Genetic and metabolic effects of ripening mutations and vine detachment on tomato fruit quality

Fruit ripening and postharvest changes in very early-harvested tomatoes

It is well known that if a fruit is harvested extremely early its development and function are interrupted, and it may never attain full maturity and optimal quality. Reports revealing insights regarding the alterations of maturation, ripening and postharvest quality in very early picked fruits are rare. We examined the effects of early harvesting on tomatoes by characterizing different accessions at the molecular, physiological, and biochemical levels. We found that even very early-harvested fruits could achieve postharvest maturation and ripening though with some defects in pigment and cuticle formation, and seeds from very early-harvested fruits could still germinate and develop as normal and healthy plants. One critical regulator of tomato cuticle integrity, SlCER1-2, was shown to contribute to cuticle defects in very early-harvested fruits. Very early fruit harvest still allowing ripening and seed development indicate that the genetic and physiological programs of later maturation and ripening are set into motion early in fruit development and are not dependent on complete fruit expansion nor attachment to the plant.

Histone H3K27 demethylase SlJMJ3 modulates fruit ripening in tomato

The histone lysine (K) demethylase 4 (KDM4/JHDM3) subfamily of jumonji domain-containing demethylases (JMJs) has been implicated in various aspects of plant development. However, their involvement in regulating the ripening of fleshy fruits remains unclear. In this study, we identified SlJMJ3, a member of the KDM4/JHDM3 family, as an H3K27me3 demethylase in tomato (Solanum lycopersicum) that plays an important role in fruit ripening regulation. Overexpression of SlJMJ3 leads to accelerated fruit ripening, whereas loss of function of SlJMJ3 delays this process. Furthermore, we determined that SlJMJ3 exerts its regulatory function by modulating the expression of multiple ripening-related genes involved in ethylene biosynthesis and response, carotenoid metabolism, cell wall modification, transcriptional control, and DNA methylation modification. SlJMJ3 binds directly to the promoters of ripening-related genes harboring the CTCTGYTY motif and activates their expression. Additionally, SlJMJ3 reduces the levels of H3K27me3 at its target genes, thereby upregulating their expression. In summary, our findings highlight the role of SlJMJ3 in the regulation of fruit ripening in tomato. By removing the methyl group from trimethylated histone H3 lysine 27 at ripening-related genes, SlJMJ3 acts as an epigenetic regulator that orchestrates the complex molecular processes underlying fruit ripening.

Polyamines: pleiotropic molecules regulating plant development and enhancing crop yield and quality

Polyamines (PAs) are pleiotropic bioorganic molecules. Cellular PA contents are determined by a balance between PA synthesis and degradation. PAs have been extensively demonstrated to play vital roles in the modulation of plant developmental processes and adaptation to various environmental stresses. In this review, the latest advances on the diverse roles of PAs in a range of developmental processes, such as morphogenesis, organogenesis, growth and development, and fruit ripening, are summarized and discussed. Besides, the crosstalk between PAs and phytohormones or other signalling molecules, including H2O2 and NO, involved in these processes is dwelled on. In addition, the attempts made to improve the yield and quality of grain and vegetable crops through altering the PA catabolism are enumerated. Finally, several other vital questions that remain unanswered are proposed and discussed. These include the mechanisms underlying the cooperative regulation of developmental processes by PAs and their interplaying partners like phytohormones, H2O2 and NO; PA transport for maintaining homeostasis; and utilization of PA anabolism/catabolism for generating high-yield and good-quality crops. This review aims to gain new insights into the pleiotropic role of PAs in the modulation of plant growth and development, which provides an alternative approach for manipulating and engineering valuable crop varieties that can be used in the future.

Impacts of Climate Change and Mitigation Strategies for Some Abiotic and Biotic Constraints Influencing Fruit Growth and Quality

Factors such as extreme temperatures, light radiation, and nutritional condition influence the physiological, biochemical, and molecular processes associated with fruit development and its quality. Besides abiotic stresses, biotic constraints can also affect fruit growth and quality. Moreover, there can be interactions between stressful conditions. However, it is challenging to predict and generalize the risks of climate change scenarios on seasonal patterns of growth, development, yield, and quality of fruit species because their responses are often highly complex and involve changes at multiple levels. Advancements in genetic editing technologies hold great potential for the agricultural sector, particularly in enhancing fruit crop traits. These improvements can be tailored to meet consumer preferences, which is crucial for commercial success. Canopy management and innovative training systems are also key factors that contribute to maximizing yield efficiency and improving fruit quality, which are essential for the competitiveness of orchards. Moreover, the creation of habitats that support pollinators is a critical aspect of sustainable agriculture, as they play a significant role in the production of many crops, including fruits. Incorporating these strategies allows fruit growers to adapt to changing climate conditions, which is increasingly important for the stability of food production. By investing in these areas, fruit growers can stay ahead of challenges and opportunities in the industry, ultimately leading to increased success and profitability. In this review, we aim to provide an updated overview of the current knowledge on this important topic. We also provide recommendations for future research.

