LeERF1 positively modulated ethylene triple response on etiolated seedling, plant development and fruit ripening and softening in tomato

Ripening and rot: How ripening processes influence disease susceptibility in fleshy fruits

Fleshy fruits become more susceptible to pathogen infection when they ripen; for example, changes in cell wall properties related to softening make it easier for pathogens to infect fruits. The need for high-quality fruit has driven extensive research on improving pathogen resistance in important fruit crops such as tomato (Solanum lycopersicum). In this review, we summarize current progress in understanding how changes in fruit properties during ripening affect infection by pathogens. These changes affect physical barriers that limit pathogen entry, such as the fruit epidermis and its cuticle, along with other defenses that limit pathogen growth, such as preformed and induced defense compounds. The plant immune system also protects ripening fruit by recognizing pathogens and initiating defense responses involving reactive oxygen species production, mitogen-activated protein kinase signaling cascades, and jasmonic acid, salicylic acid, ethylene, and abscisic acid signaling. These phytohormones regulate an intricate web of transcription factors (TFs) that activate resistance mechanisms, including the expression of pathogenesis-related genes. In tomato, ripening regulators, such as RIPENING INHIBITOR and NON_RIPENING, not only regulate ripening but also influence fruit defenses against pathogens. Moreover, members of the ETHYLENE RESPONSE FACTOR (ERF) family play pivotal and distinct roles in ripening and defense, with different members being regulated by different phytohormones. We also discuss the interaction of ripening-related and defense-related TFs with the Mediator transcription complex. As the ripening processes in climacteric and non-climacteric fruits share many similarities, these processes have broad applications across fruiting crops. Further research on the individual contributions of ERFs and other TFs will inform efforts to diminish disease susceptibility in ripe fruit, satisfy the growing demand for high-quality fruit and decrease food waste and related economic losses.

Revealing the specific regulations of nitric oxide on the postharvest ripening and senescence of bitter melon fruit

Bitter melon fruit is susceptible to yellowing, softening, and rotting under room-temperature storage conditions, resulting in reduced commercial value. Nitric oxide (NO) is an important signaling molecule and plays a crucial role in regulating the fruit postharvest quality. In this study, we investigated the effects of NO treatment on changes in sensory and firmness of bitter melon fruit during postharvest storage. Moreover, transcriptomic, metabolomic, and proteomic analyses were performed to elucidate the regulatory mechanisms through which NO treatment delays the ripening and senescence of bitter melon fruit. Our results show that differentially expressed genes (DEGs) were involved in fruit texture (CSLE, β-Gal, and PME), plant hormone signal transduction (ACS, JAR4, and AUX28), and fruit flavor and aroma (SUS2, LOX, and GDH2). In addition, proteins differentially abundant were associated with fruit texture (PLY, PME, and PGA) and plant hormone signal transduction (PBL15, JAR1, and PYL9). Moreover, NO significantly increased the abundance of key enzymes involved in the phenylpropanoid biosynthetic pathway, thus enhancing the disease resistance and alleviating softening of bitter melon fruit. Finally, differential metabolites mainly included phenolic acids, terpenoids, and flavonoids. These results provide a theoretical basis for further studies on the physiological changes associated with postharvest ripening and senescence of bitter melon fruit.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-023-00110-y.

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.

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.

SHINE clade of ERF transcription factors: A significant player in abiotic and biotic stress tolerance in plants

SHINE (SHN) clade transcription factors (TFs) represents a subfamily of APETALA2/ethylene-responsive factor (AP2/ERF) proteins. The latter, is characterized by its responsiveness to the phytohormone ethylene and the presence of AP2 DNA-binding domain. They are involved in many biological processes and in responses to different environmental constraints. SHN TFs were among the first identified regulators of cuticle formation. Cuticle plays crucial role in plant tolerance to drought, salinity and high temperature as well as in defense against pathogens. In addition, SHN were shown to be involved in the regulation of stomatal development which influences resistance to drought and diseases. Interestingly, recent studies have also shown that SHN TFs are involved in mediating the beneficial effects of arbuscular mycorrhizal fungi (AMF) as well as disease resistance conferred by nanoparticles. To fulfill their roles, SHN TFs are controlled upstream by other TFs and they control, in their turn, different downstream genes. In this review, we highlight the role of SHN TFs in different abiotic and biotic stresses through their involvement in cuticle biosynthesis, stomatal development and molecular regulation of biochemical and physiological traits. In addition, we discuss the regulation of SHN TFs by plant hormones and their influence on hormone biosynthesis and signaling pathways. Knowledge of this complex regulation can be put into contribution to increase multiple abiotic stress tolerances through transgenesis, gene editing and classical breeding.

Genome-wide identification and molecular characterization of the AP2/ERF superfamily members in sand pear (Pyrus pyrifolia)

BACKGROUND

'Whangkeumbae' (Pyrus pyrifolia) is a typical climacteric fruit variety of sand pear with excellent taste. However, the rapid postharvest ethylene production limits the shelf life of 'Whangkeumbae' fruit. AP2/ERF superfamily is a large family of transcription factors involved in plant growth and development, including fruit ripening and senescence through the ethylene signaling pathway. The numbers and functions of AP2/ERF superfamily members in sand pear remain largely unknown.

RESULTS

In this study, a total of 234 AP2/ERF family members were identified through the transcriptome of Pyrus pyrifolia 'Whangkeumbae' (17 genes) and Pyrus pyrifolia genome (223 genes) analyses. Six genes (Accession: EVM0023062.1, EVM0034833.1, EVM0027049.1, EVM0034047.1, EVM0028755.1, EVM0015862.1) identified via genome analysis shared 100% identity with PpERF14-L, PpERF5-L, PpERF3a, PpERF3, PpERF017 and PpERF098, respectively, which were identified from transcriptome sequencing. Further, the AP2/ERF superfamily members were divided into AP2, ERF, and RAV subfamilies, each comprising 38, 188, and 8 members, respectively. Tissue-specific expression analysis showed that PpERF061, PpERF113, PpERF51L-B, PpERF5-L, and PpERF017 were predominantly expressed in fruits than in other tissues. Additionally, PpERF5-L and PpERF017 showed higher expressions at the early stage of fruit development. While, PpERF51B-L exhibited higher expression during the fruit ripening stage. Besides, PpERF061 and PpERF113 had pronounced expressions during fruit senescence.

CONCLUSION

These results indicate that PpERF061, PpERF113, PpERF51L-B, PpERF5-L, and PpERF017 could play crucial roles in sand pear fruit development, ripening, and senescence. Overall, this study provides valuable information for further functional analysis of the AP2/ERF genes during fruit ripening and senescence in sand pear.

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.

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.

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.

