A tomato LATERAL ORGAN BOUNDARIES transcription factor, SlLOB1, predominantly regulates cell wall and softening components of ripening

A tomato fruit ripening–specific transcription factor, SlLOB1 predominantly influences fruit cell wall–related gene regulation and textural changes during fruit maturation and thus is distinct from broadly acting ripening transcription factors described to date that influence many ripening processes. As such, SlLOB1 is an intermediate regulator primarily influencing a physiological subdomain of the overall ripening transition.

Fruit softening is a key component of the irreversible ripening program, contributing to the palatability necessary for frugivore-mediated seed dispersal. The underlying textural changes are complex and result from cell wall remodeling and changes in both cell adhesion and turgor. While a number of transcription factors (TFs) that regulate ripening have been identified, these affect most canonical ripening-related physiological processes. Here, we show that a tomato fruit ripening–specific LATERAL ORGAN BOUNDRIES (LOB) TF, SlLOB1, up-regulates a suite of cell wall–associated genes during late maturation and ripening of locule and pericarp tissues. SlLOB1 repression in transgenic fruit impedes softening, while overexpression throughout the plant under the direction of the 35s promoter confers precocious induction of cell wall gene expression and premature softening. Transcript and protein levels of the wall-loosening protein EXPANSIN1 (EXP1) are strongly suppressed in SlLOB1 RNA interference lines, while EXP1 is induced in SlLOB1-overexpressing transgenic leaves and fruit. In contrast to the role of ethylene and previously characterized ripening TFs, which are comprehensive facilitators of ripening phenomena including softening, SlLOB1 participates in a regulatory subcircuit predominant to cell wall dynamics and softening.

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.

Semi-dominant effects of a novel ripening inhibitor (rin) locus allele on tomato fruit ripening

The tomato (Solanum lycopersicum) ripening inhibitor (rin) mutation completely represses fruit ripening, as rin fruits fail to express ripening-associated genes and remain green and firm. Moreover, heterozygous rin fruits (rin/+) ripen normally but have extended shelf life, an important consideration for this perishable fruit crop; therefore, heterozygous rin has been widely used to breed varieties that produce red tomatoes with improved shelf life. We previously used CRISPR/Cas9 to produce novel alleles at the rin locus. The wild-type allele RIN encodes a MADS-box transcription factor and the novel allele, named as rinG2, generates an early stop codon, resulting in C-terminal truncation of the transcription factor. Like rin fruits, rinG2 fruits exhibit extended shelf life, but unlike rin fruits, which remain yellow-green even after long-term storage, rinG2 fruits turn orange due to ripening-associated carotenoid production. Here, to explore the potential of the rinG2 mutation for breeding, we characterized the effects of rinG2 in the heterozygous state (rinG2/+) compared to the effects of rin/+. The softening of rinG2/+ fruits was delayed compared to the wild type but to a lesser degree than rin/+ fruits. Lycopene and β-carotene levels in rinG2/+ fruits were similar to those of the wild type, whereas rin/+ fruits accumulated half the amount of β-carotene compared to the wild type. The rinG2/+ fruits produced lower levels of ethylene than wild-type and rin/+ fruits. Expression analysis revealed that in rinG2/+ fruits, the rinG2 mutation (like rin) partially inhibited the expression of ripening-associated genes. The small differences in the inhibitory effects of rinG2 vs. rin coincided with small differences in phenotypes, such as ethylene production, softening, and carotenoid accumulation. Therefore, rinG2 represents a promising genetic resource for developing tomato cultivars with extended shelf life.

Paper-containing 1-methylcyclopropene treatment suppresses fruit decay of fresh Anxi persimmons by enhancing disease resistance

Objectives

The purpose of this work was to evaluate the potential application of papers containing 1-methylcyclopropene (1-MCP) postharvest treatment for suppressing fruit decay of fresh Anxi persimmons and its possible mechanism.

Materials and methods

Anxi persimmon fruit were treated with papers containing 1-MCP at the dosage of 1.35 μL/L and stored at 25 ± 1 °C and 85 per cent relative humidity for 35 days. During storage, the fruit decay rate and lignin content were evaluated, and the content of total phenolics, the activities of phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO), peroxidase (POD), chitinase (CHI), and β-1,3-glucanase (GLU) were determined by spectrophotometry.

Results

The 1-MCP–treated persimmons displayed a lower fruit decay rate, but higher contents of lignin and total phenolics, higher activities of PAL, PPO, POD, CHI, and GLU.

Conclusions

The treatment with 1-MCP could inhibit the fruit decay of postharvest Anxi persimmons, which might be because 1-MCP enhanced fruit disease resistance by increasing the activities of disease resistance-associated enzymes and retaining higher contents of disease resistance-related substances in postharvest fresh Anxi persimmons. These findings indicate that papers containing 1-MCP at the dosage of 1.35 μL/L have potential application in suppressing fruit decay and extending storage life of postharvest fresh Anxi persimmons.

