A tomato (Solanum lycopersicum) APETALA2/ERF gene, SlAP2a, is a negative regulator of fruit ripening

Construction of multi-targeted CRISPR libraries in tomato to overcome functional redundancy at genome-scale level

Genetic variance is vital for breeding programs and mutant screening, yet traditional mutagenesis methods wrestle with genetic redundancy and a lack of specificity in gene targeting. CRISPR-Cas9 offers precise, site-specific gene editing, but its application in crop improvement has been limited by scalability challenges. In this study, we develop genome-wide multi-targeted CRISPR libraries in tomato, enhancing the scalability of CRISPR gene editing in crops and addressing the challenges of redundancy while maintaining its precision. We design 15,804 unique single guide RNAs (sgRNAs), each targeting multiple genes within the same gene families. These sgRNAs are classified into 10 sub-libraries based on gene function. We generate approximately 1300 independent CRISPR lines and successfully identify mutants with distinct phenotypes related to fruit development, fruit flavor, nutrient uptake, and pathogen response. Additionally, we develop CRISPR-GuideMap, a double-barcode tagging system to enable large-scale sgRNA tracking in generated plants. Our results demonstrate that multi-targeted CRISPR libraries are scalable and effective for large-scale gene editing and offer an approach to overcome gene functional redundancy in basic plant research and crop breeding.

Two transcription factors play critical roles in mediating epigenetic regulation of fruit ripening in tomato

DNA methylation regulates fruit ripening in tomato, and disruption of the DNA demethylase DEMETER-LIKE 2 (DML2) results in genome-wide DNA hypermethylation and impaired ripening. We report here that the transcription factors Ripening Inhibitor (RIN) and FRUITFULL 1 (FUL1) play critical roles in mediating the effect of DNA methylation on tomato fruit ripening. RIN and FUL1 are silenced in dml2 mutant plants, and the defective ripening phenotype of dml2 is mimicked by the rin/ful1 double mutant. Restoration of RIN expression in dml2 partially rescues its ripening defects. DNA methylation controls ripening not only by regulating the expression of RIN and FUL1 but also by interfering with the genomic binding of RIN. In dml2 mutant plants, RIN cannot bind to some of its targets in vivo even though DNA methylation does not interfere with RIN binding in vitro; this inhibited binding in vivo is correlated with increased DNA methylation and histone H3 enrichment within 100 bp of the binding site. Our work uncovers the molecular mechanisms underlying DNA methylation control of fruit ripening in tomato.

Molecular breeding of tomato: Advances and challenges

The modern cultivated tomato (Solanum lycopersicum) was domesticated from Solanum pimpinellifolium native to the Andes Mountains of South America through a "two-step domestication" process. It was introduced to Europe in the 16th century and later widely cultivated worldwide. Since the late 19th century, breeders, guided by modern genetics, breeding science, and statistical theory, have improved tomatoes into an important fruit and vegetable crop that serves both fresh consumption and processing needs, satisfying diverse consumer demands. Over the past three decades, advancements in modern crop molecular breeding technologies, represented by molecular marker technology, genome sequencing, and genome editing, have significantly transformed tomato breeding paradigms. This article reviews the research progress in the field of tomato molecular breeding, encompassing genome sequencing of germplasm resources, the identification of functional genes for agronomic traits, and the development of key molecular breeding technologies. Based on these advancements, we also discuss the major challenges and perspectives in this field.

An AP2/ERF transcription factor GhERF109 negatively regulates plant growth and development in cotton

Cotton is an important source of natural fibers. The AP2/ethylene response factor (ERF) family is one of the largest plant-specific transcription factors (TFs) groups, playing key roles in plant growth and development. However, the role of ERF TFs in cotton's growth and development remains unclear. In this study, we identified GhERF109, a nuclear-localized ERF, which showed significant expression differences between ZM24 and pag1 cotton. Heterologous overexpression of GhERF109 in Arabidopsis resulted in reduced plant height, shortened root length, and reduced silique lengths compared to wild-type (WT) plants. In contrast, silencing GhERF109 in cotton led to a significant increase in plant height due to the elongation of stem cells. Overexpression of GhERF109 in cotton also produced a compact plant type with a notable reduction in height. RNA-seq analysis of GhERF109-silenced plants revealed 4123 differentially expressed genes (DEGs), with many upregulated genes involved in auxin response, polar transport, cell expansion, cell cycle regulation, brassinolide (BL) biosynthesis, and very long-chain fatty acid (VLCFA) pathways. These findings suggest that GhERF109 integrates auxin and other signaling pathways to suppress plant growth, providing valuable genetic material for breeding programs to improve mechanized cotton harvesting.

Ethylene response factor SlERF.D6 promotes ripening in part through transcription factors SlDEAR2 and SlTCP12

Ripening is crucial for the development of fleshy fruits that release their seeds following consumption by frugivores and are important contributors to human health and nutritional security. Many genetic ripening regulators have been identified, especially in the model system tomato, yet more remain to be discovered and integrated into comprehensive regulatory models. Most tomato ripening genes have been studied in pericarp tissue, though recent evidence indicates that locule tissue is a site of early ripening-gene activities. Here, we identified and functionally characterized an Ethylene Response Factor (ERF) gene, SlERF.D6, by investigating tomato transcriptome data throughout plant development, emphasizing genes elevated in the locule during fruit development and ripening. SlERF.D6 loss-of-function mutants resulting from CRISPR/Cas9 gene editing delayed ripening initiation and carotenoid accumulation in both pericarp and locule tissues. Transcriptome analysis of lines altered in SlERF.D6 expression revealed multiple classes of altered genes including ripening regulators, in addition to carotenoid, cell wall, and ethylene pathway genes, suggesting comprehensive ripening control. Distinct regulatory patterns in pericarp versus locule tissues were observed, indicating tissue-specific activity of this transcription factor (TF). Analysis of SlERF.D6 interaction with target promoters revealed an APETALA 2/ETHYLENE RESPONSE FACTOR (AP2/ERF) TF (SlDEAR2) as a target of SlERF.D6. Furthermore, we show that a third TF gene, SlTCP12, is a target of SlDEAR2, presenting a tricomponent module of ripening control residing in the larger SlERF.D6 regulatory network.

Abscission zone metabolism impacts pre- and post-harvest fruit quality: a very attaching story

The function of abscission zones (AZs) determines the timing of fleshy fruit abscission, with important consequences not only for the optimal fruit harvest, but also on the overall final fruit quality. In this context, chemical treatments are commonly used at different stages of fruit development to control fruit abscission, which can also have positive or negative effects on fruit quality. In the current review, we examine commonly used chemicals that affect the metabolic activity in the AZs of fleshy fruit, in addition to their effects on fruit quality characteristics. The main hormone metabolism and signaling in the AZ include that of ethylene, auxin, abscisic acid and jasmonates, and the molecular components that are involved are covered and discussed, in addition to how these hormones work together to regulate AZ activity and hence, affect fruit quality. We focus on studies that have provided new insight into possible protein complexes that function in the AZ, including multiple MADS-box transcription factors, with potential overlapping regulatory roles which exist between AZ development, ethylene production, AZ activation, fruit ripening and overall fruit quality. The view of the AZ as a cross roads where multiple pathways and signals are integrated is discussed.

Ethylene Signaling in Regulating Plant Growth, Development, and Stress Responses

Ethylene is a gaseous plant hormone that plays a crucial role in coordinating various physiological processes in plants. It acts as a key mediator, integrating both endogenous developmental cues and external environmental signals to regulate a wide range of functions, including growth, fruit ripening, leaf abscission, and responses to stress. The signaling pathway is initiated when ethylene binds to its receptor. After decades of research, the key components of ethylene signaling have been identified and characterized. Although the molecular mechanisms of the sensing of ethylene signal and its transduction have been studied extensively, a new area of research is how respiration and epigenetic modifications influence ethylene signaling and ethylene response. Here, we summarize the research progress in recent years and review the function and importance of ethylene signaling in plant growth and stress responses. In addition, we also describe the current understanding of how epigenetic modifications regulate ethylene signaling and the ethylene response. Together, our review sheds light on the new signaling mechanisms of ethylene.