Ethylene Signaling under Stressful Environments: Analyzing Collaborative Knowledge

Ethylene is a gaseous plant growth hormone that regulates various plant developmental processes, ranging from seed germination to senescence. The mechanisms underlying ethylene biosynthesis and signaling involve multistep mechanisms representing different control levels to regulate its production and response. Ethylene is an established phytohormone that displays various signaling processes under environmental stress in plants. Such environmental stresses trigger ethylene biosynthesis/action, which influences the growth and development of plants and opens new windows for future crop improvement. This review summarizes the current understanding of how environmental stress influences plants’ ethylene biosynthesis, signaling, and response. The review focuses on (a) ethylene biosynthesis and signaling in plants, (b) the influence of environmental stress on ethylene biosynthesis, (c) regulation of ethylene signaling for stress acclimation, (d) potential mechanisms underlying the ethylene-mediated stress tolerance in plants, and (e) summarizing ethylene formation under stress and its mechanism of action.

Contrasting Roles of Ethylene Response Factors in Pathogen Response and Ripening in Fleshy Fruit

Fleshy fruits are generally hard and unpalatable when unripe; however, as they mature, their quality is transformed by the complex and dynamic genetic and biochemical process of ripening, which affects all cell compartments. Ripening fruits are enriched with nutrients such as acids, sugars, vitamins, attractive volatiles and pigments and develop a pleasant taste and texture and become attractive to eat. Ripening also increases sensitivity to pathogens, and this presents a crucial problem for fruit postharvest transport and storage: how to enhance pathogen resistance while maintaining ripening quality. Fruit development and ripening involve many changes in gene expression regulated by transcription factors (TFs), some of which respond to hormones such as auxin, abscisic acid (ABA) and ethylene. Ethylene response factor (ERF) TFs regulate both fruit ripening and resistance to pathogen stresses. Different ERFs regulate fruit ripening and/or pathogen responses in both fleshy climacteric and non-climacteric fruits and function cooperatively or independently of other TFs. In this review, we summarize the current status of studies on ERFs that regulate fruit ripening and responses to infection by several fungal pathogens, including a systematic ERF transcriptome analysis of fungal grey mould infection of tomato caused by Botrytis cinerea. This deepening understanding of the function of ERFs in fruit ripening and pathogen responses may identify novel approaches for engineering transcriptional regulation to improve fruit quality and pathogen resistance.

SlERF.F12 modulates the transition to ripening in tomato fruit by recruiting the co-repressor TOPLESS and histone deacetylases to repress key ripening genes

Ethylene response factors (ERFs) are downstream components of ethylene-signaling pathways known to play critical roles in ethylene-controlled climacteric fruit ripening, yet little is known about the molecular mechanism underlying their mode of action. Here, we demonstrate that SlERF.F12, a member of the ERF.F subfamily containing Ethylene-responsive element-binding factor-associated Amphiphilic Repression (EAR) motifs, negatively regulates the onset of tomato (Solanum lycopersicum) fruit ripening by recruiting the co-repressor TOPLESS 2 (TPL2) and the histone deacetylases (HDAs) HDA1/HDA3 to repress the transcription of ripening-related genes. The SlERF.F12-mediated transcriptional repression of key ripening-related genes 1-AMINO-CYCLOPROPANE-1-CARBOXYLATE SYNTHASE 2 (ACS2), ACS4, POLYGALACTURONASE 2a, and PECTATE LYASE is dependent on the presence of its C-terminal EAR motif. We show that SlERF.F12 interacts with the co-repressor TPL2 via the C-terminal EAR motif and recruits HDAs SlHDA1 and SlHDA3 to form a tripartite complex in vivo that actively represses transcription of ripening genes by decreasing the level of the permissive histone acetylation marks H3K9Ac and H3K27Ac at their promoter regions. These findings provide new insights into the ripening regulatory network and uncover a direct link between repressor ERFs and histone modifiers in modulating the transition to ripening of climacteric fruit.

A tomato NAC transcription factor, SlNAM1, positively regulates ethylene biosynthesis and the onset of tomato fruit ripening

Fruit ripening in tomato (Solanum lycopersicum) is the result of selective expression of ripening-related genes, which are regulated by transcription factors (TFs). The NAC (NAM, ATAF1/2, and CUC2) TF family is one of the largest families of plant-specific TFs and members are involved in a variety of plant physiological activities, including fruit ripening. Fruit ripening-associated NAC TFs studied in tomato to date include NAC-NOR (non-ripening), SlNOR-like1 (non-ripening like1), SlNAC1, and SlNAC4. Considering the large number of NAC genes in the tomato genome, there is little information about the possible roles of other NAC members in fruit ripening, and research on their target genes is lacking. In this study, we characterize SlNAM1, a NAC TF, which positively regulates the initiation of tomato fruit ripening via its regulation of ethylene biosynthesis. The onset of fruit ripening in slnam1-deficient mutants created by CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9) technology was delayed, whereas fruit ripening in OE-SlNAM1 lines was accelerated compared with the wild type. The results of RNA-sequencing (RNA-seq) and promoter analysis suggested that SlNAM1 directly binds to the promoters of two key ethylene biosynthesis genes (1-aminocyclopropane-1-carboxylate synthase: SlACS2 and SlACS4) and activates their expression. This hypothesis was confirmed by electrophoretic mobility shift assays and dual-luciferase reporter assay. Our findings provide insights into the mechanisms of ethylene production and enrich understanding of the tomato fruit ripening regulatory network.

PpIAA1 and PpERF4 form a positive feedback loop to regulate peach fruit ripening by integrating auxin and ethylene signals

The signaling pathways of both auxin and ethylene regulate peach fruit ripening via the Aux/IAA and ERF transcription factors, respectively. However, the molecular mechanisms that coordinate both auxin and ethylene signals during peach fruit ripening remain unclear. In this study, we show that PpIAA1 and PpERF4 act as key players in a positive feedback loop, and promote peach fruit ripening by directly binding to and enhancing the activity of target gene promoters. PpIAA1 increased the expression of the ethylene biosynthesis gene PpACS1. Furthermore, PpERF4 enhanced the transcription of PpACO1 and PpIAA1 genes by binding to their promoters. Additionally, PpIAA1 and PpERF4 bound to each other to form a complex, which then enhanced the transcription of abscisic acid biosynthesis genes (PpNCED2 and PpNCED3) and the fruit softening gene (PpPG1) to levels higher than those achieved by each transcription factor individually. Moreover, overexpression of PpIAA1 in tomato accelerated fruit ripening and shortened the fruit shelf-life by increasing the production of ethylene and the expression levels of ripening regulator genes. Collectively, these results advance our understanding of the molecular mechanisms underlying peach fruit ripening and softening via auxin and ethylene signaling pathways.

AcERF1B and AcERF073 Positively Regulate Indole-3-acetic Acid Degradation by Activating AcGH3.1 Transcription during Postharvest Kiwifruit Ripening

Ethylene can accelerate the postharvest ripening process of kiwifruit, while indole-3-acetic acid (IAA) delays it. However, the molecular mechanism by which ethylene regulates IAA degradation is unclear. Here, we found that ethephon promotes the degradation of free IAA in kiwifruit. Furthermore, ethylene can promote the expression of AcGH3.1 and enhance its promoter activity. Two ethylene response factors (ERFs), AcERF1B and AcERF073, were obtained using an AcGH3.1 promoter as bait for a yeast one-hybrid screening library. Both AcERF1B and AcERF073 bind to the AcGH3.1 promoter to activate it. Also, AcERF1B/073 enhanced AcGH3.1 expression, decreased the free IAA content, and increased the IAA-Asp content in kiwifruit. In addition, we found that the AcERF1B and AcERF073 proteins directly interact, and this interaction enhanced their binding to the AcGH3.1 promoter. In summary, our results suggest that AcERF1B and AcERF073 positively regulate IAA degradation by activating AcGH3.1 transcription, which accelerated postharvest kiwifruit ripening.