Deciphering transcriptional regulators of banana fruit ripening by regulatory network analysis

Fruit ripening is a critical phase in the production and marketing of fruits. Previous studies have indicated that fruit ripening is a highly coordinated process, mainly regulated at the transcriptional level, in which transcription factors play essential roles. Thus, identifying key transcription factors regulating fruit ripening as well as their associated regulatory networks promises to contribute to a better understanding of fruit ripening. In this study, temporal gene expression analyses were performed to investigate banana fruit ripening with the aim to discern the global architecture of gene regulatory networks underlying fruit ripening. Eight time points were profiled covering dynamic changes of phenotypes, the associated physiology and levels of known ripening marker genes. Combining results from a weighted gene co-expression network analysis (WGCNA) as well as cis-motif analysis and supported by EMSA, Y1H, tobacco-, banana-transactivation experimental results, the regulatory network of banana fruit ripening was constructed, from which 25 transcription factors were identified as prime candidates to regulate the ripening process by modulating different ripening-related pathways. Our study presents the first global view of the gene regulatory network involved in banana fruit ripening, which may provide the basis for a targeted manipulation of fruit ripening to attain higher banana and loss-reduced banana commercialization.

Diverse Functions of IAA-Leucine Resistant PpILR1 Provide a Genic Basis for Auxin-Ethylene Crosstalk During Peach Fruit Ripening

Auxin and ethylene play critical roles in the ripening of peach (Prunus persica) fruit; however, the interaction between these two phytohormones is complex and not fully understood. Here, we isolated a peach ILR gene, PpILR1, which encodes an indole-3-acetic acid (IAA)-amino hydrolase. Functional analyses revealed that PpILR1 acts as a transcriptional activator of 1-amino cyclopropane-1-carboxylic acid synthase (PpACS1), and hydrolyzes auxin substrates to release free auxin. When Cys137 was changed to Ser137, PpILR1 failed to show hydrolase activity but continued to function as a transcriptional activator of PpACS1 in tobacco and peach transient expression assays. Furthermore, transgenic tomato plants overexpressing PpILR1 exhibited ethylene- and strigolactone-related phenotypes, including premature pedicel abscission, leaf and petiole epinasty, and advanced fruit ripening, which are consistent with increased expression of genes involved in ethylene biosynthesis and fruit ripening, as well as suppression of branching and growth of internodes (related to strigolactone biosynthesis). Collectively, these results provide novel insights into the role of IAA-amino acid hydrolases in plants, and position the PpILR1 protein at the junction of auxin and ethylene pathways during peach fruit ripening. These results could have substantial implications on peach fruit cultivation and storage in the future.

Ethylene and Auxin: Hormonal Regulation of Volatile Compound Production During Tomato (Solanum lycopersicum L.) Fruit Ripening

As the auxin-ethylene interaction in climacteric fruit ripening has been highlighted, the hormonal regulation of aroma changes in climacteric fruits requires clarification. The influence of both phytohormones on the volatile organic compound (VOC) metabolism was evaluated during tomato (Solanum lycopersicum L.) fruit ripening. Tomato fruits cv. Micro-Tom and Sweet Grape at the mature green stage were randomly grouped according to treatment with ethylene (ETHY), auxin (IAA), or both (ETHY + IAA). At middle ripening, Micro-Tom ETHY + IAA fruits present VOC profiles similar to those of ETHY fruits, while Sweet Grape presents VOC profiles closer to those of IAA fruits. At full ripeness, Micro-Tom and Sweet Grape ETHY + IAA fruits show profiles closer to those of IAA fruits, suggesting that the auxin overlaps the ethylene effects. Aroma compounds positively correlated with consumer preferences (2-isobutylthiazole, 6-methyl-5-hepten-2-one, and others) are identified in both cultivars and have their contents affected by both hormone treatments. The transcription of genes related to the biosynthesis of important tomato VOCs that have fatty-acid and carotenoid precursors evidences their regulation by both plant hormones. Additionally, the results indicate that the observed effects on the VOC metabolism are not restricted to the Micro-Tom cultivar, as these are also observed in the Sweet Grape cultivar. In conclusion, ethylene and auxin directly regulate the metabolic pathways related to VOC formation, impacting tomato aroma formation during ripening since Micro-Tom fruits apparently at the same maturation stage have different aromas.

Preservation of Agaricus bisporus freshness with using innovative ethylene manipulating active packaging paper

Agaricus bisporus produces substantial ethylene during storage and transportation, which accelerates ripening and senescence, thereby shortening the shelf-life. In this study, a novel food packaging material with ethylene removal property was prepared to increase storage time of Agaricus bisporus. 1-Methylcyclopropen and molecular sieves loaded with potassium permanganate were used as ethylene scavengers to coat the fresh-keeping paper. SEM, FT-IR and DSC analyses proved that these functional components were successfully coated on the fresh-keeping paper. The qualities of the mushrooms packed by prepared functional paper were then determined. The results showed that this prepared functional paper could delay the softening, browning and weight loss of mushrooms during storage by inhibiting ethylene synthesis-related enzymes and gene expression in the mushroom fruiting body, and continuous adsorption and removal of the exogenous ethylene. Consequently, the functional paper could reduce the biochemical and physicochemical quality loss of Agaricus bisporus, thus prolonging its shelf-life.