Protein phosphatase PP2C2 dephosphorylates transcription factor ZAT5 and modulates tomato fruit ripening

Although C2H2 zinc finger transcription factors are important in plant growth, development, and stress resistance, their specific roles in fruit ripening have been less explored. Here, we demonstrate that the C2H2 zinc finger transcription factor 5 (SlZAT5) regulates fruit ripening in tomato (Solanum lycopersicum L.). Overexpression of SlZAT5 delayed ripening, while its knockout accelerated it, confirming its role as a negative regulator. SlZAT5 functions as a transcriptional repressor by directly inhibiting ripening-related genes, including SlACS4, SlPL8, and SlGRAS38, thereby delaying ripening. Furthermore, SlZAT5 interacts with the type 2C protein phosphatase SlPP2C2, which regulates the repressor activity of SlZAT5 by dephosphorylating SlZAT5 at Ser-65. This interaction is crucial in modulating ethylene production, thereby influencing the ripening process. These findings reveal a regulatory function of SlZAT5 in tomato fruit development, offering insights into the SlZAT5-SlPP2C2 module and potential targets for genetic modification to improve fruit quality and extend fruit shelf life.

Red peel regulator 1 links ethylene response factor 25 and β-citraurin biosynthetic genes to regulate ethylene-induced peel reddening in citrus

The reddish apocarotenoid β-citraurin, produced by CAROTENOID CLEAVAGE DIOXYGENASE 4b (CsCCD4b), is responsible for peel reddening in citrus (Citrus spp.). Ethylene induces the characteristic red color of citrus peel, but the underlying molecular mechanism remains largely unclear. Here, we identified red peel regulator 1 (CsRP1), a trihelix transcriptional activator that regulates ethylene-induced peel reddening by directly binding to a key V-myb avian myeloblastosis viral oncogene homolog (MYB)-binding site in the CsCCD4b promoter, thus activating its transcription. Furthermore, 2 drought-responsive cis-elements in the CsRP1 promoter are bound by the ethylene response factor ethylene response factor 25 (CsERF25). We reconstructed the CsERF25-CsRP1-CsCCD4b transcriptional regulatory cascade through transient expression of CsERF25 and CsRP1 in citrus peel and via stable transformation of citrus calli. In this cascade, CsERF25 expression was induced by ethylene to activate CsRP1 expression, and then, CsRP1 directly induced CsCCD4b transcription to catalyze β-citraurin biosynthesis. CsRP1 and CsERF25 also bound to the promoters of other carotenogenic genes and induced their transcription, thereby promoting β-citraurin accumulation. Collectively, our findings reveal a complex regulatory network modulating ethylene-induced citrus peel reddening and provide innovative strategies for improving the nutritional and esthetic values of citrus and other fruit crops.

Histone deacetylase SlHDA7 impacts fruit ripening and shelf life in tomato

Fruit ripening depends on the accurate control of ripening-related genes expression, with histone deacetylases (HDACs) playing crucial roles in transcriptional regulation. However, the functions of HDACs in fruit maturation remain largely unexplored. Here, we show that SlHDA7 acts as a suppressor of fruit ripening and functions as an H4ac HDAC in tomato. Deletion of SlHDA7 accelerated fruit ripening, while overexpression of SlHDA7 delayed the maturation process. Additionally, ethylene production and carotenoid biosynthesis significantly increased in slhda7 mutant fruits but decreased in SlHDA7-overexpressing fruits. Furthermore, SlHDA7 repress the expression of ethylene production and signaling, carotenoid metabolism, cell wall modification, and transcriptional regulation-related genes. RT-qPCR and ChIP-qPCR analyses indicated that SlHDA7 may deacetylate H4ac, leading to reduced transcript levels of ACO1, GGPPS2, Z-ISO, EXP1, and XYL1 mRNA, consequently suppressing fruit ripening. Moreover, SlHDA7 suppresses fruit ripening by targeting specific ripening-associated transcription factors (TFs) like RIN, FUL1, and ERF.E1, ultimately leading to delayed ripening and prolonged fruit shelf life. In summary, our findings indicate that SlHDA7 negatively modulates tomato fruit maturation by adjusting H4ac levels of these ripening-associated genes and key TFs.

SlFSR positively regulates ethylene biosynthesis and lycopene accumulation during fruit ripening in tomato

Transcription factors (TFs) are crucial for regulating fruit ripening in tomato (Solanum lycopersicum). The GRAS (GAI, RGA, and SCR) TFs are involved in various physiological processes, but their role in fruit ripening has seldom been reported. We have previously identified a gene encoding GRAS protein named SlFSR (Fruit Shelf-life Regulator), which is implicated in fruit ripening by regulating cell wall metabolism; however, the underlying mechanism remains unclear. Here, we demonstrate that SlFSR proteins are localized to the nucleus, where they could bind to specific DNA sequences. SlFSR acts downstream of the master ripening regulator RIN and could collaborate with RIN to control the ripening process by regulating expression of ethylene biosynthesis genes. In SlFSR-CR (CRISPR/Cas9) mutants, the initiation of fruit ripening was not affected but the reduced ethylene production and a delayed coloring process occurred. RNA-sequencing (RNA-seq) and promoter analysis reveal that SlFSR directly binds to the promoters of two key ethylene biosynthesis genes (SlACO1 and SlACO3) and activates their expression. However, SlFSR-CR fruits displayed a significant down-regulation of key rate-limiting genes (SlDXS1 and SlGGPPS2) in the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, which may account for the impaired lycopene synthesis. Altogether, we propose that SlFSR positively regulates ethylene biosynthesis and lycopene accumulation, providing valuable insights into the molecular mechanisms underlying fruit ripening.

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.

Mutation of YFT3, an isomerase in the isoprenoid biosynthetic pathway, impairs its catalytic activity and carotenoid accumulation in tomato fruit

Tomato fruit colors are directly associated with their appearance quality and nutritional value. However, tomato fruit color formation is an intricate biological process that remains elusive. In this work we characterized a tomato yellow fruited tomato 3 (yft3, e9292, Solanum lycopersicum) mutant with yellow fruits. By the map-based cloning approach, we identified a transversion mutation (A2117C) in the YFT3 gene encoding a putative isopentenyl diphosphate isomerase (SlIDI1) enzyme, which may function in the isoprenoid biosynthetic pathway by catalyzing conversion between isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). The mutated YFT3 (A2117C) (designated YFT3 allele) and the YFT3 genes did not show expression difference at protein level, and their encoded YFT3 allelic (S126R) and YFT3 proteins were both localized in plastids. However, the transcript levels of eight genes (DXR, DXS, HDR, PSY1, CRTISO, CYCB, CYP97A, and NCED) associated with carotenoid synthesis were upregulated in fruits of both yft3 and YFT3 knockout (YFT3-KO) lines at 35 and 47 days post-anthesis compared with the red-fruit tomato cultivar (M82). In vitro and in vivo biochemical analyses indicated that YFT3 (S126R) possessed much lower enzymatic activities than the YFT3 protein, indicating that the S126R mutation can impair YFT3 activity. Molecular docking analysis showed that the YFT3 allele has higher ability to recruit isopentenyl pyrophosphate (IPP), but abolishes attachment of the Mg2+ cofactor to IPP, suggesting that Ser126 is a critical residue for YTF3 biochemical and physiological functions. As a result, the yft3 mutant tomato line has low carotenoid accumulation and abnormal chromoplast development, which results in yellow ripe fruits. This study provides new insights into molecular mechanisms of tomato fruit color formation and development.

Key transcription factors regulate fruit ripening and metabolite accumulation in tomato

Fruit ripening is a complex process involving dynamic changes to metabolites and is controlled by multiple factors, including transcription factors (TFs). Several TFs are reportedly essential regulators of tomato (Solanum lycopersicum) fruit ripening. To evaluate the effects of specific TFs on metabolite accumulation during fruit ripening, we combined CRISPR/Cas9-mediated mutagenesis with metabolome and transcriptome analyses to explore regulatory mechanisms. Specifically, we generated various genetically engineered tomato lines that differed regarding metabolite contents and fruit colors. The metabolite and transcript profiles indicated that the selected TFs have distinct functions that control fruit metabolite contents, especially carotenoids and sugars. Moreover, a mutation to ELONGATED HYPOCOTYL5 (HY5) increased tomato fruit fructose and glucose contents by approximately 20% (relative to the wild-type levels). Our in vitro assay showed that HY5 can bind directly to the G-box cis-element in the Sugars Will Eventually be Exported Transporter (SWEET12c) promoter to activate expression, thereby modulating sugar transport. Our findings provide insights into the mechanisms regulating tomato fruit ripening and metabolic networks, providing the theoretical basis for breeding horticultural crops that produce fruit with diverse flavors and colors.

