PG) watermelon fruits developed slower and the ripening period was extended compared to self-grafted (SG) fruits. We found that the concentrations of abscisic acid (ABA) among endogenous phytohormones were dramatically reduced by pumpkin grafting. In order to understand these changes at the gene expression level, we performed a comprehensive analysis of the fruit flesh transcriptomes between PG and SG during fruit development and ripening. A total of 1,675 and 4,102 differentially expressed genes (DEGs) were identified between PG and SG. Further functional enrichment analysis revealed that these DEGs were associated with carbohydrate biosynthesis, phytohormone signaling transmission, and cell wall metabolism categories. ABA centric phytohormone signaling and fruit quality-related genes including ABA receptor, PP2C proteins, AP2-EREBP transcription factors, sucrose transporter, and carotenoid isomerase were co-expressed with fruit ripening. These results provide the valuable resource for understanding the mechanism of pumpkin grafting effect on watermelon fruit ripening and quality development.

Contrasting dynamics in abscisic acid metabolism in different Fragaria spp. during fruit ripening and identification of the enzymes involved

Abscisic acid (ABA) is a key hormone in non-climacteric Fragaria spp, regulating multiple physiological processes throughout fruit ripening. Its concentration increases during ripening, and it promotes fruit (receptacle) development. However, its metabolism in the fruit is largely unknown. We analyzed the concentrations of ABA and its catabolites at different developmental stages of strawberry ripening in diploid and octoploid genotypes and identified two functional ABA-glucosyltransferases (FvUGT71A49 and FvUGT73AC3) and two regiospecific ABA-8'-hydroxylases (FaCYP707A4a and FaCYP707A1/3). ABA-glucose ester content increased during ripening in diploid F. vesca varieties but decreased in octoploid F.×ananassa. Dihydrophaseic acid content increased throughout ripening in all analyzed receptacles, while 7'-hydroxy-ABA and neo-phaseic acid did not show significant changes during ripening. In the studied F. vesca varieties, the receptacle seems to be the main tissue for ABA metabolism, as the concentration of ABA and its metabolites in the receptacle was generally 100 times higher than in achenes. The accumulation patterns of ABA catabolites and transcriptomic data from the literature show that all strawberry fruits produce and metabolize considerable amounts of the plant hormone ABA during ripening, which is therefore a conserved process, but also illustrate the diversity of this metabolic pathway which is species, variety, and tissue dependent.

ABI5 regulates ABA-induced anthocyanin biosynthesis by modulating the MYB1-bHLH3 complex in apple

Abscisic acid (ABA) induces anthocyanin biosynthesis in many plant species. However, the molecular mechanism of ABA-regulated anthocyanin biosynthesis remains unclear. As a crucial regulator of ABA signaling, ABSCISIC ACID-INSENSITIVE5 (ABI5) is involved in many aspects of plant growth and development, yet its regulation of anthocyanin biosynthesis has not been elucidated. In this study, we found that MdABI5, the apple homolog of Arabidopsis ABI5, positively regulated ABA-induced anthocyanin biosynthesis. A series of biochemical tests showed that MdABI5 specifically interacts with basic helix-loop-helix 3 (MdbHLH3), a positive regulator of anthocyanin biosynthesis. MdABI5 enhanced the binding of MdbHLH3 to its target genes dihydroflavonol 4-reductase (MdDFR) and UDP flavonoid glucosyl transferase (MdUF3GT). In addition, MdABI5 directly bound to the promoter of MdbHLH3 to activate its expression. Moreover, MdABI5 enhanced ABA-promoted interaction between MdMYB1 and MdbHLH3. Finally, antisense suppression of MdbHLH3 significantly reduced anthocyanin biosynthesis promoted by MdABI5, indicating that MdABI5-promoted anthocyanin biosynthesis was dependent on MdbHLH3. Taken together, our data suggest that MdABI5 plays a positive role in ABA-induced anthocyanin biosynthesis by modulating the MdbHLH3-MdMYB1 complex. Our work broadens the regulatory network of ABA-mediated anthocyanin biosynthesis, providing new insights to further study the transcriptional regulatory mechanisms behind this process.