Postharvest Properties of Ultra-Late Maturing Peach Cultivars and Their Attributions to Melting Flesh (M) Locus: Re-evaluation of M Locus in Association With Flesh Texture

The postharvest properties of two ultra-late maturing peach cultivars, "Tobihaku" (TH) and "Daijumitsuto" (DJ), were investigated. Fruit were harvested at commercial maturity and held at 25°C. TH exhibited the characteristics of normal melting flesh (MF) peach, including rapid fruit softening associated with appropriate level of endogenous ethylene production In contrast, DJ did not soften at all during 3 weeks experimental period even though considerable ethylene production was observed. Fruit of TH and DJ were treated with 5,000 ppm of propylene, an ethylene analog, continuously for 7 days. TH softened rapidly whereas DJ maintained high flesh firmness in spite of an increase in endogenous ethylene production, suggesting that DJ but not TH lacked the ability to be softened in response to endogenous and exogenous ethylene/propylene. DNA-seq analysis showed that tandem endo-polygalacturonase (endoPG) genes located at melting flesh (M) locus, Pp-endoPGM (PGM), and Pp-endoPGF (PGF), were deleted in DJ. The endoPG genes at M locus are known to control flesh texture of peach fruit, and it was suggested that the non-softening property of DJ is due to the lack of endoPG genes. On the other hand, TH possessed an unidentified M haplotype that is involved in determination of MF phenotype. Structural identification of the unknown M haplotype, designated as M ⁰, through comparison with previously reported M haplotypes revealed distinct differences between PGM on M ⁰ haplotype (PGM-M⁰ ) and PGM on other haplotypes (PGM-M¹ ). Peach M haplotypes were classified into four main haplotypes: M ⁰ with PGM-M⁰ ; M ¹ with both PGM-M¹ and PGF; M ² with PGM-M¹ ; and M ³ lacking both PGM and PGF. Re-evaluation of M locus in association with MF/non-melting flesh (NMF) phenotypes in more than 400 accessions by using whole genome shotgun sequencing data on database and/or by PCR genotyping demonstrated that M ⁰ haplotype was the common haplotype in MF accessions, and M ⁰ and M ¹ haplotypes were dominant over M ² and M ³ haplotypes and co-dominantly determined the MF trait. It was also assumed on the basis of structural comparison of M haplotypes among Prunus species that the ancestral haplotype of M ⁰ diverged from those of the other haplotypes before the speciation of Prunus persica.

CRISPR/Cas9-mediated gene-editing technology in fruit quality improvement

Fruits are an essential part of a healthy, balanced diet and it is particularly important for fibre, essential vitamins, and trace elements. Improvement in the quality of fruit and elongation of shelf life are crucial goals for researchers. However, traditional techniques have some drawbacks, such as long period, low efficiency, and difficulty in the modification of target genes, which limit the progress of the study. Recently, the clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 technique was developed and has become the most popular gene-editing technology with high efficiency, simplicity, and low cost. CRISPR/Cas9 technique is widely accepted to analyse gene function and complete genetic modification. This review introduces the latest progress of CRISPR/Cas9 technology in fruit quality improvement. For example, CRISPR/Cas9-mediated targeted mutagenesis of RIPENING INHIBITOR gene (RIN), Lycopene desaturase (PDS), Pectate lyases (PL), SlMYB12, and CLAVATA3 (CLV3) can affect fruit ripening, fruit bioactive compounds, fruit texture, fruit colouration, and fruit size. CRISPR/Cas9-mediated mutagenesis has become an efficient method to modify target genes and improve fruit quality.

ERF4 affects fruit firmness through TPL4 by reducing ethylene production

The firmness of fleshy fruit crops has a significant effect on their quality, consumer preference, shelf life and transportability. In a combined quantitative trait locus and genome-wide association studies study of apple fruit texture, we identified a mutation (C-G) in the ethylene response factor-associated amphiphilic repression (EAR) motif in the coding region of the apple ETHYLENE RESPONSE FACTOR4 (ERF4) gene. Chromatin immunoprecipitation sequencing showed that ERF4 binds to the promoter of ERF3, which is involved in regulation of ethylene biosynthesis. The EAR mutation in ERF4 results in reduced repression of ERF3 expression, which is turn promotes ethylene production and loss of fruit firmness. ERF4 acts as a transcriptional repressor whose activity is modulated by a TOPLESS co-repressor 4 (TPL4)-binding EAR repression motif. Biolayer interferometry analysis showed that the mutation in the EAR motif causes a reduction in the interaction with TPL4. Suppression of ERF4 or TPL4 promoted fruit ripening and ethylene production. Taken together, our results provide insights into how ERF4 allelic variation underlies an important fruit quality trait.