Galactinol synthase 2 influences the metabolism of chlorophyll, carotenoids, and ethylene in tomato fruits

Galactinol synthase (GolS), which catalyses the synthesis of galactinol, is the first critical enzyme in the biosynthesis of raffinose family oligosaccharides (RFOs) and contributes to plant growth and development, and resistance mechanisms. However, its role in fruit development remains largely unknown. In this study, we used CRISPR/Cas9 gene-editing technology in tomato (Solanum lycopersicum) to create the gols2 mutant showing uniformly green fruits without dark-green shoulders, and promoting fruit ripening. Analysis indicated that galactinol was undetectable in the ovaries and fruits of the mutant, and the accumulation of chlorophyll and chloroplast development was suppressed in the fruits. RNA-sequencing analysis showed that genes related to chlorophyll accumulation and chloroplast development were down-regulated, including PROTOCHLOROPHYLLIDE OXIDOREDUCTASE, GOLDEN 2-LIKE 2, and CHLOROPHYLL A/B-BINDING PROTEINS. In addition, early color transformation and ethylene release was prompted in the gols2 lines by regulation of the expression of genes involved in carotenoid and ethylene metabolism (e.g. PHYTOENE SYNTHASE 1, CAROTENE CIS-TRANS ISOMERASE, and 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHASE2/4) and fruit ripening (e.g. RIPENING INHIBITOR, NON-RIPENING, and APETALA2a). Our results provide evidence for the involvement of GolS2 in pigment and ethylene metabolism of tomato fruits.

A transcriptional cascade mediated by two APETALA2 family members orchestrates carotenoid biosynthesis in tomato

Carotenoids are important nutrients for human health that must be obtained from plants since they cannot be biosynthesized by the human body. Dissecting the regulatory mechanism of carotenoid metabolism in plants represents the first step toward manipulating carotenoid contents in plants by molecular design breeding. In this study, we determined that SlAP2c, an APETALA2 (AP2) family member, acts as a transcriptional repressor to regulate carotenoid biosynthesis in tomato (Solanum lycopersicum). Knockout of SlAP2c in both the "MicroTom" and "Ailsa Craig" backgrounds resulted in greater lycopene accumulation, whereas overexpression of this gene led to orange-ripe fruit with significantly lower lycopene contents than the wild type. We established that SlAP2c represses the expression of genes involved in lycopene biosynthesis by directly binding to the cis-elements in their promoters. Moreover, SlAP2c relies on its EAR motif to recruit the co-repressors TOPLESS (TPL)2/4 and forms a complex with histone deacetylase (had)1/3, thereby reducing the histone acetylation levels of lycopene biosynthesis genes. Furthermore, SlAP2a, a homolog of SlAP2c, acts upstream of SlAP2c and alleviates the SlAP2c-induced repression of lycopene biosynthesis genes by inhibiting SlAP2c transcription during fruit ripening. Therefore, we identified a transcriptional cascade mediated by AP2 family members that regulates lycopene biosynthesis during fruit ripening in tomato, laying the foundation for the manipulation of carotenoid metabolism in plants.

Bifunctional transcription factors SlERF.H5 and H7 activate cell wall and repress gibberellin biosynthesis genes in tomato via a conserved motif

The plant cell wall is a dynamic structure that plays an essential role in development, but the mechanism regulating cell wall formation remains poorly understood. We demonstrate that two transcription factors, SlERF.H5 and SlERF.H7, control cell wall formation and tomato fruit firmness in an additive manner. Knockout of SlERF.H5, SlERF.H7, or both genes decreased cell wall thickness, firmness, and cellulose contents in fruits during early development, especially in double-knockout lines. Overexpressing either gene resulted in thicker cell walls and greater fruit firmness with elevated cellulose levels in fruits but severely dwarf plants with lower gibberellin contents. We further identified that SlERF.H5 and SlERF.H7 activate the cellulose biosynthesis gene SlCESA3 but repress the gibberellin biosynthesis gene GA20ox1. Moreover, we identified a conserved LPL motif in these ERFs responsible for their activities as transcriptional activators and repressors, providing insight into how bifunctional transcription factors modulate distinct developmental processes.

Transcriptomic study of the role of MeFtsZ2-1 in pigment accumulation in cassava leaves

MeFtsZ2-1 is a key gene for plant plastid division, but the mechanism by which MeFtsZ2-1 affects pigment accumulation in cassava (Manihot esculenta Crantz) through plastids remains unclear. We found that MeFtsZ2-1 overexpression in cassava (OE) exhibited darker colors of leaves, with increased levels of anthocyanins and carotenoids. Further observation via Transmission Electron Microscopy (TEM) revealed no apparent defects in chloroplast structure but an increase in the number of plastoglobule in OE leaves. RNA-seq results showed 1582 differentially expressed genes (DEGs) in leaves of OE. KEGG pathway analysis indicated that these DEGs were enriched in pathways related to flavonoid, anthocyanin, and carotenoid biosynthesis. This study reveals the role of MeFtsZ2-1 in cassava pigment accumulation from a physiological and transcriptomic perspective, providing a theoretical basis for improving cassava quality.

Multi-omic analysis of the extension of broccoli quality during storage by folic acid

INTRODUCTION

Folic acid (FA) is a critical metabolite in all living organisms and an important nutritional component of broccoli. Few studies have been conducted on the impact of an exogenous application of FA on the postharvest physiology of fruits and vegetables during storage. In this regard, the mechanism by which an exogenous application of FA extends the postharvest quality of broccoli is unclear.

OBJECTIVE

This study utilized a multicomponent analysis to investigate how an exogenous application of FA effects the postharvest quality of broccoli.

METHODS

Broccoli was soaked in 5 mg/L FA for 10 min and the effect of the treatment on the appearance and nutritional quality of broccoli was evaluated. These data were combined with transcriptomic, metabolomic, and DNA methylation data to provide insight into the potential mechanism by which FA delays senescence.

RESULTS

The FA treatment inhibited the yellowing of broccoli during storage. CHH methylation was identified as the main type of methylation that occurs in broccoli and the FA treatment was found to inhibit DNA methylation, promote the accumulation of endogenous FA and chlorophyl, and inhibit ethylene biosynthesis in stored broccoli. The FA treatment also prevented the formation of off-odors by inhibiting the degradation of glucosinolate.

CONCLUSIONS

FA treatment inhibited the loss of nutrients during the storage of broccoli, delayed its yellowing, and inhibited the generation of off-odors. Our study provides deeper insight into the mechanism by which the postharvest application of FA delays postharvest senescence in broccoli and provides the foundation for further studies of postharvest metabolism in broccoli.

Promoter variations of ClERF1 gene determines flesh firmness in watermelon

BACKGROUND

Flesh firmness is a critical factor that influences fruit storability, shelf-life and consumer's preference as well. However, less is known about the key genetic factors that are associated with flesh firmness in fresh fruits like watermelon.

RESULTS

In this study, through bulk segregant analysis (BSA-seq), we identified a quantitative trait locus (QTL) that influenced variations in flesh firmness among recombinant inbred lines (RIL) developed from cross between the Citrullus mucosospermus accession ZJU152 with hard-flesh and Citrullus lanatus accession ZJU163 with soft-flesh. Fine mapping and sequence variations analyses revealed that ethylene-responsive factor 1 (ClERF1) was the most likely candidate gene for watermelon flesh firmness. Furthermore, several variations existed in the promoter region between ClERF1 of two parents, and significantly higher expressions of ClERF1 were found in hard-flesh ZJU152 compared with soft-flesh ZJU163 at key developmental stages. DUAL-LUC and GUS assays suggested much stronger promoter activity in ZJU152 over ZJU163. In addition, the kompetitive allele-specific PCR (KASP) genotyping datasets of RIL populations and germplasm accessions further supported ClERF1 as a possible candidate gene for fruit flesh firmness variability and the hard-flesh genotype might only exist in wild species C. mucosospermus. Through yeast one-hybrid (Y1H) and dual luciferase assay, we found that ClERF1 could directly bind to the promoters of auxin-responsive protein (ClAux/IAA) and exostosin family protein (ClEXT) and positively regulated their expressions influencing fruit ripening and cell wall biosynthesis.