Metabolomics to understand metabolic regulation underpinning fruit ripening, development, and quality

Classically fruit ripening and development was studied using genetic approaches, with understanding of metabolic changes that occurred in concert largely focused on a handful of metabolites including sugars, organic acids, cell wall components, and phytohormones. The advent and widespread application of metabolomics has, however, led to far greater understanding of metabolic components that play a crucial role not only in this process but also in influencing the organoleptic and nutritive properties of the fruits. Here we review how the study of natural variation, mutants, transgenics, and gene-edited fruits has led to a considerable increase in our understanding of these aspects. We focus on fleshy fruits such as tomato but also review berries, receptacle fruits, and stone-bearing fruits. Finally, we offer a perspective as to how comparative analyses and machine learning will likely further improve our comprehension of the functional importance of various metabolites in the future.

Introgression of a dominant phototropin1 mutant enhances carotenoids and boosts flavour-related volatiles in genome-edited tomato RIN mutants

The tomato (Solanum lycopersicum) ripening inhibitor (rin) mutation is known to completely repress fruit ripening. The heterozygous (RIN/rin) fruits have extended shelf life, ripen normally, but have inferior taste/flavour. To address this, we used genome editing to generate newer alleles of RIN (rinCR ) by targeting the K-domain. Unlike previously reported CRISPR alleles, the rinCR alleles displayed delayed onset of ripening, suggesting that the mutated K-domain represses the onset of ripening. The rinCR fruits had extended shelf life and accumulated carotenoids at an intermediate level between rin and progenitor line. Besides, the metabolites and hormonal levels in rinCR fruits were more akin to rin. To overcome the negative attributes of rin, we crossed the rinCR alleles with Nps1, a dominant-negative phototropin1 mutant, which enhances carotenoid levels in tomato fruits. The resulting Nps1/rinCR hybrids had extended shelf life and 4.4-7.1-fold higher carotenoid levels than the wild-type parent. The metabolome of Nps1/rinCR fruits revealed higher sucrose, malate, and volatiles associated with tomato taste and flavour. Notably, the boosted volatiles in Nps1/rinCR were only observed in fruits bearing the homozygous Nps1 mutation. The Nps1 introgression into tomato provides a promising strategy for developing cultivars with extended shelf life, improved taste, and flavour.

Redesigning the tomato fruit shape for mechanized production

Crop breeding for mechanized harvesting has driven modern agriculture. In tomato, machine harvesting for industrial processing varieties became the norm in the 1970s. However, fresh-market varieties whose fruits are suitable for mechanical harvesting are difficult to breed because of associated reduction in flavour and nutritional qualities. Here we report the cloning and functional characterization of fs8.1, which controls the elongated fruit shape and crush resistance of machine-harvestable processing tomatoes. FS8.1 encodes a non-canonical GT-2 factor that activates the expression of cell-cycle inhibitor genes through the formation of a transcriptional module with the canonical GT-2 factor SlGT-16. The fs8.1 mutation results in a lower inhibitory effect on the cell proliferation of the ovary wall, leading to elongated fruits with enhanced compression resistance. Our study provides a potential route for introducing the beneficial allele into fresh-market tomatoes without reducing quality, thereby facilitating mechanical harvesting.