Relationships between genome methylation, levels of non-coding RNAs, mRNAs and metabolites in ripening tomato fruit

Ripening of tomato fruit is a complex tightly orchestrated developmental process that involves multiple physiological and metabolic changes that render fruit attractive, palatable and nutritious. Ripening requires initiation, activation and coordination of key pathways at the transcriptional and post-transcriptional levels that lead to ethylene synthesis and downstream ripening events determining quality. We studied wild-type, Gr and r mutant fruits at the coding and non-coding transcriptomic, metabolomic and genome methylation levels. Numerous differentially expressed non-coding RNAs were identified and quantified and potential competing endogenous RNA regulation models were constructed. Multiple changes in gene methylation were linked to the ethylene pathway and ripening processes. A combined analysis of changes in genome methylation, long non-coding RNAs, circular RNAs, micro-RNAs and fruit metabolites revealed many differentially expressed genes (DEGs) with differentially methylated regions encoding transcription factors and key enzymes related to ethylene or carotenoid pathways potentially targeted by differentially expressed non-coding RNAs. These included ACO2 (targeted by MSTRG.59396.1 and miR396b), CTR1 (targeted by MSTRG.43594.1 and miR171b), ERF2 (targeted by MSTRG.183681.1), ERF5 (targeted by miR9470-3p), PSY1 (targeted by MSTRG.95226.7), ZISO (targeted by 12:66127788|66128276) and NCED (targeted by MSTRG.181568.2). Understanding the functioning of this intricate genetic regulatory network provides new insights into the underlying integration and relationships between the multiple events that collectively determine the ripe phenotype.

ETHYLENE RESPONSE FACTOR39-MYB8 complex regulates low-temperature-induced lignification of loquat fruit

Flesh lignification is a specific chilling response that causes deterioration in the quality of stored red-fleshed loquat fruit (Eribotrya japonica) and is one aspect of wider chilling injury. APETALA2/ETHLENE RESPONSIVE FACTOR (AP2/ERF) transcription factors are important regulators of plant low-temperature responses and lignin biosynthesis. In this study, the expression and action of 27 AP2/ERF genes from the red-fleshed loquat cultivar 'Luoyangqing' were investigated in order to identify transcription factors regulating low-temperature-induced lignification. EjERF27, EjERF30, EjERF36, and EjERF39 were significantly induced by storage at 0 °C but inhibited by a low-temperature conditioning treatment (pre-storage at 5 °C for 6 days before storage at 0 °C, which reduces low-temperature-induced lignification), and their transcript levels positively correlated with flesh lignification. A dual-luciferase assay indicated that EjERF39 could transactivate the promoter of the lignin biosynthetic gene Ej4CL1, and an electrophoretic mobility shift assay confirmed that EjERF39 recognizes the DRE element in the promoter region of Ej4CL1. Furthermore, the combination of EjERF39 and the previously characterized EjMYB8 synergistically transactivated the Ej4CL1 promoter, and both transcription factors showed expression patterns correlated with lignification in postharvest treatments and red-fleshed 'Luoyangqing' and white-fleshed 'Ninghaibai' cultivars with different lignification responses. Bimolecular fluorescence complementation and luciferase complementation imaging assays confirmed direct protein-protein interaction between EjERF39 and EjMYB8. These data indicate that EjERF39 is a novel cold-responsive transcriptional activator of Ej4CL1 that forms a synergistic activator complex with EjMYB8 and contributes to loquat fruit lignification at low temperatures.

Transcriptome-wide identification and characterization of the Rab GTPase family in mango

The Rab GTPase family plays a vital role in several plant physiological processes including fruit ripening. Fruit softening during ripening involves trafficking of cell wall polymers and enzymes between cellular compartments. Mango, an economically important fruit crop, is known for its delicious taste, exotic flavour and nutritional value. So far, there is a paucity of information on the mango Rab GTPase family. In this study, 23 genes encoding Rab proteins were identified in mango by a comprehensive in silico approach. Sequence alignment and similarity tree analysis with the model plant Arabidopsis as a reference enabled the bona fide assignment of the deduced mango proteins to classify into eight subfamilies. Expression analysis by RNA-Sequencing (RNA-Seq) showed that the Rab genes were differentially expressed in ripe and unripe mangoes suggesting the involvement of vesicle trafficking during ripening. Interaction analysis showed that the proteins involved in vesicle trafficking and cell wall softening were interconnected providing further evidence of the involvement of the Rab GTPases in fruit softening. Correlation analyses showed a significant relationship between the expression level of the RabA3 and RabA4 genes and fruit firmness at the unripe stage of the mango varieties suggesting that the differences in gene expression level might be associated with the contrasting firmness of these varieties. This study will not only provide new insights into the complexity of the ripening-regulated molecular mechanism but also facilitate the identification of potential Rab GTPases to address excessive fruit softening.

Noncoding RNAs: functional regulatory factors in tomato fruit ripening

Tomato has emerged as the model system for investigations into the regulation of fleshy-fruit ripening and senescence, and the ripening process involving the coordinated regulation at the gene/chromatin/epigenetic, transcriptional, post-transcriptional and protein levels. Noncoding RNAs play important roles in fruit ripening as important transcriptional and post-transcriptional regulatory factors. In this review, we systematically summarize the recent advances in the regulation of tomato fruit ripening involved in ethylene biosynthesis and signal transduction, fruit pigment accumulation, fruit flavor and aroma, fruit texture by noncoding RNAs and their coordinate regulatory network model were set up and also suggest future directions for the functional regulations of noncoding RNAs on tomato fruit ripening.