Comparative Transcriptomic Analysis Provides Novel Insights into the Blanched Stem of Oenanthe javanica

In the agricultural field, blanching is a technique used to obtain tender, sweet, and delicious water dropwort stems by blocking sunlight. The physiological and nutritional parameters of blanched water dropwort have been previously investigated. However, the molecular mechanism of blanching remains unclear. In the present study, we investigated transcriptomic variations for different blanching periods in the stem of water dropwort (pre, mid, post-blanching, and control). The results showed that many genes in pathways, such as photosynthesis, carbon fixation, and phytohormone signal transduction as well as transcription factors (TFs) were significantly dysregulated. Blanched stems of water dropwort showed the higher number of downregulated genes in pathways, such as photosynthesis, antenna protein, carbon fixation in photosynthetic organisms, and porphyrin and chlorophyll metabolism, which ultimately affect the photosynthesis in water dropwort. The genes of hormone signal transduction pathways (ethylene, jasmonic acid, brassinosteroid, and indole-3-acetic acid) showed upregulation in the post-blanched water dropwort plants. Overall, a higher number of genes coding for TFs, such as ERF, BHLH, MYB, zinc-finger, bZIP, and WRKY were overexpressed in blanched samples in comparison with the control. These genes and pathways participate in inducing the length, developmental processes, pale color, and stress tolerance of the blanched stem. Overall, the genes responsive to blanching, which were identified in this study, provide an effective foundation for further studies on the molecular mechanisms of blanching and photosynthesis regulations in water dropwort and other species.

L2, a chloroplast metalloproteinase, regulates fruit ripening by participating in ethylene autocatalysis under the control of ethylene response factors

Although autocatalytic ethylene biosynthesis plays an important role in the ripening of climacteric fruits, our knowledge of the network that promotes it remains limited. We identified white fruit (wf), a tomato mutant that produces immature fruit that are white and that ripen slowly. We found that an inversion on chromosome 10 disrupts the LUTESCENT2 (L2) gene, and that white fruit is allelic to lutescent2. Using CRISPR/Cas9 technology we knocked out L2 in wild type tomato and found that the l2-cr mutants produced phenotypes that were very similar to white fruit (lutescent2). In the l2-cr fruit, chloroplast development was impaired and the accumulation of carotenoids and lycopene occurred more slowly than in wild type. During fruit ripening in l2-cr mutants, the peak of ethylene release was delayed, less ethylene was produced, and the expression of ACO genes was significantly suppressed. We also found that exogenous ethylene induces the expression of L2 and that ERF.B3, an ethylene response factor, binds to the promoter of the L2 gene and activates its transcription. Thus, the expression of L2 is regulated by exogenous ethylene. Taken together, our results indicate that ethylene may affect the expression of L2 gene and that L2 participates in autocatalytic ethylene biosynthesis during tomato fruit ripening.

The NAM/ATAF1/2/CUC2 transcription factor PpNAC.A59 enhances PpERF.A16 expression to promote ethylene biosynthesis during peach fruit ripening

Peach is a typical climacteric fruit that releases ethylene during fruit ripening. Several studies have been conducted on the transcriptional regulation of ethylene biosynthesis in peach fruit. Herein, an ethylene response factor, PpERF.A16, which was induced by exogenous ethylene, could enhance ethylene biosynthesis by directly inducing the expression of 1-aminocyclopropane-1-carboxylic acid synthase (PpACS1) and 1-aminocyclopropane-1-carboxylic acid oxidase (PpACO1) genes. Moreover, the NAM/ATAF1/2/CUC2 (NAC) transcription factor (TF) PpNAC.A59 was coexpressed with PpERF.A16 in all tested peach cultivars. Interestingly, PpNAC.A59 can directly interact with the promoter of PpERF.A16 to induce its expression but not enhance LUC activity driven by any promoter of PpACS1 or PpACO1. Thus, PpNAC.A59 can indirectly mediate ethylene biosynthesis via the NAC-ERF signaling cascade to induce the expression of both PpACS1 and PpACO1. These results enrich the genetic network of fruit ripening in peach and provide new insight into the ripening mechanism of other perennial fruits.

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

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

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

Identification of tomato root growth regulatory genes and transcription factors through comparative transcriptomic profiling of different tissues

Tomato is an economically important vegetable crop and a model for development and stress response studies. Although studied extensively for understanding fruit ripening and pathogen responses, its role as a model for root development remains less explored. In this study, an Illumina-based comparative differential transcriptomic analysis of tomato root with different aerial tissues was carried out to identify genes that are predominantly expressed during root growth. Sequential comparisons revealed ~ 15,000 commonly expressed genes and ~ 3000 genes of several classes that were mainly expressed or regulated in roots. These included 1069 transcription factors (TFs) of which 100 were differentially regulated. Prominent amongst these were members of families encoding Zn finger, MYB, ARM, bHLH, AP2/ERF, WRKY and NAC proteins. A large number of kinases, phosphatases and F-box proteins were also expressed in the root transcriptome. The major hormones regulating root growth were represented by the auxin, ethylene, JA, ABA and GA pathways with root-specific expression of certain components. Genes encoding carbon metabolism and photosynthetic components showed reduced expression while several protease inhibitors were amongst the most highly expressed. Overall, the study sheds light on genes governing root growth in tomato and provides a resource for manipulation of root growth for plant improvement.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01015-0.

The transcription factor SlHY5 regulates the ripening of tomato fruit at both the transcriptional and translational levels

Light plays a critical role in plant growth and development, but the mechanisms through which light regulates fruit ripening and nutritional quality in horticultural crops remain largely unknown. Here, we found that ELONGATED HYPOCOTYL 5 (HY5), a master regulator in the light signaling pathway, is required for normal fruit ripening in tomato (Solanum lycopersicum). Loss of function of tomato HY5 (SlHY5) impairs pigment accumulation and ethylene biosynthesis. Transcriptome profiling identified 2948 differentially expressed genes, which included 1424 downregulated and 1524 upregulated genes, in the Slhy5 mutants. In addition, genes involved in carotenoid and anthocyanin biosynthesis and ethylene signaling were revealed as direct targets of SlHY5 by chromatin immunoprecipitation. Surprisingly, the expression of a large proportion of genes encoding ribosomal proteins was downregulated in the Slhy5 mutants, and this downregulation pattern was accompanied by a decrease in the abundance of ribosomal proteins. Further analysis demonstrated that SlHY5 affected the translation efficiency of numerous ripening-related genes. These data indicate that SlHY5 regulates fruit ripening both at the transcriptional level by targeting specific molecular pathways and at the translational level by affecting the protein translation machinery. Our findings unravel the regulatory mechanisms of SlHY5 in controlling fruit ripening and nutritional quality and uncover the multifaceted regulation of gene expression by transcription factors.