CONCLUSIONS

Our results indicate that ClERF1 encoding an ethylene-responsive factor 1 is associated with flesh firmness in watermelon and provide mechanistic insight into the regulation of flesh firmness, and the ClERF1 gene is potentially applicable to the molecular improvement of fruit-flesh firmness by design breeding.

The apple transcription factor MdZF-HD11 regulates fruit softening by promoting Mdβ-GAL18 expression

Fruit ripening and the associated softening are major determinants of fruit quality and post-harvest shelf life. Although the mechanisms underlying fruit softening have been intensively studied, there are limited reports on the regulation of fruit softening in apples (Malus domestica). Here, we identified a zinc finger homeodomain transcription factor MdZF-HD11that trans-activates the promoter of Mdβ-GAL18, which encodes a pectin-degradation enzyme associated with cell wall metabolism. Both MdZF-HD11 and Mdβ-GAL18 genes were up-regulated by exogenous ethylene treatment and repressed by 1-methylcyclopropene treatment. Further experiments revealed that MdZF-HD11 binds directly to the Mdβ-GAL18 promoter and up-regulates its transcription. Moreover, using transgenic apple fruit calli, we found that overexpression of Mdβ-GAL18 or MdZF-HD11 significantly enhanced β-galactosidase activity, and overexpression of MdZF-HD11 induced the expression of Mdβ-GAL18. We also discovered that transient overexpression of Mdβ-GAL18 or MdZF-HD11 in 'Golden Delicious' apple significantly increased the release of ethylene, reduced fruit firmness, promoted the transformation of skin color from green to yellow, and accelerated ripening and softening of the fruit. Finally, the overexpression of MdZF-HD11 in tomato also promoted fruit softening. Collectively, these results indicate that ethylene-induced MdZF-HD11 interacts with Mdβ-GAL18 to promote the post-harvest softening of apple.

Transcriptional regulation of tomato fruit ripening

An intrinsic and genetically determined ripening program of tomato fruits often depends upon the appropriate activation of tissue- and stage-specific transcription factors in space and time. The past two decades have yielded considerable progress in detailing these complex transcriptional as well as hormonal regulatory circuits paramount to fleshy fruit ripening. This non-linear ripening process is strongly controlled by the MADS-box and NOR family of proteins, triggering a transcriptional response associated with the progression of fruit ripening. Deepening insights into the connection between MADS-RIN and plant hormones related transcription factors, such as ERFs and ARFs, further conjugates the idea that several signaling units work in parallel to define an output fruit ripening transcriptome. Besides these TFs, the role of other families of transcription factors such as MYB, GLK, WRKY, GRAS and bHLH have also emerged as important ripening regulators. Other regulators such as EIN and EIL proteins also determine the transcriptional landscape of ripening fruits. Despite the abundant knowledge of the complex spectrum of ripening networks in the scientific domain, identifying more ripening effectors would pave the way for a better understanding of fleshy fruit ripening at the molecular level. This review provides an update on the transcriptional regulators of tomato fruit ripening.

NtERF4 promotes the biosynthesis of chlorogenic acid and flavonoids by targeting PAL genes in Nicotiana tabacum

Chlorogenic acid (CGA) and flavonoids are important secondary metabolites, which modulate plant growth and development, and contribute to plant resistance to various environmental stresses. ERF4 has been shown to be a repressor of anthocyanin accumulation in grape, but its full roles in regulating the biosynthesis of other phenylpropanoid compounds still needs to be further studied. In the present study, two NtERF4 genes were identified from N. tabacum genome. The expression level of NtERF4a was higher than that of NtERF4b in all the tobacco tissues examined. Over-expression of NtERF4a significantly promoted the accumulation of CGA and flavonoids in tobacco leaves, while silencing of NtERF4a significantly repressed the biosynthesis of CGA and flavonoids. RNA-seq analysis of NtERF4a-OE and WT plants revealed 8 phenylpropanoids-related differentially expressed genes (DEGs), including 4 NtPAL genes that encode key enzymes in the phenylpropanoid pathway. Activation of NtERF4a-GR fusion protein in tobacco significantly induced the transcription of NtPAL1 and NtPAL2 in the presence of protein synthesis inhibitor. Chromatin immunoprecipitation and Dual-Luc assays further indicated that NtERF4a could bind to the GCC box presented in the promoters of NtPAL1 and NtPAL2, thereby activating their transcription. Moreover, ectopic expression of NtERF4a induced the transcription of NtGSK1, NtMYC2, and NtJAZ3 genes, and enhanced the resistance of tobacco seedlings to salt and drought stresses, indicating multiple roles of NtERF4a in plants. Our findings revealed new roles of NtERF4a in modulating the accumulation of phenylpropanoid compounds in tobacco, and provided a putative target for improving phenylpropanoids synthesis and stress resistance in plants.

ERF5.1 modulates carotenoid accumulation by interacting with CCD4.1 in Lycium

Carotenoids are important natural pigments and have medical and health functions for humans. Carotenoid cleavage dioxygenase 4 (CCD4) and ethylene responsive factor (ERF) participate in carotenoid metabolism, but their roles in Lycium have not been discovered. Here, we annotated LbCCDs from the Lycium reference genome and found that LbCCD4.1 expression was significantly correlated with the carotenoid metabolites during Lycium five fruit developmental stages. Over-expression of LbCCD4.1 in NQ's leaves resulted in a series of significantly lower contents of carotenoid metabolites, including β-carotene and β-cryptoxanthin. Moreover, LbERF5.1, a transcription factor belonging to the ERF family that was located in the nucleus, was isolated. Significant reductions in the carotenoids, especially lutein, violaxanthin and their derivatives, were observed in over-expressing ERF5.1 transgenic NQ's leaves. Over-expression or virus-induced gene silencing of LbERF5.1 in NQ's leaves induced a consistent up- or down-expression, respectively, of LbCCD4.1. Furthermore, yeast one-hybrid and dual-luciferase reporter assays showed that ERF5.1 interacted with the promoter of CCD4.1 to increase its expression, and LbERF5.1 could bind to any one of the three predicted binding sites in the promoter of LbCCD4.1. A transcriptome analysis of LbERF5.1 and LbCCD4.1 over-expressed lines showed similar global transcript expression, and geranylgeranyl diphosphate synthase, phytoene synthase, lycopene δ-cyclase cytochrome, cytochrome P450-type monooxygenase 97A, cytochrome P450-type monooxygenase 97C, and zeaxanthin epoxidase in the carotenoid biosynthesis pathway were differentially expressed. In summary, we uncovered a novel molecular mechanism of carotenoid accumulation that involved an interaction between ERF5.1 and CCD4.1, which may be used to enhance carotenoid in Lycium.

Insights into microRNA-mediated interaction and regulation of metabolites in tomato

microRNAs direct regulation of various metabolic pathways in plants and animals. miRNAs may be useful in developing novel/elite genotypes, with enhanced metabolites and disease resistance. We examined miRNAs in tomato. In tomato, miRNAs in the carotenoid pathway have not been fully elucidated. We examined the potential role of miRNAs in biosynthesis of carotenoids, transcript profiling of miRNAs and their possible targets (genes and transcription factors) at different development stages of tomato using stem-loop PCR and RT-qPCR. We also identified miRNAs targeting key flavonoid genes, such as chalcone isomerase (CHI), and dihydroflavonol-4-reductase (DFR). Distinct expression profiles of miRNAs and their targets were found in fruits of three tomato accessions, suggesting carotenoid regulation by miRNAs at various stages of fruit development. This was also confirmed using HPLC of the carotenoids. The present study may help in understanding possible regulation of carotenoid biosynthesis. The identified miRNAs can be exploited to enhance biosynthesis of different carotenoids in plants.

Identification of watermelon H3K4 and H3K27 genes and their expression profiles during watermelon fruit development

BACKGROUND

The ClaH3K4s and ClaH3K27s gene families are subfamilies of the SET family, each with a highly conserved SET structure domain and a PHD structural domain. Both participate in histone protein methylation, which affects the chromosome structure and gene expression, and is essential for fruit growth and development.