Roles of RIN and ethylene in tomato fruit ripening and ripening-associated traits

RIPENING INHIBITOR (RIN)-deficient fruits generated by CRISPR/Cas9 initiated partial ripening at a similar time to wild-type (WT) fruits but only 10% WT concentrations of carotenoids and ethylene (ET) were synthesized. RIN-deficient fruit never ripened completely, even when supplied with exogenous ET. The low amount of endogenous ET that they did produce was sufficient to enable ripening initiation and this could be suppressed by the ET perception inhibitor 1-MCP. The reduced ET production by RIN-deficient tomatoes was due to an inability to induce autocatalytic system-2 ET synthesis, a characteristic feature of climacteric ripening. Production of volatiles and transcripts of key volatile biosynthetic genes also were greatly reduced in the absence of RIN. By contrast, the initial extent and rates of softening in the absence of RIN were similar to WT fruits, although detailed analysis showed that the expression of some cell wall-modifying enzymes was delayed and others increased in the absence of RIN. These results support a model where RIN and ET, via ERFs, are required for full expression of ripening genes. Ethylene initiates ripening of mature green fruit, upregulates RIN expression and other changes, including system-2 ET production. RIN, ET and other factors are required for completion of the full fruit-ripening programme.

Role of ethylene response factors (ERFs) in fruit ripening

The ethylene response factors (ERFs) belong to the APETALA2/ethylene response factor (AP2/ERF) superfamily and act downstream of the ethylene signalling pathway to regulate the expression of ethylene responsive genes. In different species, ERFs have been reported to be involved in plant development, flower abscission, fruit ripening, and defense responses. In this review, based on the new progress made by recent studies, we summarize the specific role and mode of action of ERFs in regulating different aspects of ripening in both climacteric and non-climacteric fruits, and provide new insights into the role of ethylene in non-climacteric fruit ripening.

Banana MaBZR1/2 associate with MaMPK14 to modulate cell wall modifying genes during fruit ripening

Banana MaBZR1/2 interact with MaMPK14 to enhance the transcriptional inhibition of cell wall modifying genes including MaEXP2, MaPL2 and MaXET5. Fruit ripening and softening, the major attributes to perishability in fleshy fruits, are modulated by various plant hormones and gene expression. Banana MaBZR1/2, the central transcription factors of brassinosteroid (BR) signaling, mediate fruit ripening through regulation of ethylene biosynthesis, but their possible roles in fruit softening as well as the underlying mechanisms remain to be determined. In this work, we found that MaBZR1/2 directly bound to and repressed the promoters of several cell wall modifying genes such as MaEXP2, MaPL2 and MaXET5, whose transcripts were elevated concomitant with fruit ripening. Moreover, yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays indicated that MaBZR1/2 physically interacted with a mitogen-activated protein kinase MaMPK14, and this interaction strengthened the MaBZR1/2-mediated transcriptional inhibitory abilities. Collectively, our study provides insight into the mechanism of MaBZR1/2 contributing to fruit ripening and softening, which may have potential for banana molecular improvement.

Primary Metabolism in Fresh Fruits During Storage

The extension of commercial life and the reduction of postharvest losses of perishable fruits is mainly based on storage at low temperatures alone or in combination with modified atmospheres (MAs) and controlled atmospheres (CAs), directed primarily at reducing their overall metabolism thus delaying ripening and senescence. Fruits react to postharvest conditions with desirable changes if appropriate protocols are applied, but otherwise can develop negative and unacceptable traits due to the onset of physiological disorders. Extended cold storage periods and/or inappropriate temperatures can result in development of chilling injuries (CIs). The etiology, incidence, and severity of such symptoms vary even within cultivars of the same species, indicating the genotype significance. Carbohydrates and amino acids have protective/regulating roles in CI development. MA/CA storage protocols involve storage under hypoxic conditions and high carbon dioxide concentrations that can maximize quality over extended storage periods but are also affected by the cultivar, exposure time, and storage temperatures. Pyruvate metabolism is highly reactive to changes in oxygen concentration and is greatly affected by the shift from aerobic to anaerobic metabolism. Ethylene-induced changes in fruits can also have deleterious effects under cold storage and MA/CA conditions, affecting susceptibility to chilling and carbon dioxide injuries. The availability of the inhibitor of ethylene perception 1-methylcyclopropene (1-MCP) has not only resulted in development of a new technology but has also been used to increase understanding of the role of ethylene in ripening of both non-climacteric and climacteric fruits. Temperature, MA/CA, and 1-MCP alter fruit physiology and biochemistry, resulting in compositional changes in carbon- and nitrogen-related metabolisms and compounds. Successful application of these storage technologies to fruits must consider their effects on the metabolism of carbohydrates, organic acids, amino acids and lipids.