RIN enhances plant disease resistance via root exudate-mediated assembly of disease-suppressive rhizosphere microbiota

The RIPENING-INHIBITOR (RIN) transcriptional factor is a key regulator governing fruit ripening. While RIN also affects other physiological processes, its potential roles in triggering interactions with the rhizosphere microbiome and plant health are unknown. Here we show that RIN affects microbiome-mediated disease resistance via root exudation, leading to recruitment of microbiota that suppress the soil-borne, phytopathogenic Ralstonia solanacearum bacterium. Compared with the wild-type (WT) plant, RIN mutants had different root exudate profiles, which were associated with distinct changes in microbiome composition and diversity. Specifically, the relative abundances of antibiosis-associated genes and pathogen-suppressing Actinobacteria (Streptomyces) were clearly lower in the rhizosphere of rin mutants. The composition, diversity, and suppressiveness of rin plant microbiomes could be restored by the application of 3-hydroxyflavone and riboflavin, which were exuded in much lower concentrations by the rin mutant. Interestingly, RIN-mediated effects on root exudates, Actinobacteria, and disease suppression were evident from the seedling stage, indicating that RIN plays a dual role in the early assembly of disease-suppressive microbiota and late fruit development. Collectively, our work suggests that, while plant disease resistance is a complex trait driven by interactions between the plant, rhizosphere microbiome, and the pathogen, it can be indirectly manipulated using "prebiotic" compounds that promote the recruitment of disease-suppressive microbiota.

Metabolic profiling and gene expression analysis reveal the quality deterioration of postharvest toon buds between two different storage temperatures

Toon buds, a popular woody vegetable, contain large amounts of nutrients. However, toon buds have strong respiratory metabolism after harvest and are highly prone to decay, resulting in quality deterioration. Low temperature can effectively inhibit postharvest senescence of toon buds. GC-TOF-MS combined with quantitative real-time PCR was used to elucidate the toon bud deterioration mechanism after harvest by analyzing the difference in the relative contents of primary metabolites and their derivatives, and the expression of key genes associated with metabolic pathways in toon buds between low temperature and room temperature storages for 72 h. Results showed that the ethylene synthesis in toon buds accelerated under room temperature storage, along with significant changes in the primary metabolic pathway. The catabolism of amino acids, fatty acids, and cell membrane phospholipids was accelerated, and the gluconeogenesis synthesis was strengthened. Moreover, the sucrose synthesis was increased, the glycolysis and TCA cycle were broken down, and the pentose phosphate pathway was vigorous. As metabolic intermediates, organic acids were considerably accumulated. Moreover, varieties of toxic compounds were produced in parallel with the activation of aromatic compounds. This work provided a comprehensive understanding of the metabolic regulation, thereby revealing how low and room temperatures differentially influenced the quality deterioration of postharvest toon buds.

An overview of ethylene insensitive tomato mutants: Advantages and disadvantages for postharvest fruit shelf-life and future perspective

The presence of ethylene during postharvest handling of tomatoes can be the main problem in maintaining fruit shelf-life by accelerating the ripening process and causing several quality changes in fruit. Several researchers have studied the methods for improving the postharvest life of tomato fruit by controlling ethylene response, such as by mutation. New ethylene receptor mutants have been identified, namely Sletr1-1, Sletr1-2, Nr (Never ripe), Sletr4-1, and Sletr5-1. This review identifies the favorable and undesirable effects of several ethylene receptor mutants. Also, the impact of those mutations on the metabolite alteration of tomatoes and the future perspectives of those ethylene receptor mutants. The review data is taken from the primary data of our experiment related to ethylene receptor mutants and the secondary data from numerous publications in Google Scholar and other sources pertaining to ethylene physiology. This review concluded that mutation in the SlETR1 gene was more effective than mutation in NR, SLETR4, and SLETR5 genes in generating a new ethylene mutant. Sletr1-2 mutant is a potential ethylene receptor mutant for developing new tomato cultivars with prolonged fruit-shelf life without any undesirable effect. Therefore, that has many challenges to using the Sletr1-2 mutant for future purposes in breeding programs.

Enrichment of health-promoting lutein and zeaxanthin in tomato fruit through metabolic engineering