Identification, classification, and characterization of AP2/ERF superfamily genes in Masson pine (Pinus massoniana Lamb.)

Transcription factors (TFs) play crucial regulatory roles in controlling the expression of the target genes in plants. APETALA2/Ethylene-responsive factors (AP2/ERF) are part of a large superfamily of plant-specific TFs whose members are involved in the control of plant metabolism, development and responses to various biotic and abiotic stresses. However, the AP2/ERF superfamily has not been identified systematically in Masson pine (Pinus massoniana), which is one of the most important conifer in southern China. Therefore, we performed systematic identification of the AP2/ERF superfamily using transcriptome sequencing data from Masson pine. In the current study, we obtained 88 members of the AP2/ERF superfamily. All PmAP2/ERF members could be classified into 3 main families, AP2 (7 members), RAV (7 members), ERF (73 members) families, and a soloist protein. Subcellular localization assays suggested that two members of PmAP2/ERF were nuclear proteins. Based on pine wood nematode (PWN) inoculated transcriptome and qPCR analysis, we found that many members of PmAP2/ERF could respond to PWN inoculation and PWN related treatment conditions in vitro. In general, members of the AP2/ERF superfamily play an important role in the response of Masson pine responds to PWN. Furthermore, the roles of the AP2/ERF superfamily in other physiological activities of Masson pine remain to be further studied.

Citrus CitERF6 Contributes to Citric Acid Degradation via Upregulation of CitAclα1, Encoding ATP-Citrate Lyase Subunit α

Citric acid is the most abundant organic acid in citrus fruit, and the acetyl-CoA pathway potentially plays an important role in citric acid degradation, which occurs during fruit ripening. Analysis of transcripts during fruit development of key genes in the acetyl-CoA pathway and transient overexpression assay in citrus leaves indicated that CitAclα1 could be a potential target gene involved in citrate degradation. In order to understand more about CitAclα1, 23 transcription factors coexpressed with CitAclα1 in citrus fruit were identified by RNA-seq. Using dual-luciferase assays, CitERF6 was shown to trans-activate the promoter of CitAclα1 and electrophoretic mobility shift assays (EMSAs) showed that CitERF6 directly bound to a 5'-CAACA-3' motif in the CitAclα1 promoter. Furthermore, citric acid content was significantly reduced when CitERF6 was overexpressed in transgenic tobacco leaves. Taken together, these results indicate an important role for CitERF6 in transcriptional regulation of CitAclα1 and control of citrate degradation.

Dynamic Changes of DNA Methylation Induced by Heat Treatment Were Involved in Ethylene Signal Transmission and Delayed the Postharvest Ripening of Tomato Fruit

Deoxyribonucleic acid (DNA) methylation plays an important role in fruit ripening and senescence. Here, the role of DNA methylation of the CpG island of SlACS10, LeCTR1, LeEIN3, LeERT10, and SlERF-A1 genes induced by heat treatment (37 °C) in postharvest ripening of tomato fruit was studied. After heat treatment, the firmness and vitamin C content showed higher levels, the loss of aldehydes in volatile components was delayed, and the activities of methylase and demethylase decreased in tomato fruit. Moreover, in heat-treated fruit, significant changes in DNA methylation of SlACS10, LeCTR1, LeEIN3, LeERT10, and SlERF-A1 were induced, the expression of LeERT10 and LeEIN3 was inhibited, the expression of SlERF-A1 was increased, by which ethylene signal transmission might be suppressed and the postharvest ripening of tomato fruit was delayed. The present study provided valuable information for understanding the essential role of DNA methylation in the postharvest ripening of tomato fruit.

DNA methylation is involved in the regulation of pepper fruit ripening and interacts with phytohormones

There is growing evidence to suggest that epigenetic tags, especially DNA methylation, are critical regulators of fruit ripening. To examine whether this is the case in sweet pepper (Capsicum annuum) we conducted experiments at the transcriptional, epigenetic, and physiological levels. McrBC PCR, bisulfite sequencing, and real-time PCR demonstrated that DNA hypomethylation occurred in the upstream region of the transcription start site of some genes related to pepper ripening at the turning stage, which may be attributed to up-regulation of CaDML2-like and down-regulation of CaMET1-like1, CaMET1-like2, CaCMT2-like, and CaCMT4-like. Silencing of CaMET1-like1 by virus-induced gene silencing led to DNA hypomethylation, increased content of soluble solids, and accumulation of carotenoids in the fruit, which was accompanied by changes in expression of genes involved in capsanthin/capsorubin biosynthesis, cell wall degradation, and phytohormone metabolism and signaling. Endogenous ABA increased during fruit ripening, whereas endogenous IAA showed an opposite trend. No ethylene signal was detected during ripening. DNA hypomethylation repressed the expression of auxin and gibberellin biosynthesis genes as well as cytokinin degradation genes, but induced the expression of ABA biosynthesis genes. In mature-green pericarp, exogenous ABA induced expression of CaDML2-like but repressed that of CaCMT4-like. IAA treatment promoted the transcription of CaMET1-like1 and CaCMT3-like. Ethephon significantly up-regulated the expression of CaDML2-like. Treatment with GA3 and 6-BA showed indistinct effects on DNA methylation at the transcriptional level. On the basis of the results, a model is proposed that suggests a high likelihood of a role for DNA methylation in the regulation of ripening in the non-climacteric pepper fruit.

PeERF1, a SHINE-Like Transcription Factor, Is Involved in Nanoridge Development on Lip Epidermis of Phalaenopsis Flowers

Phalaenopsis orchids have a spectacular floral morphology with a highly evolved lip that offers a landing platform for pollinators. The typical morphological orchid lip features are essential for the special pollination mechanism of Phalaenopsis flowers. Previously, we found that in the lip, a member of the AP2/EREBP protein family was highly expressed. Here, we further confirmed its high expression and characterized its function during lip development. Phylogenetic analysis showed that AP2/EREBP belongs to the Va2 subgroup of ERF transcription factors. We named it PeERF1. We found that PeERF1 was only expressed at stage 5, as flowers opened. This coincided with both thickening of the cuticle and development of nanoridges. We performed knockdown expression of PeERF1 using CymMV-based virus-induced gene silencing in either the AP2 conserved domain, producing PeERF1_AP2-silenced plants, or the SHN specific domain, producing PeERF1_SHN-silenced plants. Using cryo-SEM, we found that the number of nanoridges was reduced only in the PeERF1_AP2-silenced group. This change was found on both the abaxial and adaxial surfaces of the central lip lobe. Expression of PeERF1 was reduced significantly in PeERF1_AP2-silenced plants. In cutin biosynthesis genes, expression of both PeCYP86A2 and PeDCR was significantly decreased in both groups. The expression of PeCYP77A4 was reduced significantly only in the PeERF1_AP2-silenced plants. Although PeGPAT expression was reduced in both silenced plants, but to a lesser degree. The expression of PeERF1 was significantly reduced in the petal-like lip of a big-lip variant. PeCYP77A4 and PeGPAT in the lip were also reduced, but PeDCR was not. Furthermore, heterologous overexpression of PeERF1 in the genus Arabidopsis produced leaves that were shiny on the adaxial surface. Taken together, our results show that in Phalaenopsis orchids PeERF1 plays an important role in formation of nanoridges during lip epidermis development.