METHODS AND RESULTS

In order to demonstrate the structure and expression characteristics of ClaH3K4s and ClaH3K27s in watermelon, members of the watermelon H3K4 and H3K27 gene families were identified, and their chromosomal localization, gene structure, and protein structural domains were analyzed. The phylogeny and covariance of the gene families with other species were subsequently determined, and the expression profiles were obtained by performing RNA-Seq and qRT-PCR. The watermelon genome had five H3K4 genes with 3207-8043 bp nucleotide sequence lengths and four H3K27 genes with a 1107-5499 bp nucleotide sequence. Synteny analysis revealed the close relationship between watermelon and cucumber, with the majority of members displaying a one-to-one covariance. Approximately half of the 'Hua-Jing 13 watermelon' ClaH3K4s and ClaH3K27s genes were expressed more in the late fruit development stages, while the changes were minimal for the remaining half. H3K4-2 expression was observed to be slightly greater on day 21 compared to other periods. Moreover, ClaH3K27-1 and ClaH3K27-2 were hardly expressed throughout the developing period, and ClaH3K27-4 exhibited the highest expression.

CONCLUSION

These results serve as a basis for further functional characterization of the H3K4 and H3K27 genes in the fruit development of watermelon.

Fruit ripening under heat stress: The intriguing role of ethylene-mediated signaling

Crop production is significantly influenced by climate, and even minor climate changes can have a substantial impact on crop yields. Rising temperature due to climate change can lead to heat stress (HS) in plants, which not only hinders plant growth and development but also result in significant losses in crop yields. To cope with the different stresses including HS, plants have evolved a variety of adaptive mechanisms. In response to these stresses, phytohormones play a crucial role by generating endogenous signals that regulate the plant's defensive response. Among these, Ethylene (ET), a key phytohormone, stands out as a major regulator of stress responses in plants and regulates many plant traits, which are critical for crop productivity and nutritional quality. ET is also known as a ripening hormone for decades in climacteric fruit and many studies are available deciphering the function of different ET biosynthesis and signaling components in the ripening process. Recent studies suggest that HS significantly affects fruit quality traits and perturbs fruit ripening by altering the regulation of many ethylene biosynthesis and signaling genes resulting in substantial loss of fruit yield, quality, and postharvest stability. Despite the significant progress in this field in recent years the interplay between ET, ripening, and HS is elusive. In this review, we summarized the recent advances and current understanding of ET in regulating the ripening process under HS and explored their crosstalk at physiological and molecular levels to shed light on intricate relationships.

Trihelix transcription factor SlGT31 regulates fruit ripening mediated by ethylene in tomato

Trihelix proteins are plant-specific transcription factors that are classified as GT factors due to their binding specificity for GT elements, and they play crucial roles in development and stress responses. However, their involvement in fruit ripening and transcriptional regulatory mechanisms remains largely unclear. In this study, we cloned SlGT31, encoding a trihelix protein in tomato (Solanum lycopersicum), and determined that its relative expression was significantly induced by the application of exogenous ethylene whereas it was repressed by the ethylene-inhibitor 1-methylcyclopropene. Suppression of SlGT31 expression resulted in delayed fruit ripening, decreased accumulation of total carotenoids, and reduced ethylene content, together with inhibition of expression of genes related to ethylene and fruit ripening. Conversely, SlGT31-overexpression lines showed opposite results. Yeast one-hybrid and dual-luciferase assays indicated that SlGT31 can bind to the promoters of two key ethylene-biosynthesis genes, ACO1 and ACS4. Taken together, our results indicate that SlGT31 might act as a positive modulator during fruit ripening.

Integrative Analysis of Metabolome and Transcriptome Revealed Lutein Metabolism Contributed to Yellow Flower Formation in Prunus mume

Prunus mume is a famous ornamental woody tree with colorful flowers. P. mume with yellow flowers is one of the most precious varieties. Regretfully, metabolites and regulatory mechanisms of yellow flowers in P. mume are still unclear. This hinders innovation of flower color breeding in P. mume. To elucidate the metabolic components and molecular mechanisms of yellow flowers, we analyzed transcriptome and metabolome between 'HJH' with yellow flowers and 'ZLE' with white flowers. Comparing the metabolome of the two varieties, we determined that carotenoids made contributions to the yellow flowers rather than flavonoids. Lutein was the key differential metabolite to cause yellow coloration of 'HJH'. Transcriptome analysis revealed significant differences in the expression of carotenoid cleavage dioxygenase (CCD) between the two varieties. Specifically, the expression level of PmCCD4 was higher in 'ZLE' than that in 'HJH'. Moreover, we identified six major transcription factors that probably regulated PmCCD4 to affect lutein accumulation. We speculated that carotenoid cleavage genes might be closely related to the yellow flower phenotype in P. mume. Further, the coding sequence of PmCCD4 has been cloned from the 'HJH' petals, and bioinformatics analysis revealed that PmCCD4 possessed conserved histidine residues, ensuring its enzymatic activity. PmCCD4 was closely related to PpCCD4, with a homology of 98.16%. Instantaneous transformation analysis in petal protoplasts of P. mume revealed PmCCD4 localization in the plastid. The overexpression of PmCCD4 significantly reduced the carotenoid content in tobacco plants, especially the lutein content, indicating that lutein might be the primary substrate for PmCCD4. We speculated that PmCCD4 might be involved in the cleavage of lutein in plastids, thereby affecting the formation of yellow flowers in P. mume. This work could establish a material and molecular basis of molecular breeding in P. mume for improving the flower color.

Suppression of the target of rapamycin kinase accelerates tomato fruit ripening through reprogramming the transcription profile and promoting ethylene biosynthesis

Tomato fruit ripening is a unique process of nutritional and energy metabolism. Target of rapamycin (TOR), a conserved serine/threonine protein kinase in eukaryotes, controls cell growth and metabolism by integrating nutrient, energy, and hormone signals. However, it remains unclear whether TOR participates in the modulation of tomato fruit ripening. Here, we showed that the manipulation of SlTOR by chemical or genetic methods greatly alters the process of tomato fruit maturation. Expression pattern analysis revealed that the transcripts of SlTOR declined as fruit ripening progressed. Moreover, suppression of SlTOR by TOR inhibitor AZD8055 or knock down of its transcripts by inducible RNA interference, accelerated fruit ripening, and led to overall effects on fruit maturity, including changes in colour and metabolism, fruit softening, and expression of ripening-related genes. Genome-wide transcription analysis indicated that silencing SlTOR reprogrammed the transcript profile associated with ripening, including cell wall and phytohormone pathways, elevated the expression of ethylene biosynthetic genes, and further promoted ethylene production. In contrast, the ethylene action inhibitor 1-MCP efficiently blocked fruit maturation, even following SlTOR inhibition. These results suggest that accelerated fruit ripening caused by SlTOR inhibition depends on ethylene, and that SlTOR may function as a regulator in ethylene metabolism.

Tomato MED25 regulates fruit ripening by interacting with EIN3-like transcription factors

Fruit ripening relies on the precise spatiotemporal control of RNA polymerase II (Pol II)-dependent gene transcription, and the evolutionarily conserved Mediator (MED) coactivator complex plays an essential role in this process. In tomato (Solanum lycopersicum), a model climacteric fruit, ripening is tightly coordinated by ethylene and several key transcription factors. However, the mechanism underlying the transmission of context-specific regulatory signals from these ripening-related transcription factors to the Pol II transcription machinery remains unknown. Here, we report the mechanistic function of MED25, a subunit of the plant Mediator transcriptional coactivator complex, in controlling the ethylene-mediated transcriptional program during fruit ripening. Multiple lines of evidence indicate that MED25 physically interacts with the master transcription factors of the ETHYLENE-INSENSITIVE 3 (EIN3)/EIN3-LIKE (EIL) family, thereby playing an essential role in pre-initiation complex formation during ethylene-induced gene transcription. We also show that MED25 forms a transcriptional module with EIL1 to regulate the expression of ripening-related regulatory as well as structural genes through promoter binding. Furthermore, the EIL1-MED25 module orchestrates both positive and negative feedback transcriptional circuits, along with its downstream regulators, to fine-tune ethylene homeostasis during fruit ripening.