Evidence for pre-climacteric activation of AOX transcription during cold-induced conditioning to ripen in European pear (Pyrus communis L.)

European pears (Pyrus communis L.) require a range of cold-temperature exposure to induce ethylene biosynthesis and fruit ripening. Physiological and hormonal responses to cold temperature storage in pear have been well characterized, but the molecular underpinnings of these phenomena remain unclear. An established low-temperature conditioning model was used to induce ripening of 'D'Anjou' and 'Bartlett' pear cultivars and quantify the expression of key genes representing ripening-related metabolic pathways in comparison to non-conditioned fruit. Physiological indicators of pear ripening were recorded, and fruit peel tissue sampled in parallel, during the cold-conditioning and ripening time-course experiment to correlate gene expression to ontogeny. Two complementary approaches, Nonparametric Multi-Dimensional Scaling and efficiency-corrected 2-(ΔΔCt), were used to identify genes exhibiting the most variability in expression. Interestingly, the enhanced alternative oxidase (AOX) transcript abundance at the pre-climacteric stage in 'Bartlett' and 'D'Anjou' at the peak of the conditioning treatments suggests that AOX may play a key and a novel role in the achievement of ripening competency. There were indications that cold-sensing and signaling elements from ABA and auxin pathways modulate the S1-S2 ethylene transition in European pears, and that the S1-S2 ethylene biosynthesis transition is more pronounced in 'Bartlett' as compared to 'D'Anjou' pear. This information has implications in preventing post-harvest losses of this important crop.

Auto- and mutual-regulation between two CitERFs contribute to ethylene-induced citrus fruit degreening

Citrus fruit postharvest degreening is a critical stage in marketing, carried out by exposure to ethylene or ethephon. Genome-wide screening of the AP2/ERF superfamily indicated that a novel ERF-II (CitERF6) was shown to trans-activate the CitPPH promoter. Expression of CitERF6 is associated with both developmental and postharvest degreening in citrus fruit. Transient and stable over-expression of CitERF6 in Nicotiana tabacum leaves and 'Ponkan' fruit also results in rapid chlorophyll degradation. Auto- and mutual-regulation was also found between CitERF6 and the previously characterized CitERF13 using the dual-luciferase and yeast one-hybrid assays. Moreover, substitution of the 35S promoter for endogenous promoters showed that both pCitERF6::CitERF6 and pCitERF13::CitERF13 were effective in trans-activating their promoters or triggering chlorophyll degradation. It is proposed that ethylene is one of the triggers activating promoters of CitERF6 and CitERF13, and subsequent auto- and mutual-regulation between CitERF6 and CitERF13 might facilitate the effect of ethylene, leading to fruit degreening.

MaMPK2 enhances MabZIP93-mediated transcriptional activation of cell wall modifying genes during banana fruit ripening

Transcriptional regulation is an essential molecular machinery in controlling gene expression in diverse plant developmental processes including fruit ripening. This involves the interaction of transcription factors (TFs) and promoters of target genes. In banana, although a number of fruit ripening-associated TFs have been characterized, their number is relatively small. Here we identified a nuclear-localized basic leucine zipper (bZIP) TF, MabZIP93, associated with banana ripening. MabZIP93 activated cell wall modifying genes MaPL2, MaPE1, MaXTH23 and MaXGT1 by directly binding to their promoters. Transient over-expression of MabZIP93 in banana fruit resulted in the increased expression of MaPL2, MaPE1, MaXTH23 and MaXGT1. Moreover, a mitogen-activated protein kinase MaMPK2 and MabZIP93 were found to interact with MabZIP93. The interaction of MabZIP93 with MaMPK2 enhanced MabZIP93 activation of cell wall modifying genes, which was likely due to the phosphorylation of MabZIP93 mediated by MaMPK2. Overall, this study shows that MaMPK2 interacts with and phosphorylates MabZIP93 to promote MabZIP93-mediated transcriptional activation of cell wall modifying genes, thereby expanding our understanding of gene networks associated with banana fruit ripening.

Ternary complex EjbHLH1-EjMYB2-EjAP2-1 retards low temperature-induced flesh lignification in loquat fruit

Many transcription factors (TFs), including NACs and MYBs, are involved in regulation of lignin biosynthesis during plant development and in responses to biotic and abiotic stresses. The lignin biosynthesis gene Ej4CL1 has been identified as a target for cold-induced TFs. We isolated a bHLH gene from loquat, EjbHLH1, the expression of which was negatively correlated with cold-induced fruit lignification. During low temperature storage (0 ^°C), EjbHLH1 transcripts were stable but accumulated during low-temperature conditioning (LTC) treatment, an acclimation process that reduces lignification during subsequent storage at 0 ^°C. Dual luciferase assays showed EjbHLH1 could repress Ej4CL1 promoter, but yeast one hybrid assay indicated EjbHLH1 is not able to bind to the Ej4CL1 promoter. Bimolecular fluorescence complementation (BIFC) indicated that EjbHLH1 could interact with EjAP2-1 and EjMYB2, two previously characterized fruit lignification related transcription factors and firefly luciferase complementation imaging assay indicated EjbHLH1, EjMYB2 and EjAP2-1 could form a ternary complex which enhanced repression of transcription from the Ej4CL1 promoter, reducing lignification at 0 ^°C.