Expression of galactinol synthase from Ammopiptanthus nanus in tomato improves tolerance to cold stress

Soluble carbohydrates not only directly affect plant growth and development but also act as signal molecules in processes that enhance tolerance to cold stress. Raffinose family oligosaccharides (RFOs) are an example and play an important role in abiotic stress tolerance. This study aimed to determine whether galactinol, a key limiting factor in RFO biosynthesis, functions as a signal molecule in triggering cold tolerance. Exposure to low temperatures induces the expression of galactinol synthase (AnGolS1) in Ammopiptanthus nanus, a desert plant that survives temperatures between -30 °C to 47 °C. AnGolS1 has a greater catalytic activity than tomato galactinol synthase (SlGolS2). Moreover, SlGolS2 is expressed only at low levels. Expression of AnGolS1 in tomato enhanced cold tolerance and led to changes in the sugar composition of the seeds and seedlings. AnGolS1 transgenic tomato lines exhibited an enhanced capacity for ethylene (ET) signaling. The application of galactinol abolished the repression of the ET signaling pathway by 1-methylcyclopropene during seed germination. In addition, the expression of ERF transcription factors was increased. Galactinol may therefore act as a signal molecule affecting the ET pathway.

Tomato Fruit Development and Metabolism

Tomato (Solanum lycopersicum L.) belongs to the Solanaceae family and is the second most important fruit or vegetable crop next to potato (Solanum tuberosum L.). It is cultivated for fresh fruit and processed products. Tomatoes contain many health-promoting compounds including vitamins, carotenoids, and phenolic compounds. In addition to its economic and nutritional importance, tomatoes have become the model for the study of fleshy fruit development. Tomato is a climacteric fruit and dramatic metabolic changes occur during its fruit development. In this review, we provide an overview of our current understanding of tomato fruit metabolism. We begin by detailing the genetic and hormonal control of fruit development and ripening, after which we document the primary metabolism of tomato fruits, with a special focus on sugar, organic acid, and amino acid metabolism. Links between primary and secondary metabolic pathways are further highlighted by the importance of pigments, flavonoids, and volatiles for tomato fruit quality. Finally, as tomato plants are sensitive to several abiotic stresses, we briefly summarize the effects of adverse environmental conditions on tomato fruit metabolism and quality.

The expanding roles of APETALA2/Ethylene Responsive Factors and their potential applications in crop improvement

Understanding the molecular basis of the gene-regulatory networks underlying agronomic traits or plant responses to abiotic/biotic stresses is very important for crop improvement. In this context, transcription factors, which either singularly or in conjugation directly control the expression of many target genes, are suitable candidates for improving agronomic traits via genetic engineering. In this regard, members of one of the largest class of plant-specific APETALA2/Ethylene Response Factor (AP2/ERF) superfamily, which is implicated in various aspects of development and plant stress adaptation responses, are considered high-value targets for crop improvement. Besides their long-known regulatory roles in mediating plant responses to abiotic stresses such as drought and submergence, the novel roles of AP2/ERFs during fruit ripening or secondary metabolites production have also recently emerged. The astounding functional plasticity of AP2/ERF members is considered to be achieved by their interplay with other regulatory networks and signalling pathways. In this review, we have integrated the recently accumulated evidence from functional genomics studies and described their newly emerged functions in plants. The key structural features of AP2/ERF proteins and the modes of their action are briefly summarized. The importance of AP2/ERFs in plant development and stress responses and a summary of the event of their successful applications in crop improvement programs are also provided. Altogether, we envisage that the synthesized information presented in this review will be useful to design effective strategies for improving agronomic traits in crop plants.

Genome-wide identification and expression analysis reveal the potential function of ethylene responsive factor gene family in response to Botrytis cinerea infection and ovule development in grapes (Vitis vinifera L.)

The prevention of Botrytis cinerea infection and the study of grape seedlessness are very important for grape industries. Finding correlated regulatory genes is an important approach towards understanding their molecular mechanisms. Ethylene responsive factor (ERF) gene family play critical roles in defence networks and the growth of plants. To date, no large-scale study of the ERF proteins associated with pathogen defence and ovule development has been performed in grape (Vitis vinifera L.). In the present study, we identified 113 ERF genes (VvERF) and named them based on their chromosome locations. The ERF genes could be divided into 11 groups based on a multiple sequence alignment and a phylogenetic comparison with homologues from Arabidopsis thaliana. Synteny analysis and Ka/Ks ratio calculation suggested that segmental and tandem duplications contributed to the expansion of the ERF gene family. The evolutionary relationships between the VvERF genes were investigated by exon-intron structure characterisation, and an analysis of the cis-acting regulatory elements in their promoters suggested potential regulation after stress or hormone treatments. Expression profiling after infection with the fungus, B. cinerea, indicated that ERF genes function in responses to pathogen attack. In addition, the expression levels of most ERF genes were much higher during ovule development in seedless grapes, suggesting a role in ovule abortion related to seedlessness. Taken together, these results indicate that VvERF proteins are involved in responses to Botrytis cinerea infection and in grape ovule development. This information may help guide strategies to improve grape production.

Copy number variants in kiwifruit ETHYLENE RESPONSE FACTOR/APETALA2 (ERF/AP2)-like genes show divergence in fruit ripening associated cold and ethylene responses in C-REPEAT/DRE BINDING FACTOR-like genes

The ETYHLENE RESPONSE FACTOR/APETALA2 (ERF/AP2) transcription factors have been shown to control a wide range of developmental and environmental responses in plants. These include hormonal responses to ethylene and Abscisic Acid (ABA) as well as to cold and drought. In Actinidia chinensis (kiwifruit), ripening is unusual: although it is sometimes classed as a climacteric fruit (ethylene-associated ripening), much of fruit ripening occurs independently from autocatalytic ethylene production. Initiation of ripening appears to be strongly developmentally controlled and modulated by low temperature. In this study, fruit treated with different temperatures showed an increase in soluble sugar accumulation, and a corresponding increase in ß-AMYLASE (BAM) genes (predominantly BAM3.2 and BAM9) with lower temperatures. To investigate the potential role of the AP2/ERF gene family in the control of fruit ripening in kiwifruit this family was investigated further. Using the new genome annotation and further genome sequence analysis we identified 226 ERF-like genes, 10 AP2L/RAV-like genes and 32 AP2-like genes. An RNA-seq screen from kiwifruit of different maturities, and following treatment with ethylene and temperatures between 0 and 16°C, revealed 4%, 26% and 18% of the ERF-like genes were upregulated by maturation, ethylene and cold temperatures, respectively. Focusing on the C-REPEAT/DRE BINDING FACTOR (CBF) cold master regulators, nine potential genes were identified based on sequence similarity. Five of these CBF-like genes were found in a copy number variant (CNV) cluster of six genes on chromosome 14. Expression analysis showed that two homeologous genes (ERF41 and ERF180) increased in abundance with cold and ethylene, while the cluster of CNV CBF-like genes had lost the ability to respond to cold and increased only with ethylene, suggesting an evolutionary progression of function of these genes.