Integrative Analysis of Metabolome and Transcriptome Reveals the Mechanism of Color Formation in Yellow-Fleshed Kiwifruit

During the development of yellow-fleshed kiwifruit (Actinidia chinensis), the flesh appeared light pink at the initial stage, the pink faded at the fastest growth stage, and gradually changed into green. At the maturity stage, it showed bright yellow. In order to analyze the mechanism of flesh color change at the metabolic and gene transcription level, the relationship between color and changes of metabolites and key enzyme genes was studied. In this study, five time points (20 d, 58 d, 97 d, 136 d, and 175 d) of yellow-fleshed kiwifruit were used for flavonoid metabolites detection and transcriptome, and four time points (20 d, 97 d, 136 d, and 175 d) were used for targeted detection of carotenoids. Through the analysis of the content changes of flavonoid metabolites, it was found that the accumulation of pelargonidin and cyanidin and their respective anthocyanin derivatives was related to the pink flesh of young fruit, but not to delphinidin and its derivative anthocyanins. A total of 140 flavonoid compounds were detected in the flesh, among which anthocyanin and 76% of the flavonoid compounds had the highest content at 20 d, and began to decrease significantly at 58 d until 175 d, resulting in the pale-pink fading of the flesh. At the mature stage of fruit development (175 d), the degradation of chlorophyll and the increase of carotenoids jointly led to the change of flesh color from green to yellow, in addition to chlorophyll degradation. In kiwifruit flesh, 10 carotenoids were detected, with none of them being linear carotenoids. During the whole development process of kiwifruit, the content of β-carotene was always higher than that of α-carotene. In addition, β-cryptoxanthin was the most-accumulated pigment in the kiwifruit at 175 d. Through transcriptome analysis of kiwifruit flesh, seven key transcription factors for flavonoid biosynthesis and ten key transcription factors for carotenoid synthesis were screened. This study was the first to analyze the effect of flavonoid accumulation on the pink color of yellow-fleshed kiwifruit. The high proportion of β-cryptoxanthin in yellow-fleshed kiwifruit was preliminarily found. This provides information on metabolite accumulation for further revealing the pink color of yellow-fleshed kiwifruit, and also provides a new direction for the study of carotenoid biosynthesis and regulation in yellow-fleshed kiwifruit.

Transition to ripening in tomato requires hormone-controlled genetic reprogramming initiated in gel tissue

Ripening is the last stage of the developmental program in fleshy fruits. During this phase, fruits become edible and acquire their unique sensory qualities and post-harvest potential. Although our knowledge of the mechanisms that regulate fruit ripening has improved considerably over the past decades, the processes that trigger the transition to ripening remain poorly deciphered. While transcriptomic profiling of tomato (Solanum lycopersicum L.) fruit ripening to date has mainly focused on the changes occurring in pericarp tissues between the Mature Green and Breaker stages, our study addresses the changes between the Early Mature Green and Late Mature Green stages in the gel and pericarp separately. The data showed that the shift from an inability to initiate ripening to the capacity to undergo full ripening requires extensive transcriptomic reprogramming that takes place first in the locular tissues before extending to the pericarp. Genome-wide transcriptomic profiling revealed the wide diversity of transcription factor (TF) families engaged in the global reprogramming of gene expression and identified those specifically regulated at the Mature Green stage in the gel but not in the pericarp, thereby providing potential targets toward deciphering the initial factors and events that trigger the transition to ripening. The study also uncovered an extensive reformed homeostasis for most plant hormones, highlighting the multihormonal control of ripening initiation. Our data unveil the antagonistic roles of ethylene and auxin during the onset of ripening and show that auxin treatment delays fruit ripening via impairing the expression of genes required for System-2 autocatalytic ethylene production that is essential for climacteric ripening. This study unveils the detailed features of the transcriptomic reprogramming associated with the transition to ripening of tomato fruit and shows that the first changes occur in the locular gel before extending to pericarp and that a reformed auxin homeostasis is essential for the ripening to proceed.

The APETALA2a/DWARF/BRASSINAZOLE-RESISTANT 1 module contributes to carotenoid synthesis in tomato fruits

Ethylene (ET) signaling plays a critical role in the ripening of climacteric fruits such as tomato. Brassinosteroids (BRs) were found to promote the ripening of both climacteric and non-climacteric fruits. However, the mechanism of interaction between ET and BRs during fruit ripening is unclear. Here, we found that BR synthesis and signaling increased after the onset of fruit ripening. Overexpression of the BR synthesis gene DWARF (DWF) promotedfruit softening, lycopene synthesis and ET production, whereas defect of DWF inhibited them. BRASSINAZOLE RESISTANT 1 (BZR1) as a key component of BR signaling, enhanced fruit lycopene content by directly activating the transcription of PSY1 gene. Interestingly, the increases in BR synthesis and BZR1 protein levels were dependent on ET signaling. Knocking out the ET-induced APETALA2a (AP2a) suppressed the expression of DWF and BR accumulation. Molecular assays demonstrated that AP2a was a positive regulator of DWF expression. Furthermore, 28-homobrassinolide, a bioactive BR, partially compensated the defects of lycopene accumulation and expression of PSY1 in ap2a mutant fruits. The results demonstrated that AP2a mediated ET signaling to regulate BR synthesis and signaling. BRs played critical roles in lycopene synthesis after onset of fruit ripening.

Ethylene response factor ERF.D7 activates auxin response factor 2 paralogs to regulate tomato fruit ripening

Despite the obligatory role of ethylene in climacteric fruit ripening and the identification of 77 ethylene response factors (ERFs) in the tomato (Solanum lycopersicum) genome, the role of few ERFs has been validated in the ripening process. Here, using a comprehensive morpho-physiological, molecular, and biochemical approach, we demonstrate the regulatory role of ERF D7 (SlERF.D7) in tomato fruit ripening. SlERF.D7 expression positively responded to exogenous ethylene and auxin treatments, most likely in a ripening inhibitor-independent manner. SlERF.D7 overexpression (OE) promoted ripening, and its silencing had the opposite effect. Alterations in its expression modulated ethylene production, pigment accumulation, and fruit firmness. Consistently, genes involved in ethylene biosynthesis and signaling, lycopene biosynthesis, and cell wall loosening were upregulated in the OE lines and downregulated in RNAi lines. These transgenic lines also accumulated altered levels of indole-3-acetic acid at late-breaker stages. A positive association between auxin response factor 2 (ARF2) paralog's transcripts and SlERF.D7 mRNA levels and that SlARF2A and SlARF2B are direct targets of SlERF.D7 underpinned the perturbed auxin-ethylene crosstalk for the altered ripening program observed in the transgenic fruits. Overall, this study uncovers that SlERF.D7 positively regulates SlARF2A/B abundance to amalgamate auxin and ethylene signaling pathways for controlling tomato fruit ripening.

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.

A banana transcriptional repressor MaAP2a participates in fruit starch degradation during postharvest ripening

Fruit postharvest ripening is a crucial course for many fruits with significant conversion of biosubstance, which forms an intricate regulatory network. Ethylene facilitates the ripening process in banana with a remarkable change of fruit starch, but the mechanism adjusting the expression of starch degradation-related enzyme genes is incompletely discovered. Here, we describe a banana APETALA2 transcription factor (MaAP2a) identified as a transcriptional repressor with its powerful transcriptional inhibitory activity. The transcriptional level of MaAP2a gradually decreased with the transition of banana fruit ripening, suggesting a passive role of MaAP2a in banana fruit ripening. Moreover, MaAP2a is a classic nucleoprotein and encompasses transcriptional repressor domain (EAR, LxLxLx). More specifically, protein-DNA interaction assays found that MaAP2a repressed the expression of 15 starch degradation-related genes comprising MaGWD1, MaPWD1, MaSEX4, MaLSF1, MaBAM1-MaBAM3, MaAMY2B/2C/3A/3C, MaMEX1/2, and MapGlcT2-1/2-2 via binding to the GCC-box or AT-rich motif of their promoters. Overall, these results reveal an original MaAP2a-mediated negative regulatory network involved in banana postharvest starch breakdown, which advances our cognition on banana fruit ripening and offers additional reference values for banana varietal improvement.