Carotenoids constitute a large group of natural pigments widely distributed in nature. These compounds not only provide fruits and flowers with distinctive colors, but also have significant health benefits for humans. Lutein and zeaxanthin, both oxygen-containing carotenoids, are considered to play vital roles in promoting ocular development and maintaining eye health. However, humans and mammals cannot synthesize these carotenoid derivatives, which can only be taken from certain fruits or vegetables. Here, by introducing four endogenous synthetic genes, SlLCYE, SlLCYB, SlHYDB, and SlHYDE under fruit-specific promoters, we report the metabolic engineering of lutein/zeaxanthin biosynthesis in tomato fruit. Transgenic lines overexpression of one (SlLCYE), two (SlLCYE and SlLCYB; SlLCYB and SlHYDB), and all these four synthetic genes re-established the lutein/zeaxanthin biosynthetic pathways in the ripe tomato fruit and thus resulted in various types of carotenoid riched lines. Metabolic analyses of these engineered tomato fruits showed the strategy involved expression of SlLCYE tends to produce α-carotene and lutein, as well as a higher content of β-carotene and zeaxanthin was detected in lines overexpressing SlLCYB. In addition, the different combinations of engineered tomatoes with riched carotenoids showed higher antioxidant capacity and were associated with a significantly extended shelf life during postharvest storage. This work provides a successful example of accurate metabolic engineering in tomato fruit, suggesting the potential utility for synthetic biology to improve agronomic traits in crops. These biofortified tomato fruits could be also exploited as new research subjects for studying the health benefits of carotenoid derivatives.

Ethylene: Management and breeding for postharvest quality in vegetable crops. A review

Ethylene is a two-carbon gaseous plant growth regulator that involved in several important physiological events, including growth, development, ripening and senescence of fruits, vegetables, and ornamental crops. The hormone accelerates ripening of ethylene sensitive fruits, leafy greens and vegetables at micromolar concentrations, and its accumulation can led to fruit decay and waste during the postharvest stage. Several strategies of crops management and techniques of plant breeding have been attempted in the last decades to understand ethylene regulation pathways and ethylene-dependent biochemical and physiological processes, with the final aim to extend the produce shelf-life and improve the postharvest quality of fruits and vegetables. These investigation approaches involve the use of conventional and new breeding techniques, including precise genome-editing. This review paper aims to provide a relevant overview on the state of the art related to the use of modern breeding techniques focused on ethylene and ethylene-related metabolism, as well as on the possible postharvest technological applications for the postharvest management of ethylene-sensitive crops. An updated view and perspective on the implications of new breeding and management strategies to maintain the quality and the marketability of different crops during postharvest are given, with particular focus on: postharvest physiology (ethylene dependent) for mature and immature fruits and vegetables; postharvest quality management of vegetables: fresh and fresh cut products, focusing on the most important ethylene-dependent biochemical pathways; evolution of breeding technologies for facing old and new challenges in postharvest quality of vegetable crops: from conventional breeding and marker assisted selection to new breeding technologies focusing on transgenesis and gene editing. Examples of applied breeding techniques for model plants (tomato, zucchini and brocccoli) are given to elucidate ethylene metabolism, as well as beneficial and detrimental ethylene effects.

DNA and coding/non-coding RNA methylation analysis provide insights into tomato fruit ripening

Ripening is the last, irreversible developmental stage during which fruit become palatable, thus promoting seed dispersal by frugivory. In Alisa Craig fruit, mRNAs with increasing m5C levels, such as STPK and WRKY 40, were identified as being involved in response to biotic and abiotic stresses. Furthermore, two mRNAs involved in cell wall metabolism, PG and EXP-B1, also presented increased m5C levels. In the Nr mutant, several m5C-modified mRNAs involved in fruit ripening, including those encoding WRKY and MADS-box proteins, were found. Targets of long non-coding RNAs and circular RNAs with different m5C sites were also found; these targets included 2-alkenal reductase, soluble starch synthase 1, WRKY, MADS-box, and F-box/ketch-repeat protein SKIP11. A combined analysis of changes in 5mC methylation and mRNA revealed many differentially expressed genes with differentially methylated regions encoding transcription factors and key enzymes related to ethylene biosynthesis and signal transduction; these included ERF084, EIN3, AP2/ERF, ACO5, ACS7, EIN3/4, EBF1, MADS-box, AP2/ERF, and ETR1. Taken together, our findings contribute to the global understanding of the mechanisms underlying fruit ripening, thereby providing new information for both fruit and post-harvest behavior.

Metabolomics reveals key resistant responses in tomato fruit induced by Cryptococcus laurentii

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Cryptococcus laurentii induces resistance through in concert with key metabolic changes in tomato fruit.

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A total of 59 metabolites were differently abundant in C. laurentii-treated tomato fruit.

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Key metabolites chlorogenic acid, caffeic acid and ferulic acid are involved in phenylpropanoid biosynthesis pathway may play a key role in resistance induction by C. Laurentii in tomato.