Fruit ripening: the role of hormones, cell wall modifications, and their relationship with pathogens

Fruits result from complex biological processes that begin soon after fertilization. Among these processes are cell division and expansion, accumulation of secondary metabolites, and an increase in carbohydrate biosynthesis. Later fruit ripening is accomplished by chlorophyll degradation and cell wall lysis. Fruit maturation is an essential step to optimize seed dispersal, and is controlled by a complex network of transcription factors and genetic regulators that are strongly influenced by phytohormones. Abscisic acid (ABA) and ethylene are the major regulators of ripening and senescence in both dry and fleshy fruits, as demonstrated by numerous ripening-defective mutants, effects of exogenous hormone application, and transcriptome analyses. While ethylene is the best characterized player in the final step of a fruit's life, ABA also has a key regulatory role, promoting ethylene production and acting as a stress-related hormone in response to drought and pathogen attack. In this review, we focus on the role of ABA and ethylene in relation to the interconnected biotic and abiotic phenomena that affect ripening and senescence. We integrate and discuss the most recent data available regarding these biological processes, which are crucial for post-harvest fruit conservation and for food safety.

Fast and Furious: Ethylene-Triggered Changes in the Metabolism of Papaya Fruit During Ripening

Papaya is a climacteric fleshy fruit characterized by fast ripening after harvest. During the relatively short postharvest period, papaya fruit undergoes several changes in metabolism that result in pulp softening and sweetening, as well as the development of a characteristic aroma. Since papaya is one of the most cultivated and appreciated tropical fruit crops worldwide, extensive research has been conducted to not only understand the formation of the quality and nutritional attributes of ripe fruit but also to develop methods for controlling the ripening process. However, most strategies to postpone papaya ripening, and therefore to increase shelf life, have failed to maintain fruit quality. Ethylene blockage precludes carotenoid biosynthesis, while cold storage can induce chilling injury and negatively affect the volatile profile of papaya. As a climacteric fruit, the fast ripening of papaya is triggered by ethylene biosynthesis. The generation of the climacteric ethylene positive feedback loop is elicited by the expression of a specific transcription factor that leads to an up-regulation of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC-oxidase (ACO) expression, triggering the system II ethylene biosynthesis. The ethylene burst occurs about 3 to 4 days after harvest and induces pectinase expression. The disassembling of the papaya cell wall appears to help in fruit sweetness, while glucose and fructose are also produced by acidic invertases. The increase in ethylene production also results in carotenoid accumulation due to the induction of cyclases and hydroxylases, leading to yellow and red/orange-colored pulp phenotypes. Moreover, the production of volatile terpene linalool, an important biological marker for papaya's sensorial quality, is also induced by ethylene. All these mentioned processes are related to papaya's sensorial and nutritional quality. We describe the understanding of ethylene-triggered events that influence papaya quality and nutritional traits, as these characteristics are a consequence of an accelerated metabolism during fruit ripening.

Transcriptome analysis provides insights into the regulation of metabolic processes during postharvest cold storage of loquat (Eriobotrya japonica) fruit

Loquat (Eriobotrya japonica) fruit accumulates lignin during postharvest storage under chilling conditions (0 °C), while low-temperature conditioning (LTC; 5 °C for 6 days followed by transfer to 0 °C) or heat treatment (HT; 40 °C for 4 h followed by transfer to 0 °C) can alleviate lignification. Here we compared transcriptome profiles of loquat fruit samples under LTC or HT to those stored at 0 °C at five time points from day 1 to day 8 after treatment. High-throughput transcriptome sequences were de novo assembled into 53,319 unique transcripts with an N50 length of 1306 bp. A total of 2235 differentially expressed genes were identified in LTC, and 1020 were identified in HT compared to 0 °C. Key genes in the lignin biosynthetic pathway, including EjPAL2, EjCAD1, EjCAD3, 4CL, COMT, and HCT, were responsive to LTC or HT treatment, but they showed different expression patterns during the treatments, indicating that different structural genes could regulate lignification at different treatment stages. Coexpression network analysis showed that these candidate biosynthetic genes were associated with a number of transcription factors, including those belonging to the AP2, MYB, and NAC families. Gene ontology (GO) enrichment analysis of differentially expressed genes indicated that biological processes such as stress responses, cell wall and lignin metabolism, hormone metabolism, and metal ion transport were significantly affected under LTC or HT treatment when compared to 0 °C. Our analyses provide insights into transcriptome responses to postharvest treatments in loquat fruit.