A tomato HD-zip I transcription factor, VAHOX1, acts as a negative regulator of fruit ripening

Homeodomain-leucine zipper (HD-Zip) transcription factors are only present in higher plants and are involved in plant development and stress responses. However, our understanding of their participation in the fruit ripening of economical plants, such as tomato (Solanum lycopersicum), remains largely unclear. Here, we report that VAHOX1, a member of the tomato HD-Zip I subfamily, was expressed in all tissues, was highly expressed in breaker+4 fruits, and could be induced by ethylene. RNAi repression of VAHOX1 (VAHOX1-RNAi) resulted in accelerated fruit ripening, enhanced sensitivity to ethylene, and increased total carotenoid content and ethylene production. Conversely, VAHOX1 overexpression (VAHOX1-OE) in tomato had the opposite effect. RNA-Seq results showed that altering VAHOX1 expression affected the transcript accumulation of a series of genes involved in ethylene biosynthesis and signal transduction and cell wall modification. Additionally, a dual-luciferase reporter assay, histochemical analysis of GUS activity and a yeast one-hybrid (Y1H) assay revealed that VAHOX1 could activate the expression of AP2a. Our findings may expand our knowledge about the physiological functions of HD-Zip transcription factors in tomato and highlight the diversities of transcriptional regulation during the fruit ripening process.

Molecular and Genetic Events Determining the Softening of Fleshy Fruits: A Comprehensive Review

Fruit softening that occurs during fruit ripening and postharvest storage determines the fruit quality, shelf life and commercial value and makes fruits more attractive for seed dispersal. In addition, over-softening results in fruit eventual decay, render fruit susceptible to invasion by opportunistic pathogens. Many studies have been conducted to reveal how fruit softens and how to control softening. However, softening is a complex and delicate life process, including physiological, biochemical and metabolic changes, which are closely related to each other and are affected by environmental conditions such as temperature, humidity and light. In this review, the current knowledge regarding fruit softening mechanisms is summarized from cell wall metabolism (cell wall structure changes and cell-wall-degrading enzymes), plant hormones (ETH, ABA, IAA and BR et al.), transcription factors (MADS-Box, AP2/ERF, NAC, MYB and BZR) and epigenetics (DNA methylation, histone demethylation and histone acetylation) and a diagram of the regulatory relationship between these factors is provided. It will provide reference for the cultivation of anti-softening fruits.

Tomato MicroRNAs and Their Functions

MicroRNAs (miRNAs) define an essential class of non-coding small RNAs that function as posttranscriptional modulators of gene expression. They are coded by MIR genes, several hundreds of which exist in the genomes of Arabidopsis and rice model plants. The functional analysis of Arabidopsis and rice miRNAs indicate that their miRNAs regulate a wide range of processes including development, reproduction, metabolism, and stress. Tomato serves as a major model crop for the study of fleshy fruit development and ripening but until recently, information on the identity of its MIR genes and their coded miRNAs was limited and occasionally contradictory. As a result, the majority of tomato miRNAs remained uncharacterized. Recently, a comprehensive annotation of tomato MIR genes has been carried out by several labs and us. In this review, we curate and organize the resulting partially overlapping MIR annotations into an exhaustive and non-redundant atlas of tomato MIR genes. There are 538 candidate and validated MIR genes in the atlas, of which, 169, 18, and 351 code for highly conserved, Solanaceae-specific, and tomato-specific miRNAs, respectively. Furthermore, a critical review of functional studies on tomato miRNAs is presented, highlighting validated and possible functions, creating a useful resource for future tomato miRNA research.

Overexpression of PSY1 increases fruit skin and flesh carotenoid content and reveals associated transcription factors in apple (Malus × domestica)

Knowledge of the transcriptional regulation of the carotenoid metabolic pathway is still emerging and here, we have misexpressed a key biosynthetic gene in apple to highlight potential transcriptional regulators of this pathway. We overexpressed phytoene synthase (PSY1), which controls the key rate-limiting biosynthetic step, in apple and analyzed its effects in transgenic fruit skin and flesh using two approaches. Firstly, the effects of PSY overexpression on carotenoid accumulation and gene expression was assessed in fruit at different development stages. Secondly, the effect of light exclusion on PSY1-induced fruit carotenoid accumulation was examined. PSY1 overexpression increased carotenoid content in transgenic fruit skin and flesh, with beta-carotene being the most prevalent carotenoid compound. Light exclusion by fruit bagging reduced carotenoid content overall, but carotenoid content was still higher in bagged PSY fruit than in bagged controls. In tissues overexpressing PSY1, plastids showed accelerated chloroplast to chromoplast transition as well as high fluorescence intensity, consistent with increased number of chromoplasts and carotenoid accumulation. Surprisingly, the expression of other carotenoid pathway genes was elevated in PSY fruit, suggesting a feed-forward regulation of carotenogenesis when this enzyme step is mis-expressed. Transcriptome profiling of fruit flesh identified differentially expressed transcription factors (TFs) that also were co-expressed with carotenoid pathway genes. A comparison of differentially expressed genes from both the developmental series and light exclusion  treatment revealed six candidate TFs exhibiting strong correlation with carotenoid accumulation. This combination of physiological, transcriptomic and metabolite data sheds new light on plant carotenogenesis and TFs that may play a role in regulating apple carotenoid biosynthesis.

Overexpression of PagERF072 from Poplar Improves Salt Tolerance

Extreme environments, especially drought and high salt conditions, seriously affect plant growth and development. Ethylene-responsive factor (ERF) transcription factors play an important role in salt stress response. In this study, a significantly upregulated ERF gene was identified in 84K (Populus alba × P. glandulosa), which was named PagERF072. PagERF072 was confirmed to be a nuclear-localized protein. The results of yeast two-hybrid (Y2H) assay showed that PagERF072 protein exhibited no self-activating activity, and yeast one-hybrid (Y1H) demonstrated that PagERF072 could specifically bind to GCC-box element. Under salt stress, the transgenic poplar lines overexpressing PagERF072 showed improved salt tolerance. The activities of peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT) in transgenic poplars were significantly increased relative to those of wild-type (WT) plants, whereas malondialdehyde (MDA) content showed an opposite trend. In addition, reactive oxygen species (ROS) was significantly reduced, and the expression levels of POD- and SOD-related genes were significantly increased in transgenic poplars under salt stress compared with WT. All results indicate that overexpression of the PagERF072 gene can improve the salt tolerance of transgenic poplars.

Silicon inhibits gummosis in peach via ethylene and PpERF-PpPG1 pathway

Silicon (Si) is abundant in nature, and it has been proved to be beneficial for the healthy growth and development of many plant species, improve plant stress resistance. Gummosis in peach is an invasive disease that causes widespread and serious damage. Mechanical damage and ethylene (ETH) can induce gummosis in peach shoots in the field. In this research, we found that Si as a chemical substance or signal to enhance plant resistance can reduce the synthesis of ETH, thereby inhibiting gummosis in peach. The results showed that Si can decrease the rate of gummosis, reduce the expression level of PpACS1 (1-aminocyclopropane -1-carboxylate synthase gene) and reduce the enzyme activity of polygalacturonase (PG). It was further discovered that Si can regulate the gene expression of PpERF21 and PpERF27. Yeast one-hybrid and dual-luciferase reporter assays showed that PpERF21 and PpERF27, through direct interaction with the promoter of PpPG1, inhibited the transcriptional activation of PpPG1. Overexpression of PpERF21 and PpERF27 effectively reduced fruit colloid production when bacterial cells harbouring the expression vector were used to instantaneously infect peach fruit. These results show that Si can inhibit the synthesis of ETH and mediate PpERF21 and PpERF27 expression to inhibit the expression of PpPG1, thereby inhibiting gummosis in peach.

Transcriptional regulation of fleshy fruit texture

Fleshy fruit texture is a critically important quality characteristic of ripe fruit. Softening is an irreversible process which operates in most fleshy fruits during ripening which, together with changes in color and taste, contributes to improvements in mouthfeel and general attractiveness. Softening results mainly from the expression of genes encoding enzymes responsible for cell wall modifications but starch degradation and high levels of flavonoids can also contribute to texture change. Some fleshy fruit undergo lignification during development and post-harvest, which negatively affects eating quality. Excessive softening can also lead to physical damage and infection, particularly during transport and storage which causes severe supply chain losses. Many transcription factors (TFs) that regulate fruit texture by controlling the expression of genes involved in cell wall and starch metabolism have been characterized. Some TFs directly regulate cell wall targets, while others act as part of a broader regulatory program governing several aspects of the ripening process. In this review, we focus on advances in our understanding of the transcriptional regulatory mechanisms governing fruit textural change during fruit development, ripening and post-harvest. Potential targets for breeding and future research directions for the control of texture and quality improvement are discussed.