To investigate the mechanisms underlying inducible resistance in postharvest tomato fruit, non-targeted metabolome analysis was performed to uncover metabolic changes in tomato fruit upon Cryptococcus laurentii treatment. 289 and 149 metabolites were identified in positive and negative ion modes, respectively. A total of 59 metabolites, mainly including phenylpropanoids, flavonoids and phenolic acids, were differently abundant in C. laurentii-treated tomato fruit. Moreover, key metabolites involved in phenylpropanoid biosynthesis pathway, especially chlorogenic acid, caffeic acid and ferulic acid were identified through KEGG enrichment analysis. Enhanced levels of phenolic acids indicated activation of the phenylpropanoid biosynthesis pathway, which is a classic metabolic pathway associated with inducible resistance, suggesting that its activation and consequent metabolic changes contributed to inducible resistence induced by C. laurentii. Our findings would provide new understanding of resistance induction mechanism in tomato fruit from the metabolic perspective, and offer novel insights for new approaches reducing postharvest loss on tomato.

Deciphering Precise Gene Transcriptional Expression Using gwINTACT in Tomato

Functional gene transcription mainly occurs in the nucleus and has a significant role in plant physiology. The isolation of nuclei tagged in specific cell type (INTACT) technique provides an efficient and stable nucleus purification method to investigate the dynamic changes of nuclear gene transcriptional expression. However, the application of traditional INTACT in plants is still limited to seedlings or root cells because of severe chloroplast pollution. In this study, we proposed a newly designed and simplified INTACT based on mas-enhanced GFP (eGFP)-SlWIP2 (gwINTACT) for nuclear purification in tomato (Solanum lycopersicum) leaves, flowers, and fruits for the first time. The yield of the nucleus purified using gwINTACT from transgenic tomato leaves was doubled compared with using a traditional INTACT procedure, accompanied by more than 95% removal of chloroplasts. Relative gene expression of ethylene-related genes with ethylene treatment was reevaluated in gwINTACT leaves to reveal more different results from the traditional gene expression assay based on total RNA. Therefore, establishing the gwINTACT system in this study facilitates the precise deciphering of the transcriptional status in various tomato tissues, which lays the foundation for the further experimental study of nucleus-related molecular regulation on fruit ripening, such as ChIP-seq and ATAC-seq.

An efficient chromatin immunoprecipitation (ChIP) protocol for studying histone modifications in peach reproductive tissues

BACKGROUND

Perennial fruit trees display a growth behaviour characterized by annual cycling between growth and dormancy, with complex physiological features. Rosaceae fruit trees represent excellent models for studying not only the fruit growth/patterning but also the progression of the reproductive cycle depending upon the impact of climate conditions. Additionally, current developments in high-throughput technologies have impacted Rosaceae tree research while investigating genome structure and function as well as (epi)genetic mechanisms involved in important developmental and environmental response processes during fruit tree growth. Among epigenetic mechanisms, chromatin remodelling mediated by histone modifications and other chromatin-related processes play a crucial role in gene modulation, controlling gene expression. Chromatin immunoprecipitation is an effective technique to investigate chromatin dynamics in plants. This technique is generally applied for studies on chromatin states and enrichment of post-transcriptional modifications (PTMs) in histone proteins.

RESULTS

Peach is considered a model organism among climacteric fruits in the Rosaceae family for studies on bud formation, dormancy, and organ differentiation. In our work, we have primarily established specific protocols for chromatin extraction and immunoprecipitation in reproductive tissues of peach (Prunus persica). Subsequently, we focused our investigations on the role of two chromatin marks, namely the trimethylation of histone H3 at lysine in position 4 (H3K4me3) and trimethylation of histone H3 at lysine 27 (H3K27me3) in modulating specific gene expression. Bud dormancy and fruit growth were investigated in a nectarine genotype called Fantasia as our model system.

CONCLUSIONS

We present general strategies to optimize ChIP protocols for buds and mesocarp tissues of peach and analyze the correlation between gene expression and chromatin mark enrichment/depletion. The procedures proposed may be useful to evaluate any involvement of histone modifications in the regulation of gene expression during bud dormancy progression and core ripening in fruits.