The Constitutive Expression of a Chrysanthemum ERF Transcription Factor Influences Flowering Time in Arabidopsis thaliana

AP2/ERF transcription factors (TFs) represent valuable targets for the genetic manipulation of crop plants, as they participate in the control of metabolism, growth and development, as well as in the plants' response to environmental stimuli. Here, an ERF TF encoded by the chrysanthemum (Chrysanthemum morifolium) genome, designated CmERF110, was cloned and functionally characterized. The predicted CmERF110 polypeptide included a conserved DNA-binding AP2/ERF domain. A transient expression experiment revealed that the protein was deposited in the nucleus, and a transactivation experiment in yeast suggested that it had no transcriptional activity. The gene was transcribed in the chrysanthemum root, stem and leaf, with its transcript level following a circadian rhythm under both long and short days. The effect of constitutively expressing the gene in Arabidopsis thaliana was to accelerate flowering. Transcriptional profiling implied that its effect on floral initiation operated through the photoperiod pathway.

Recognition of candidate transcription factors related to bilberry fruit ripening by de novo transcriptome and qRT-PCR analyses

Bilberry (Vaccinium myrtillus L.) fruits are an excellent natural resource for human diet because of their special flavor, taste and nutritional value as well as medical properties. Bilberries are recognized for their high anthocyanin content and many of the genes involved in the anthocyanin biosynthesis have been characterized. So far, neither genomic nor RNA-seq data have been available for the species. In the present study, we de novo sequenced two bilberry fruit developmental stages, unripe green (G) and ripening (R). A total of 57,919 unigenes were assembled of which 80.2% were annotated against six public protein databases. The transcriptome served as exploratory data to identify putative transcription factors related to fruit ripening. Differentially expressed genes (DEGs) between G and R stages were prominently upregulated in R stage with the functional annotation indicating their main roles in active metabolism and catalysis. The unigenes encoding putative ripening-related regulatory genes, including members of NAC, WRKY, LOB, ERF, ARF and ABI families, were analysed by qRT-PCR at five bilberry developmental stages. Our de novo transcriptome database contributes to the understanding of the regulatory network associated with the fruit ripening in bilberry and provides the first dataset for wild Vaccinium species acquired by NGS technology.

Analysis of long-non-coding RNAs associated with ethylene in tomato

Long-Non-Coding RNAs (LncRNAs) are a class of non-coding endogenous RNAs contributing to numerous biological processes. LeERF1 is a tomato ethylene response factor (ERF) near the end of the ethylene signal transduction pathway. To identify lncRNAs in tomato and elucidate their roles in ethylene signaling, deep sequencing was deployed in over-expression and repression LeERF1 transgenic and control tomato fruits. A total of 397 lncRNAs were identified, including 169 tomato lncRNAs that had not previously been identified. Among these, 12 were differentially expressed between the transgenic and control tomato fruits. Numerous lncRNA target genes were identified including many associated with ethylene signaling including auxin response factors and auxin-induced proteins, F-box proteins, ERFs and MADS-box proteins. In addition, two lncRNAs were found to be the precursor of three miRNAs and four lncRNAs could be targeted by five miRNAs. We propose a regulatory model highlighting the relationships between lncRNAs and their targets involved in ethylene signal transduction which establishes a foundation for addressing the role of LncRNAs in ethylene response.

Comparative Analysis of DNA Methylation Reveals Specific Regulations on Ethylene Pathway in Tomato Fruit

DNA methylation is an essential feature of epigenetic regulation and plays a role in various physiological and biochemical processes at CG, CHG, and CHH sites in plants. LeERF1 is an ethylene response factor (ERF) found in tomatoes which plays an important role in ethylene signal transduction. To explore the characteristics of DNA methylation in the ethylene pathway, sense-/antisense-LeERF1 transgenic tomato fruit were chosen for deep sequencing and bioinformatics parsing. The methylation type with the greatest distribution was CG, (71.60–72.80%) and CHH was found least frequently (10.70–12.50%). The level of DNA methylation was different among different tomato genomic regions. The differentially methylated regions (DMRs) and the differentially expressed genes (DEGs) were conjointly analyzed and 3030 different expressed genes were found, of which several are involved in ethylene synthesis and signaling transduction (such as ACS, ACO, MADS-Box, ERFs, and F-box). Furthermore, the relationships between DNA methylation and microRNAs (miRNAs) were also deciphered, providing basic information for the further study of DNA methylation and small RNAs involved in the ethylene pathway.

Transcriptome analysis unravels an ethylene response factor involved in regulating fruit ripening in pear

Ethylene response factor (ERF) has been widely studied in regulating fruit ripening in tomato, apple, banana and kiwifruit, but little is known in pear. In this study 1-methylcyclopropene (1-MCP) treatment, an inhibitor of ethylene perception, was conducted at approximately 30 days before harvest to delay fruit ripening in a climacteric white pear cultivar Yali. Transcriptome libraries were constructed and sequenced in pre-ripening, ripening, and 1-MCP treated fruits. Data analysis showed that 73 candidate genes related to fruit ripening were induced by 1-MCP, among which two were positively related, namely 1-aminocyclopropane-1-carboxyla oxidase and an ERF gene (designated as ACO54 and ERF24). Transient transformations in pear fruit revealed that over-expression of ACO54 enhance transcription level of ERF24 and most ripening-related genes. Meanwhile, over-expression of ERF24 raises expression level of ACO54 and partially ripening-related genes. Moreover, dual-luciferase and yeast-one-hybrid assays unravel an interaction between ERF24 and the ACO54 promoter. Therefore, the ERF24 could directly regulate ACO54 expression by binding to its promoter. These results suggested that the first identified ERF24 is involved in regulating fruit ripening in Chinese white pear.

Multiple regulatory roles of AP2/ERF transcription factor in angiosperm

APETALA2/ethylene response factor (AP2/ERF) transcription factor (TF) is a superfamily in plant kingdom, which has been reported to be involved in regulation of plant growth and development, fruit ripening, defense response, and metabolism. As the final response gene in ethylene signaling pathway, AP2/ERF TF could feedback modulate phytohormone biosynthesis, including ethylene, cytokinin, gibberellin, and abscisic acid. Moreover, AP2/ERF TF also participates in response to the signals of auxin, cytokinin, abscisic acid, and jasmonate. Thus, this superfamily is key regulator for connecting the phytohormonal signals. In this review, based on the evidence of structural and functional studies, we discussed the multiple regulator roles of AP2/ERF TF in angiosperm, and then constructed the network model of AP2/ERF TF in response to various phytohormonal signals and regulatory mechanism of the cross-talk.