Identification of essential element determining fruit-specific transcriptional activity in the tomato HISTIDINE DECARBOXYLASE A gene promoter

In SlHDC-A promoter, SlHDC-A core-ES is an essential region for fruit-specific expression and interacts with GATA, HSF and AP1. Triplication of essential region was proposed as a minimal fruit-specific promoter. In plant biotechnology, fruit-specific promoter is an important tool for the improvement and utilization of tomato fruit. To expand our understanding on fruit-specific expression, it is necessary to determine the promoter region involved in fruit-specific transcriptional activity and transcriptional regulations of the promoter. In previous study, we isolated a fruit-specific SlHDC-A core promoter specifically expressed during tomato ripening stages. In this study, we identified SlHDC-A promoter region (SlHDC-A core-ES) that is essential for fruit-specific expression of the SlHDC-A. To understand the molecular mechanisms of fruit-specific expression of the SlHDC-A promoter, we first identified the putative transcription factor binding elements in the SlHDC-A core promoter region and corresponding putative transcription factors which are highly expressed during fruit maturation. Yeast one hybrid analysis confirmed that GATA, HSF, and AP1 interact with the SlHDC-A core-ES promoter region. Further transactivation analysis revealed that expression of the three transcription factors significantly activated expression of a reporter gene driven by SlHDC-A core-ES promoter. These results suggest that GATA, HSF, and AP1 are involved in the fruit-specific expression of SlHDC-A promoter. Furthermore, the synthetic promoter composed of three tandem repeats of SlHDC-A core-ES showed relatively higher activity than the constitutive 35S promoter in the transgenic tomato fruits at the orange stage. Taken together, we propose a new synthetic promoter that is specifically expressed during fruit ripening stage.

Transcriptome analysis of peach fruit under 1-MCP treatment provides insights into regulation network in melting peach softening

Peach (Prunus persica L.) displays distinguish texture phenotype during postharvest, which could be classified into three types, including melting flesh (MF), non-melting flesh (NMF) and stony-hard (SH). Of that MF peach would soften rapidly with an outbreak of ethylene production, which cause a huge waste during fruit transportation and storage. 1-methylcyclopropene (1-MCP) was used to alleviate fruit softening. In this study, we performed RNA-sequencing on two MF peach cultivars (‘YuLu’ and ‘Yanhong’) after 1-MCP treatment to identify the candidate genes participating in peach fruit softening. 167 genes were identified by WGCNA and correlation analysis, which could respond to 1-MCP treatment and might be related to softening. Among them, 5 auxin related genes including 2 IAAs, 1 ARF and 2 SAURs, and 4 cell wall modifying genes (PpPG1, PpPG2, PpPG24 and PpPMEI) were characterized as key genes participating in MF peach softening. Furthermore, 2 transcription factors, which belong to HD-ZIP and MYB were predicted as candidates regulating softening process by constructing transcriptional network of these 4 cell wall modifying genes combined with expression pattern analysis, of that the HD-ZIP could trans-activate promoter of PpPG1.

Reduced auxin signalling through the cyclophilin gene DIAGEOTROPICA impacts tomato fruit development and metabolism during ripening

Auxin is an important hormone playing crucial roles during fruit growth and ripening; however, the metabolic impact of changes in auxin signalling during tomato (Solanum lycopersicum L.) ripening remains unclear. Here, we investigated the significance of changes in auxin signalling during different stages of fruit development by analysing changes in tomato fruit quality and primary metabolism using mutants with either lower or higher auxin sensitivity [diageotropica (dgt) and entire mutants, respectively]. Altered auxin sensitivity modifies metabolism, through direct impacts on fruit respiration and fruit growth. We verified that the dgt mutant plants exhibit reductions in fruit set, total fruit dry weight, fruit size, number of seeds per fruit, and fresh weight loss during post-harvest. Sugar accumulation was associated with delayed fruit ripening in dgt, probably connected with reduced ethylene levels and respiration, coupled with a lower rate of starch degradation. In contrast, despite exhibiting parthenocarpy, increased auxin perception (entire) did not alter fruit ripening, leading to only minor changes in primary metabolism. By performing a comprehensive analysis, our results connect auxin signalling and metabolic changes during tomato fruit development, indicating that reduced auxin signalling led to extensive changes in sugar concentration and starch metabolism during tomato fruit ripening.

Genome-wide cis-decoding for expression design in tomato using cistrome data and explainable deep learning

In the evolutionary history of plants, variation in cis-regulatory elements (CREs) resulting in diversification of gene expression has played a central role in driving the evolution of lineage-specific traits. However, it is difficult to predict expression behaviors from CRE patterns to properly harness them, mainly because the biological processes are complex. In this study, we used cistrome datasets and explainable convolutional neural network (CNN) frameworks to predict genome-wide expression patterns in tomato (Solanum lycopersicum) fruit from the DNA sequences in gene regulatory regions. By fixing the effects of trans-acting factors using single cell-type spatiotemporal transcriptome data for the response variables, we developed a prediction model for crucial expression patterns in the initiation of tomato fruit ripening. Feature visualization of the CNNs identified nucleotide residues critical to the objective expression pattern in each gene, and their effects were validated experimentally in ripening tomato fruit. This cis-decoding framework will not only contribute to the understanding of the regulatory networks derived from CREs and transcription factor interactions, but also provides a flexible means of designing alleles for optimized expression.

Histone deacetylase gene SlHDT1 regulates tomato fruit ripening by affecting carotenoid accumulation and ethylene biosynthesis

Fruit development and ripening is a complicated biological process, that is not only regulated by plant hormones and transcription factors, but also affected by epigenetic modifications. Histone deacetylation is an important way of epigenetic modification, and little information about it is available. In this study, an RNAi vector was constructed and transferred successfully into wild-type tomato for further research on the detailed functions of the histone deacetylation gene SlHDT1. The expression level of PSY1 was upregulated, and the transcription levels of LCY-B, LCY-E and CYC-B were downregulated, which was consistent with the increased accumulation of carotenoids. In addition, the expression levels of ethylene biosynthetic genes (ACS2, ACS4 and ACO1, ACO3), ripening-associated genes (RIN, E4, E8, PG, Pti4 and LOXB) and fruit cell wall metabolism genes (HEX, MAN, TBG4, XTH5 and XYL) were significantly upregulated further strengthening the results, including an increased ethylene content, advanced fruit ripening time and a shortened shelf life of tomato fruits. In addition, the increased total histone H3 acetylation level also provides evidence of a connection between epigenetic regulation by histone deacetylation and fruit development and ripening. Hence, SlHDT1 is a negative regulator and plays an essential role in regulating ethylene and carotenoid biosynthesis during fruit ripening through influences on the acetylation level.

Multi-Omics Analysis Reveals That SlERF.D6 Synergistically Regulates SGAs and Fruit Development

Steroidal glycoalkaloids (SGAs) are cholesterol-derived molecules that contribute to the pathogen defense in tomato but are toxic and considered to be antinutritional compounds to humans. APETALA2/Ethylene Responsive Factor (AP2/ERF) family transcription factors (TFs) play an indispensable role in various biological processes, such as plant growth and development, fruit ripening, biotic and abiotic stresses responses, and SGA biosynthesis. In this study, we identified 176 AP2/ERF genes that were domesticated or improved SlAP2/ERF in the tomato variome (Solanum lycopersicum) within either domestication or improvement sweeps, respectively. According to the RNA-sequencing data, 93 of the ERF genes with high transcriptional level (Transcripts Per Million, TPM > 1) belong to six clusters. Weighted gene co-expression network analysis (WGCNA) and metabolite-based genome-wide association study (mGWAS) analyses revealed that the expression level of the Solyc04g071770 (SlERF.D6) gene in the cluster six gradually increased as the fruit matured. Transient transformation verified that the overexpression of SlERF.D6 significantly promoted fruit ripening and regulated the expression of multiple genes in the SGA synthesis pathway, thereby affecting the SGA content of the fruit. Virus-induced gene silencing (VIGS) showed that the silencing of SlERF.D6 delayed fruit ripening and influenced the content of SGAs. Our data provide new insights into AP2/ERF TFs in tomato, offer a candidate TF for fruit development and steroidal glycoalkaloids, and provide new resources for tomato breeding and improvement.