Interaction of two MADS-box genes leads to growth phenotype divergence of all-flesh type of tomatoes

All-flesh tomato cultivars are devoid of locular gel and exhibit enhanced firmness and improved postharvest storage. Here, we show that SlMBP3 is a master regulator of locular tissue in tomato fruit and that a deletion at the gene locus underpins the All-flesh trait. Intriguingly, All-flesh varieties lack the deleterious phenotypes reported previously for SlMBP3 under-expressing lines and which preclude any potential commercial use. We resolve the causal factor for this phenotypic divergence through the discovery of a natural mutation at the SlAGL11 locus, a close homolog of SlMBP3. Misexpressing SlMBP3 impairs locular gel formation through massive transcriptomic reprogramming at initial phases of fruit development. SlMBP3 influences locule gel formation by controlling cell cycle and cell expansion genes, indicating that important components of fruit softening are determined at early pre-ripening stages. Our findings define potential breeding targets for improved texture in tomato and possibly other fleshy fruits.

Effect of Elevated Temperature on Tomato Post-Harvest Properties

The fleshy fruit of tomato (Solanum lycopersicum) is a commodity used worldwide as a fresh or processed product. Like many crops, tomato plants and harvested fruits are susceptible to the onset of climate change. Temperature plays a key role in tomato fruit production and ripening, including softening, development of fruit colour, flavour and aroma. The combination of climate change and the drive to reduce carbon emission and energy consumption is likely to affect tomato post-harvest storage conditions. In this study, we investigated the effect of an elevated storage temperature on tomato shelf life and fungal susceptibility. A collection of 41 genotypes with low and high field performance at elevated temperature, including different growth, fruit and market types, was used to assess post-harvest performances. A temperature increase from 18–20 °C to 26 °C reduced average shelf life of fruit by 4 days ± 1 day and increased fungal susceptibility by 11% ± 5% across all genotypes. We identified tomato varieties that exhibit both favourable post-harvest fruit quality and high field performance at elevated temperature. This work contributes to efforts to enhance crop resilience by selecting for thermotolerance combined with traits suitable to maintain and improve fruit quality, shelf life and pathogen susceptibility under changing climate conditions.

Distribution of Triterpenoids and Steroids in Developing Rugosa Rose (Rosarugosa Thunb.) Accessory Fruit

Triterpenoids and steroids are considered to be important for the fruit quality and health-promoting properties for the consumers. The aim of the study was the determination of the changes in triterpenoid and steroid biosynthesis and the accumulation in hypanthium and achenes of rugosa rose (Rosa rugosa Thunb.) hip during fruit development and ripening at three different phenological stages (young fruits, fully developed unripe fruits, and matured fruits). Triterpenoids and steroids were also determined in the peel and the pulp of the matured hips. The obtained results indicated that the distribution of the analyzed compounds in different fruit tissues is a selective process. The increased rate of hydroxylation of triterpenoids, the deposition of hydroxylated acids in fruit surface layer, and the continuous biosynthesis of phytosterols in achenes versus its gradual repression in hypanthium accompanied by the accumulation of their biosynthetic intermediates and ketone derivatives seem to be characteristic metabolic features of maturation of rugosa rose accessory fruit. These observations, apart from providing the important data on metabolic modifications occurring in developing fruits, might have a practical application in defining fruit parts, particularly rich in bioactive constituents, to enable the development of novel functional products.

SlBES1 promotes tomato fruit softening through transcriptional inhibition of PMEU1

Fruit softening indicated by firmness determines the texture, transportability, and shelf life of tomato products. However, the regulatory mechanism underlying firmness formation in tomato fruit is poorly understood. Here, we report the regulatory role of SlBES1, an essential component of brassinosteroid hormone signaling, in tomato fruit softening. We found that SlBES1 promotes fruit softening during tomato fruit ripening and postharvest storage. RNA-seq analysis suggested that PMEU1, which encodes a pectin methylesterase, might participate in SlBES1-mediated softening. Biochemical and immunofluorescence assays indicated that SlBES1 inhibited PMEU1-related pectin de-methylesterification. Further molecular and genetic evidence verified that SlBES1 directly binds to the E-box of PMEU1 to repress its expression, leading to fruits softening. Loss-of-function SlBES1 mutant generated by CRISPR-Cas9 showed firmer fruits and longer shelf life during postharvest storage without other quality alteration. Collectively, our results indicated the potential of manipulating SlBES1 to regulate firmness without negative consequence on visual and nutrition quality.