A critical evaluation of the role of ethylene and MADS transcription factors in the network controlling fleshy fruit ripening

Contents Summary 1724 I. Introduction 1725 II. Ripening genes 1725 III. The importance of ethylene in controlling ripening 1727 IV. The importance of MADS-RIN in controlling ripening 1729 V. Interactions between components of the ripening regulatory network 1734 VI. Conclusions 1736 Acknowledgements 1738 Author contributions 1738 References 1738 SUMMARY: Understanding the regulation of fleshy fruit ripening is biologically important and provides insights and opportunities for controlling fruit quality, enhancing nutritional value for animals and humans, and improving storage and waste reduction. The ripening regulatory network involves master and downstream transcription factors (TFs) and hormones. Tomato is a model for ripening regulation, which requires ethylene and master TFs including NAC-NOR and the MADS-box protein MADS-RIN. Recent functional characterization showed that the classical RIN-MC gene fusion, previously believed to be a loss-of-function mutation, is an active TF with repressor activity. This, and other evidence, has highlighted the possibility that MADS-RIN itself is not important for ripening initiation but is required for full ripening. In this review, we discuss the diversity of components in the control network, their targets, and how they interact to control initiation and progression of ripening. Both hormones and individual TFs affect the status and activity of other network participants, which changes overall network signaling and ripening outcomes. MADS-RIN, NAC-NOR and ethylene play critical roles but there are still unanswered questions about these and other TFs. Further attention should be paid to relationships between ethylene, MADS-RIN and NACs in ripening control.

A Comprehensive Review on Kiwifruit Allergy: Pathogenesis, Diagnosis, Management, and Potential Modification of Allergens Through Processing

Kiwifruit is rich in bioactive components including dietary fibers, carbohydrates, natural sugars, vitamins, minerals, omega-3 fatty acids, and antioxidants. These components are beneficial to boost the human immune system and prevent cancer and heart diseases. However, kiwifruit is emerging as one of the most common elicitors of food allergies worldwide. Kiwifruit allergy results from an abnormal immune response to kiwifruit proteins and occur after consuming this fruit. Symptoms range from the oral allergy syndrome (OAS) to the life-threatening anaphylaxis. Thirteen different allergens have been identified in green kiwifruit and, among these allergens, Act d 1, Act d 2, Act d 8, Act d 11, and Act d 12 are defined as the "major allergens." Act d 1 and Act d 2 are ripening-related allergens and are found in abundance in fully ripe kiwifruit. Structures of several kiwifruit allergens may be altered under high temperatures or strong acidic conditions. This review discusses the pathogenesis, clinical features, and diagnosis of kiwifruit allergy and evaluates food processing methods including thermal, ultrasound, and chemical processing which may be used to reduce the allergenicity of kiwifruit. Management and medical treatments for kiwifruit allergy are also summarized.

Lignin Involvement in Programmed Changes in Peach-Fruit Texture Indicated by Metabolite and Transcriptome Analyses

Texture is an important component of peach-fruit quality. In the present study, an analysis of metabolite and transcriptome profiles during storage of a nonmelting-flesh cultivar, 'Baili', and a melting-flesh cultivar, 'Hongli', was conducted to explore the molecular mechanisms underlying different fruit textures in peach. Results indicated that higher levels of anthocyanins were present in 'Hongli' peach, whereas lignin monomers and ethylene precursors were higher in 'Baili'. A transcriptome analysis indicated that genes associated with lignin synthesis were more highly expressed in 'Baili' than in 'Hongli', especially Pp4CL2, Pp4CL3, and PpCOMT2. Texture differences between the two varieties may be the result of differential expression of two branches of the phenylpropanoid metabolic pathway. One branch regulates flavonoid metabolism and was highly active in 'Hongli' fruit, whereas the other branch regulates lignin synthesis and was more highly active in 'Baili' fruit.

A NAC transcription factor, NOR-like1, is a new positive regulator of tomato fruit ripening

Ripening of the model fruit tomato (Solanum lycopersicum) is controlled by a transcription factor network including NAC (NAM, ATAF1/2, and CUC2) domain proteins such as No-ripening (NOR), SlNAC1, and SlNAC4, but very little is known about the NAC targets or how they regulate ripening. Here, we conducted a systematic search of fruit-expressed NAC genes and showed that silencing NOR-like1 (Solyc07g063420) using virus-induced gene silencing (VIGS) inhibited specific aspects of ripening. Ripening initiation was delayed by 14 days when NOR-like1 function was inactivated by CRISPR/Cas9 and fruits showed obviously reduced ethylene production, retarded softening and chlorophyll loss, and reduced lycopene accumulation. RNA-sequencing profiling and gene promoter analysis suggested that genes involved in ethylene biosynthesis (SlACS2, SlACS4), color formation (SlGgpps2, SlSGR1), and cell wall metabolism (SlPG2a, SlPL, SlCEL2, and SlEXP1) are direct targets of NOR-like1. Electrophoretic mobility shift assays (EMSA), chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR), and dual-luciferase reporter assay (DLR) confirmed that NOR-like1 bound to the promoters of these genes both in vitro and in vivo, and activated their expression. Our findings demonstrate that NOR-like1 is a new positive regulator of tomato fruit ripening, with an important role in the transcriptional regulatory network.