Grapevine U-Box E3 Ubiquitin Ligase VlPUB38 Negatively Regulates Fruit Ripening by Facilitating Abscisic-Aldehyde Oxidase Degradation

The plant U-box E3 ubiquitin ligase-mediated ubiquitin/26S proteasome degradation system plays a key role in plant growth and development. Previously identified as a member of the grape PUB gene family, PUB38 was shown to participate in the berry-ripening progress. Here, we demonstrate that the E3 ligase VlPUB38 mediates abscisic acid (ABA) synthesis via 26S proteasome degradation and its involvement in regulating fruit-ripening processes. Strawberry-overexpressing VlPUB38 lines displayed obvious inhibition of mature phenotype, and this was rescued by exogenous ABA treatment and MG132. Post-ABA treatment, expression levels of ABA response-related genes in VlPUB38-overexpressed Arabidopsis significantly exceeded controls. Strawberry and Arabidopsis ectopic expression assays suggest that VlPUB38 negatively regulates fruit ripening in an ABA-dependent manner. Moreover, VlPUB38 has ubiquitin ligase activity, which depends on the U-box-conserved domain. VlPUB38 interacts with abscisic-aldehyde oxidase (VlAAO), targeting VlAAO proteolysis via the 26S proteasome system. These results indicate that VlPUB38 negatively regulates grape fruit ripening by mediating the degradation of key factor VlAAO in the ABA synthesis pathway.

Auxin-Responsive R2R3-MYB Transcription Factors HcMYB1 and HcMYB2 Activate Volatile Biosynthesis in Hedychium coronarium Flowers

Auxin, an important plant hormone, induces the biosynthesis of various secondary metabolites by modulating the expression of auxin-responsive genes. In the ornamental plant Hedychium coronarium, linalool and methyl benzoate are biosynthesized by the terpene synthase (TPS) HcTPS5 and the benzoic/salicylic acid methyltransferase (BSMT) HcBSMT2, respectively. However, the transcriptional regulation of this process remains unclear. Here, we identified and functionally characterized the R2R3-MYB transcription factors HcMYB1 and HcMYB2 in regulating the biosynthesis of these floral aroma compounds. HcMYB1 and HcMYB2 are specifically expressed in flowers, their expression is correlated with the emission of volatile compounds in flowers, and is induced by auxin. Moreover, HcMYB1 and HcMYB2 interact with the HcBSMT2 promoter region. HcMYB2 activates the expression of the linalool synthase gene HcTPS5. In flowers with HcMYB1 or HcMYB2 silenced, the levels of floral scent compounds were significantly reduced, and HcBSMT2 and HcTPS5 were downregulated compared with the wild type. Moreover, HcMYB1 form protein-protein interaction with key scent-related HcIAA4 protein to regulate floral aroma production. Taken together, these results indicate that HcMYB1 and HcMYB2 play crucial roles in regulating the formation of scent compounds in Hedychium coronarium (H. coronarium) flowers in response to auxin signaling.

MdKNOX19, a class II knotted-like transcription factor of apple, plays roles in ABA signalling/sensitivity by targeting ABI5 during organ development

The ABI5 transcription factor, which is a core component of the ABA signaling pathway, affects various plant processes, including seed development and germination and responses to environmental cues. The knotted1-like homeobox (KNOX) transcription factor has crucial functions related to plant development, including the regulation of various hormones. In this study, an ABA-responsive KNOX gene, MdKNOX19, was identified in apple (Malus domestica). The overexpression of MdKNOX19 increased the ABA sensitivity of apple calli, resulting in a dramatic up-regulation in the transcription of the Arabidopsis ABI5-like MdABI5 gene. Additionally, MdKNOX19 overexpression in Micro-Tom adversely affected fruit size and seed yield as well as enhanced ABA sensitivity and up-regulated SlABI5 transcription during seed germination and early seedling development. An examination of MdKNOX19-overexpressing Arabidopsis plants also revealed severe defects in seed development and up-regulated expression of ABA-responsive genes. Furthermore, we further confirmed that MdKNOX19 binds directly to the MdABI5 promoter to activate expression. Our findings suggest MdKNOX19 is a positive regulator of ABI5 expression, and the conserved module MdKNOX19-MdABI5-ABA may contribute to organ development.