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SlBES1 promotes tomato fruit softening without affecting nutritional quality

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SlBES1 inhibits PMEU1-related fruit pectin de-methylesterification

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SlBES1 represses PMEU1 expression through directly binding to the E-box

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Knockout of SlBES1 by CRISPR-Cas9 enhances fruit firmness and extends shelf life

The interplay between ABA/ethylene and NAC TFs in tomato fruit ripening: a review

This review contains functional roles of NAC transcription factors in the transcriptional regulation of ripening in tomato fruit, describes the interplay between ABA/ethylene and NAC TFs in tomato fruit ripening. Fruit ripening is regulated by a complex network of transcription factors (TFs) and genetic regulators in response to endogenous hormones and external signals. Studying the regulation of fruit ripening has important significance for controlling fruit quality, enhancing nutritional value, improving storage conditions and extending shelf-life. Plant-specific NAC (named after no apical meristem (NAM), Arabidopsis transcription activator factor 1/2 (ATAF1/2) and Cup-shaped cotyledon (CUC2)) TFs play essential roles in plant development, ripening and stress responses. In this review, we summarize the recent progress on the regulation of NAC TFs in fruit ripening, discuss the interactions between NAC and other factors in controlling fruit development and ripening, and emphasize how NAC TFs are involved in tomato fruit ripening through the ethylene and abscisic acid (ABA) pathways. The signaling network regulating ripening is complex, and both hormones and individual TFs can affect the status or activity of other network participants, which can alter the overall ripening network regulation, including response signals and fruit ripening. Our review helps in the systematic understanding of the regulation of NAC TFs involved in fruit ripening and provides a basis for the development or establishment of complex ripening regulatory network models.

Perspectives for epigenetic editing in crops

Site-specific nucleases (SSNs) have drawn much attention in plant biotechnology due to their ability to drive precision mutagenesis, gene targeting or allele replacement. However, when devoid of its nuclease activity, the underlying DNA-binding activity of SSNs can be used to bring other protein functional domains close to specific genomic sites, thus expanding further the range of applications of the technology. In particular, the addition of functional domains encoding epigenetic effectors and chromatin modifiers to the CRISPR/Cas ribonucleoprotein complex opens the possibility to introduce targeted epigenomic modifications in plants in an easily programmable manner. Here we examine some of the most important agronomic traits known to be controlled epigenetically and review the best studied epigenetic catalytic effectors in plants, such as DNA methylases/demethylases or histone acetylases/deacetylases and their associated marks. We also review the most efficient strategies developed to date to functionalize Cas proteins with both catalytic and non-catalytic epigenetic effectors, and the ability of these domains to influence the expression of endogenous genes in a regulatable manner. Based on these new technical developments, we discuss the possibilities offered by epigenetic editing tools in plant biotechnology and their implications in crop breeding.

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.

Update on the Roles of Polyamines in Fleshy Fruit Ripening, Senescence, and Quality

Ripening of fleshy fruits involves complex physiological, biochemical, and molecular processes that coincide with various changes of the fruit, including texture, color, flavor, and aroma. The processes of ripening are controlled by ethylene in climacteric fruits and abscisic acid (ABA) in non-climacteric fruits. Increasing evidence is also uncovering an essential role for polyamines (PAs) in fruit ripening, especially in climacteric fruits. However, until recently breakthroughs have been made in understanding PA roles in the ripening of non-climacteric fruits. In this review, we compare the mechanisms underlying PA biosynthesis, metabolism, and action during ripening in climacteric and non-climacteric fruits at the physiological and molecular levels. The PA putrescine (Put) has a role opposite to that of spermidine/spermine (Spd/Spm) in cellular metabolism. Arginine decarboxylase (ADC) is crucial to Put biosynthesis in both climacteric and non-climacteric fruits. S-adenosylmethionine decarboxylase (SAMDC) catalyzes the conversion of Put to Spd/Spm, which marks a metabolic transition that is concomitant with the onset of fruit ripening, induced by Spd in climacteric fruits and by Spm in non-climacteric fruits. Once PA catabolism is activated by polyamine oxidase (PAO), fruit ripening and senescence are facilitated by the coordination of mechanisms that involve PAs, hydrogen peroxide (H₂O₂), ABA, ethylene, nitric oxide (NO), and calcium ions (Ca²⁺). Notably, a signal derived from PAO5-mediated PA metabolism has recently been identified in strawberry, a model system for non-climacteric fruits, providing a deeper understanding of the regulatory roles played by PAs in fleshy fruit ripening.