Regulation of ethylene-responsive SlWRKYs involved in color change during tomato fruit ripening

WRKY transcription factors (TFs) play important roles in stress responses in planta. However, the function of WRKY TFs in the regulation of fruit ripening is unclear. Here, 23 tomato SlWRKYs that are similar to ethylene-responsive WRKY genes from other plant species, or show up-regulation during fruit ripening in previous genome-wide study, were selected, and their function in fruit ripening was investigated. Twelve SlWRKYs were found to be responsive to ethylene (SlER-WRKYs), showing expression patterns similar to those of genes related to fruit ripening. Eight SlER-WRKYs—SlWRKY16, 17, 22, 25, 31, 33, 53, and 54, detected in the nuclei—interacted with and activated the promoters of 4 genes related to color change: Pheophytin Pheophorbide Hydrolase (SlPPH), Pheophorbide a Oxygenase (SlPAO), Phytoene Synthase 1 (SlPSY1) and Phytoene Desaturase (SlPDS). Yeast two-hybrid and bimolecular fluorescence complement (BiFC) assays in Arabidopsis protoplasts indicated that protein interactions occurred between SlWRKY17 and SlRIN, SlERF2b or SlERF7; SlWRKY33 and SlERF7; SlWRKY54 and SlERF2b; and SlWRKY16 and SlWRKY17. Suppression of SlWRKY 16, 17, 53 or 54 by virus-induced gene silencing (VIGS) retarded the red coloration of the fruit. Our study provides comprehensive molecular evidence that WRKY TFs function in fruit ripening, particularly in color change, and are linked to the intricate regulatory network of other ripening regulators.

The novel ethylene-responsive factor CsERF025 affects the development of fruit bending in cucumber

Overexpression of CsERF025 induces fruit bending by promoting the production of ethylene. Cucumber fruit bending critically affects cucumber quality, but the mechanism that causes fruit bending remains unclear. To better understand this mechanism, we performed transcriptome analyses on tissues from the convex (C1) and concave (C2) sides of bending and straight (S) fruit at 2 days post anthesis (DPA). We identified a total of 281 differentially expressed genes (DEGs) from both the convex and concave sides of bent fruit that showed significantly different expression profiles relative to straight fruits. Of these 281 DEGs, 196 were up-regulated (C1/S_C2/S) and 85 were down-regulated (C1/S_C2/S). Among the 196 up-regulated DEGs, the transcriptional levels of genes related to ethylene biosynthesis and signaling pathways were significantly higher in bending fruit compared with straight fruit. CsERF025 showed the largest difference in expression between bending and straight fruit. CsERF025 is an AP2/ERF gene encoding a protein that localizes to the nucleus. Overexpression of this gene increased the bending rate of cucumber fruits and increased the angle of bending. CsERF025 increased both the expression of ethylene biosynthesis-related genes and the production of ethylene. The application of exogenous 1-aminocyclopropane-l-carboxylic acid (ACC) to straight fruits from control plants promoted fruit bending. Thus, CsERF025 enhances the production of ethylene and thereby promotes fruit bending in cucumber.

Integrative analysis of circRNAs acting as ceRNAs involved in ethylene pathway in tomato

Circular RNAs (circRNAs) are a large class of non-coding endogenous RNAs that could act as competing endogenous RNAs (ceRNAs) to terminate the mRNA targets' suppression of miRNAs. To elucidate the intricate regulatory roles of circRNAs in the ethylene pathway in tomato fruit, deep sequencing and bioinformatics methods were performed. After strict screening, a total of 318 circRNAs were identified. Among these circRNAs, 282 were significantly differentially expressed among wild-type and sense-/antisense-LeERF1 transgenic tomato fruits. Besides, 1254 target genes were identified and a large amount of them were found to be involved in ethylene pathway. In addition, a sophisticated regulatory model consisting of circRNAs, target genes and ethylene was set up. Importantly, 61 circRNAs were found to be potential ceRNAs to combine with miRNAs and some of the miRNAs had been revealed to participate in the ethylene signaling pathway. This research further raised the possibility that the ethylene pathway in tomato fruit may be under the regulation of various circRNAs and provided a new perspective of the roles of circRNAs.

Hypoxia-responsive ERFs involved in postdeastringency softening of persimmon fruit

Removal of astringency by endogenously formed acetaldehyde, achieved by postharvest anaerobic treatment, is of critical importance for many types of persimmon fruit. Although an anaerobic environment accelerates de-astringency, it also has the deleterious effect of promoting excessive softening, reducing shelf life and marketability. Some hypoxia-responsive ethylene response factors (ERFs) participate in anaerobic de-astringency, but their role in accelerated softening was unclear. Undesirable rapid softening induced by high CO₂ (95%) was ameliorated by adding the ethylene inhibitor 1-MCP (1 μL/L), resulting in reduced astringency while maintaining firmness, suggesting that CO₂ -induced softening involves ethylene signalling. Among the hypoxia-responsive genes, expression of eight involved in fruit cell wall metabolism (Dkβ-gal1/4, DkEGase1, DkPE1/2, DkPG1, DkXTH9/10) and three ethylene response factor genes (DkERF8/16/19) showed significant correlations with postdeastringency fruit softening. Dual-luciferase assay indicated that DkERF8/16/19 could trans-activate the DkXTH9 promoter and this interaction was abolished by a mutation introduced into the C-repeat/dehydration-responsive element of the DkXTH9 promoter, supporting the conclusion that these DkERFs bind directly to the DkXTH9 promoter and regulate this gene, which encodes an important cell wall metabolism enzyme. Some hypoxia-responsive ERF genes are involved in deastringency and softening, and this linkage was uncoupled by 1-MCP. Fruit of the Japanese cultivar 'Tonewase' provide a model for altered anaerobic response, as they lost astringency yet maintained firmness after CO₂ treatment without 1-MCP and changes in cell wall enzymes and ERFs did not occur.

Biotechnological Advancements for Improving Floral Attributes in Ornamental Plants

Developing new ornamental cultivars with improved floral attributes is a major goal in floriculture. Biotechnological approach together with classical breeding methods has been used to modify floral color, appearance as well as for increasing disease resistance. Transgenic strategies possess immense potential to produce novel flower phenotypes that are not found in nature. Adoption of Genetic engineering has supported the idea of floral trait modification. Ornamental plant attributes like floral color, fragrance, disease resistance, and vase life can be improved by means of genetic manipulation. Therefore, we witness transgenic plant varieties of high aesthetic and commercial value. This review focuses on biotechnological advancements in manipulating key floral traits that contribute in development of diverse ornamental plant lines. Data clearly reveals that regulation of biosynthetic pathways related to characteristics like pigment production, flower morphology and fragrance is both possible and predictable. In spite of their great significance, small number of genetically engineered varieties of ornamental plants has been field tested. Today, novel flower colors production is regarded as chief commercial benefit obtained from transgenic plants. But certain other floral traits are much more important and have high commercial potential. Other than achievements such as novel architecture, modified flower color, etc., very few reports are available regarding successful transformation of other valuable horticultural characteristics. Our review also summarized biotechnological efforts related to enhancement of fragrance and induction of early flowering along with changes in floral anatomy and morphology.