Gibberellic Acid Modifies the Transcript Abundance of ABA Pathway Orthologs and Modulates Sweet Cherry (Prunus avium) Fruit Ripening in Early- and Mid-Season Varieties

Several phytohormones modulate ripening in non-climacteric fruits, which is triggered by abscisic acid (ABA). Gibberellins (GAs) are present during the onset of ripening in sweet cherry fruits, and exogenous gibberellic acid (GA3) application delays ripening, though this effect is variety-dependent. Although an ABA accumulation delay has been reported following GA3 treatment, the mechanism by which GA modulates this process has not been investigated at the molecular level in sweet cherry. Therefore, the aim of this work is to analyze the effect of GA3 on the fruit ripening process and the transcript levels of ABA pathway orthologs in two varieties having different maturity time phenotypes. The early-season variety had a rapid transition from yellow to pink fruit color, whereas pink color initiation took longer in the mid-season variety. GA3 increased the proportion of lighter colored fruits at ripeness in both varieties, but it produced a delay in IAD-a ripening index-only in the mid-season variety. This delay was accompanied by an increased transcript abundance of PavPP2Cs, which are putative negative regulators of the ABA pathway. On the other hand, the early-season variety had increased expression of PavCYP707A2-a putative ABA catabolic gene-, and reduced transcript levels of PavPP2Cs and SnRK2s after the GA3 treatment. Together these results show that GA modulates fruit ripening, exerting its action in part by interacting with the ABA pathway in sweet cherry.

Visualizing the distribution of strawberry plant metabolites at different maturity stages by MALDI-TOF imaging mass spectrometry

This study aimed to visualize differences in the distribution of citric acid, soluble sugars, and anthocyanins in strawberries at four different maturity stages (green to red strawberries) by matrix-assisted laser desorption/ionization time-of-flight imaging mass spectrometry (MALDI-TOF IMS). Results demonstrated citric acid and sugars are evenly distributed in the entire fruit at all maturity stages, while most of anthocyanins are mainly located in the periphery of fruit with increased abundance in red strawberries, indicating a correlation with the colour attributes. Sugar in red strawberries (11.92 brix) increased by two-fold compared to the green ones (6.23 brix). Finally, absolute quantitation of each compound from HPLC analyses support the quantitative results from MALDI-TOF IMS. The results provide a deeper understanding in the changes and distribution of phytochemicals during the growth of strawberries, and demonstrates the usefulness of IMS for plant breeding and postharvest technology.

Reshaping of the Arabidopsis thaliana Proteome Landscape and Co-regulation of Proteins in Development and Immunity

Proteome remodeling is a fundamental adaptive response, and proteins in complexes and functionally related proteins are often co-expressed. Using a deep sampling strategy we define core proteomes of Arabidopsis thaliana tissues with around 10 000 proteins per tissue, and absolutely quantify (copy numbers per cell) nearly 16 000 proteins throughout the plant lifecycle. A proteome-wide survey of global post-translational modification revealed amino acid exchanges pointing to potential conservation of translational infidelity in eukaryotes. Correlation analysis of protein abundance uncovered potentially new tissue- and age-specific roles of entire signaling modules regulating transcription in photosynthesis, seed development, and senescence and abscission. Among others, the data suggest a potential function of RD26 and other NAC transcription factors in seed development related to desiccation tolerance as well as a possible function of cysteine-rich receptor-like kinases (CRKs) as ROS sensors in senescence. All of the components of ribosome biogenesis factor (RBF) complexes were found to be co-expressed in a tissue- and age-specific manner, indicating functional promiscuity in the assembly of these less-studied protein complexes in Arabidopsis.Furthermore, we characterized detailed proteome remodeling in basal immunity by treating Arabidopsis seeldings with flg22. Through simultaneously monitoring phytohormone and transcript changes upon flg22 treatment, we obtained strong evidence of suppression of jasmonate (JA) and JA-isoleucine (JA-Ile) levels by deconjugation and hydroxylation by IAA-ALA RESISTANT3 (IAR3) and JASMONATE-INDUCED OXYGENASE 2 (JOX2), respectively, under the control of JASMONATE INSENSITIVE 1 (MYC2), suggesting an unrecognized role of a new JA regulatory switch in pattern-triggered immunity. Taken together, the datasets generated in this study present extensive coverage of the Arabidopsis proteome in various biological scenarios, providing a rich resource available to the whole plant science community.