Single and Double Mutations in Tomato Ripening Transcription Factors Have Distinct Effects on Fruit Development and Quality Traits

Spontaneous mutations associated with the tomato transcription factors COLORLESS NON-RIPENING (SPL-CNR), NON-RIPENING (NAC-NOR), and RIPENING-INHIBITOR (MADS-RIN) result in fruit that do not undergo the normal hallmarks of ripening but are phenotypically distinguishable. Here, we expanded knowledge of the physiological, molecular, and genetic impacts of the ripening mutations on fruit development beyond ripening. We demonstrated through phenotypic and transcriptome analyses that Cnr fruit exhibit a broad range of developmental defects before the onset of fruit ripening, but fruit still undergo some ripening changes similar to wild type. Thus, Cnr should be considered as a fruit developmental mutant and not just a ripening mutant. Additionally, we showed that some ripening processes occur during senescence in the nor and rin mutant fruit, indicating that while some ripening processes are inhibited in these mutants, others are merely delayed. Through gene expression analysis and direct measurement of hormones, we found that Cnr, nor, and rin have alterations in the metabolism and signaling of plant hormones. Cnr mutants produce more than basal levels of ethylene, while nor and rin accumulate high concentrations of abscisic acid. To determine genetic interactions between the mutations, we created for the first time homozygous double mutants. Phenotypic analyses of the double ripening mutants revealed that Cnr has a strong influence on fruit traits and that combining nor and rin leads to an intermediate ripening mutant phenotype. However, we found that the genetic interactions between the mutations are more complex than anticipated, as the Cnr/nor double mutant fruit has a Cnr phenotype but displayed inhibition of ripening-related gene expression just like nor fruit. Our reevaluation of the Cnr, nor, and rin mutants provides new insights into the utilization of the mutants for studying fruit development and their implications in breeding for tomato fruit quality.

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.

Metabolite Changes during Postharvest Storage: Effects on Fruit Quality Traits

Metabolic changes occurring in ripe or senescent fruits during postharvest storage lead to a general deterioration in quality attributes, including decreased flavor and ‘off-aroma’ compound generation. As a consequence, measures to reduce economic losses have to be taken by the fruit industry and have mostly consisted of storage at cold temperatures and the use of controlled atmospheres or ripening inhibitors. However, the biochemical pathways and molecular mechanisms underlying fruit senescence in commercial storage conditions are still poorly understood. In this sense, metabolomic platforms, enabling the profiling of key metabolites responsible for organoleptic and health-promoting traits, such as volatiles, sugars, acids, polyphenols and carotenoids, can be a powerful tool for further understanding the biochemical basis of postharvest physiology and have the potential to play a critical role in the identification of the pathways affected by fruit senescence. Here, we provide an overview of the metabolic changes during postharvest storage, with special attention to key metabolites related to fruit quality. The potential use of metabolomic approaches to yield metabolic markers useful for chemical phenotyping or even storage and marketing decisions is highlighted.

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.

Metabolite Profiling of Sorghum Seeds of Different Colors from Different Sweet Sorghum Cultivars Using a Widely Targeted Metabolomics Approach

Sweet sorghum (Sorghum bicolor) is one of the most important cereal crops in the world with colorful seeds. To study the diversity and cultivar-specificity of phytochemicals in sweet sorghum seeds, widely targeted metabolomics was used to analyze the metabolic profiles of the white, red, and purple seeds from three sweet sorghum cultivars Z6, Z27, and HC4. We identified 651 metabolites that were divided into 24 categories, including fatty acids, glycerolipids, flavonoids, benzoic acid derivatives, anthocyanins, and nucleotides and its derivatives. Among them, 217 metabolites were selected as significantly differential metabolites which could be related to the seed color by clustering analysis, principal component analysis (PCA), and orthogonal signal correction and partial least squares-discriminant analysis (OPLS-DA). A significant difference was shown between the red seed and purple seed samples, Z27 and HC4, in which 106 were downregulated and 111 were upregulated. The result indicated that 240 metabolites were significantly different, which could be related to the purple color with 58 metabolites downregulated and 182 metabolites upregulated. And 199 metabolites might be involved in the red phenotype with 54 downregulated and 135 upregulated. There were 45 metabolites that were common to all three cultivars, while cyanidin O-malonyl-malonyl hexoside, cyanidin O-acetylhexoside, and cyanidin 3-O-glucosyl-malonylglucoside were significantly upregulated red seeds, which could be the basis for the variety of seed colors. Generally, our findings provide a comprehensive comparison of the metabolites between the three phenotypes of S. bicolor and an interpretation of phenotypic differences from the point of metabolomics.