Different Preclimacteric Events in Apple Cultivars with Modified Ripening Physiology

"Anna" is an early season apple cultivar exhibiting a fast softening and juiciness loss during storage, in comparison to two mid-late season cultivars "Galaxy" and "GD." The poor storage capacity of "Anna" was correlated with high lipid oxidation-related autoluminescence, high respiration and ethylene production rates, associated with high expression of MdACO1, 2, 4, 7, and MdACS1. All cultivars at harvest responded to exogenous ethylene by enhancing ethylene production, typical of system-II. The contribution of pre-climacteric events to the poor storage capacity of "Anna" was examined by comparing respiration and ethylene production rates, response to exogenous ethylene, expression of genes responsible for ethylene biosynthesis and response, and developmental regulators in the three cultivars throughout fruit development. In contrast to the "Galaxy" and "GD," "Anna" showed higher ethylene production and respiration rates during fruit development, and exhibited auto-stimulatory (system II-like) effect in response to exogenous ethylene. The higher ethylene production rate in "Anna" was correlated with higher expression of ethylene biosynthesis genes, MdACS3a MdACO2, 4, and 7 during early fruit development. The expression of negative regulators of ripening (AP2/ERF) and ethylene response pathway, (MdETR1,2 and MdCTR1) was lower in "Anna" in comparison to the other two cultivars throughout development and ripening. Similar pattern of gene expression was found for SQUAMOSA promoter binding protein (SBP)-box genes, including MdCNR and for MdFUL. Taken together, this study provides new understanding on pre-climacteric events in "Anna" that might affect its ripening behavior and physiology following storage.

Parsing the Regulatory Network between Small RNAs and Target Genes in Ethylene Pathway in Tomato

Small RNAs are a class of short non-coding endogenous RNAs that play essential roles in many biological processes. Recent studies have reported that microRNAs (miRNAs) are also involved in ethylene signaling in plants. LeERF1 is one of the ethylene response factors (ERFs) in tomato that locates in the downstream of ethylene signal transduction pathway. To elucidate the intricate regulatory roles of small RNAs in ethylene signaling pathway in tomato, the deep sequencing and bioinformatics methods were combined to decipher the small RNAs landscape in wild and sense-/antisense-LeERF1 transgenic tomato fruits. Except for the known miRNAs, 36 putative novel miRNAs, 6 trans-acting short interfering RNAs (ta-siRNAs), and 958 natural antisense small interfering RNAs (nat-siRNAs) were also found in our results, which enriched the tomato small RNAs repository. Among these small RNAs, 9 miRNAs, and 12 nat-siRNAs were differentially expressed between the wild and transgenic tomato fruits significantly. A large amount of target genes of the small RNAs were identified and some of them were involved in ethylene pathway, including AP2 TFs, auxin response factors, F-box proteins, ERF TFs, APETALA2-like protein, and MADS-box TFs. Degradome sequencing further confirmed the targets of miRNAs and six novel targets were also discovered. Furthermore, a regulatory model which reveals the regulation relationships between the small RNAs and their targets involved in ethylene signaling was set up. This work provides basic information for further investigation of the function of small RNAs in ethylene pathway and fruit ripening.

Genetically modified (GM) crops: milestones and new advances in crop improvement

New advances in crop genetic engineering can significantly pace up the development of genetically improved varieties with enhanced yield, nutrition and tolerance to biotic and abiotic stresses. Genetically modified (GM) crops can act as powerful complement to the crops produced by laborious and time consuming conventional breeding methods to meet the worldwide demand for quality foods. GM crops can help fight malnutrition due to enhanced yield, nutritional quality and increased resistance to various biotic and abiotic stresses. However, several biosafety issues and public concerns are associated with cultivation of GM crops developed by transgenesis, i.e., introduction of genes from distantly related organism. To meet these concerns, researchers have developed alternative concepts of cisgenesis and intragenesis which involve transformation of plants with genetic material derived from the species itself or from closely related species capable of sexual hybridization, respectively. Recombinase technology aimed at site-specific integration of transgene can help to overcome limitations of traditional genetic engineering methods based on random integration of multiple copy of transgene into plant genome leading to gene silencing and unpredictable expression pattern. Besides, recently developed technology of genome editing using engineered nucleases, permit the modification or mutation of genes of interest without involving foreign DNA, and as a result, plants developed with this technology might be considered as non-transgenic genetically altered plants. This would open the doors for the development and commercialization of transgenic plants with superior phenotypes even in countries where GM crops are poorly accepted. This review is an attempt to summarize various past achievements of GM technology in crop improvement, recent progress and new advances in the field to develop improved varieties aimed for better consumer acceptance.

Tomato SlERF.A1, SlERF.B4, SlERF.C3 and SlERF.A3, Members of B3 Group of ERF Family, Are Required for Resistance to Botrytis cinerea

The Ethylene-Responsive Factors (ERFs) comprise a large family of transcriptional factors that play critical roles in plant immunity. Gray mold disease caused by Botrytis cinerea, a typical necrotrophic fungal pathogen, is the serious disease that threatens tomato production worldwide. However, littler is known about the molecular mechanism regulating the immunity to B. cinerea in tomato. In the present study, virus-induced gene silencing (VIGS)-based functional analyses of 18 members of B3 group (also called Group IX) in tomato ERF family were performed to identify putative ERFs that are involved in disease resistance against B. cinerea. VIGS-based silencing of either SlERF.B1 or SlERF.C2 had lethal effect while silencing of SlERF.A3 (Pit4) significantly suppressed vegetative growth of tomato plants. Importantly, silencing of SlERF.A1, SlERF.A3, SlERF.B4, or SlERF.C3 resulted in increased susceptibility to B. cinerea, attenuated the B. cinerea-induced expression of jasmonic acid/ethylene-mediated signaling responsive defense genes and promoted the B. cinerea-induced H₂O₂ accumulation. However, silencing of SlERF.A3 also decreased the resistance against Pseudomonas syringae pv. tomato (Pst) DC3000 but silencing of SlERF.A1, SlERF.B4 or SlERF.C3 did not affect the resistance to this bacterial pathogen. Expression of SlERF.A1, SlERF.A3, SlERF.B4, or SlERF.C3 was induced by B. cinerea and by defense signaling hormones such as salicylic acid, methyl jasmonate, and 1-aminocyclopropane-1-carboxylic acid (an ethylene precursor). SlERF.A1, SlERF.B4, SlERF.C3, and SlERF.A3 proteins were found to localize in nucleus of cells and possess transactivation activity in yeasts. These data suggest that SlERF.A1, SlERF.B4, and SlERF.C3, three previously uncharacterized ERFs in B3 group, and SlERF.A3, a previously identified ERF with function in immunity to Pst DC3000, play important roles in resistance against B. cinerea in tomato.