Comparative transcriptome and metabolome analyses of two strawberry cultivars with different storability

Postharvest storability is an important trait for breeding strawberry (Fragaria × ananassa Duch.). We evaluated the postharvest fruit quality of five strawberry cultivars (‘Durihyang’, ‘Kingsberry’, ‘Maehyang’, ‘Seolhyang’, and ‘Sunnyberry’) and identified differences in their fruit ripening during the transition from the big-green to fully-red stage between two cultivars with the highest (‘Sunnyberry’) and lowest (‘Kingsberry’) storability, using comparative transcriptome and -metabolome analysis. The differentially expressed genes revealed transcriptome changes related to anthocyanin biosynthesis and cell walls. Consistently, the metabolites of both cultivars showed general changes during ripening along with cultivar-specific characteristics in sugar and amino acid profiles. To identify the genes responsible for storability differences, we surveyed the expression of transcription factors, and found that the expression levels of WRKY31, WRKY70, and NAC83 correlated with delayed senescence and increased storability. Among them, the expression levels of NAC83, and its downstream target genes, in the five cultivars suggested that NAC83 expression can be used to predict postharvest strawberry fruit storability.

Allelic Variation of MYB10 Is the Major Force Controlling Natural Variation in Skin and Flesh Color in Strawberry (Fragaria spp.) Fruit

The fruits of diploid and octoploid strawberry (Fragaria spp) show substantial natural variation in color due to distinct anthocyanin accumulation and distribution patterns. Anthocyanin biosynthesis is controlled by a clade of R2R3 MYB transcription factors, among which MYB10 is the main activator in strawberry fruit. Here, we show that mutations in MYB10 cause most of the variation in anthocyanin accumulation and distribution observed in diploid woodland strawberry (F. vesca) and octoploid cultivated strawberry (F ×ananassa). Using a mapping-by-sequencing approach, we identified a gypsy-transposon in MYB10 that truncates the protein and knocks out anthocyanin biosynthesis in a white-fruited F. vesca ecotype. Two additional loss-of-function mutations in MYB10 were identified among geographically diverse white-fruited F. vesca ecotypes. Genetic and transcriptomic analyses of octoploid Fragaria spp revealed that FaMYB10-2, one of three MYB10 homoeologs identified, regulates anthocyanin biosynthesis in developing fruit. Furthermore, independent mutations in MYB10-2 are the underlying cause of natural variation in fruit skin and flesh color in octoploid strawberry. We identified a CACTA-like transposon (FaEnSpm-2) insertion in the MYB10-2 promoter of red-fleshed accessions that was associated with enhanced expression. Our findings suggest that cis-regulatory elements in FaEnSpm-2 are responsible for enhanced MYB10-2 expression and anthocyanin biosynthesis in strawberry fruit flesh.

PaMADS7, a MADS-box transcription factor, regulates sweet cherry fruit ripening and softening

E-class MADS-box transcription factors, SEPALLATA (SEP) genes have an important role in floral organ initiation and development and fruit ripening. In this study, four sweet cherry SEP-like genes (PaMADS2, PaMADS4, PaMADS5, and PaMADS7) were cloned and functionally characterized. Gene expression analysis showed that the differential expression levels of PaMADS4 and PaMADS7 coincided with fruit ripening, and expression of PaMADS2 and PaMADS5 did not. Expression of PaMADS7 was affected by ABA and IAA. Subcellular localization assay demonstrated that four sweet cherry SEP-like proteins were all localized inside the nucleus. Silencing PaMADS7 using TRV-mediated virus-induced gene silencing inhibited fruit ripening and influenced major ripening-related physiological processes, such as ABA content, soluble sugar contents, fruit firmness, and anthocyanin content, as well as expression of ripening-related genes. In addition, silencing of PaMADS7 induced phenotype defects that suppressed fruit ripening, which were rescued by exogenous ABA. Furthermore, yeast one-hybrid assay (Y1H) and transient expression analyses revealed that PaMADS7 directly binds to the promoter of PaPG1, which is involved in sweet cherry fruit softening, and positively activated PaPG1expression. These results showed that PaMADS7 is an indispensable positive regulator of sweet cherry fruit ripening and softening.