MdMYC2 and MdERF3 Positively Co-Regulate α-Farnesene Biosynthesis in Apple

Ethylene-responsive VviERF003 modulates glycosylated monoterpenoid synthesis by upregulating VviGT14 in grapes

Terpenoids are important contributors to the aroma of grapes and wines. Grapes contain terpenoids in both volatile free form and non-volatile glycosidic form, with the latter being more abundant. Glycosylated terpenoids are deemed as latent aromatic potentials for their essential role in adding to the flowery and fruity bouquet of wines. However, the transcriptional regulatory mechanism underlying glycosylated terpenoid biosynthesis remains poorly understood. Our prior study identified an AP2/ERF transcription factor, VviERF003, through DNA pull-down screening using the promoter of terpenoid glycosyltransferase VviGT14 gene. This study demonstrated that both genes were co-expressed and synchronized with the accumulation of glycosylated monoterpenoids during grape maturation. VviERF003 can bind to the VviGT14 promoter and promote its activity according to yeast one-hybrid and dual-luciferase assays. VviERF003 upregulated VviGT14 expression in vivo, leading to increased production of glycosylated monoterpenoids based on the evidence from overexpression or RNA interference in leaves, berry skins, and calli of grapes, as well as tomato fruits. Additionally, VviERF003 and VviGT14 expressions and glycosylated monoterpenoid levels were induced by ethylene in grapes. The findings suggest that VviERF003 is ethylene-responsive and stimulates glycosylated monoterpenoid biosynthesis through upregulating VviGT14 expression.

Contemporary understanding of transcription factor regulation of terpenoid biosynthesis in plants

KEY MESSAGE

This review summarized how TFs function independently or in response to environmental factors to regulate terpenoid biosynthesis via fine-tuning the expression of rate-limiting enzymes. Terpenoids are derived from various species and sources. They are essential for interacting with the environment and defense mechanisms, such as antimicrobial, antifungal, antiviral, and antiparasitic properties. Almost all terpenoids have high medicinal value and economic performance. Recently, the control of enzyme genes on terpenoid biosynthesis has received a great deal of attention, but transcriptional factors regulatory network on terpenoid biosynthesis and accumulation has yet to get a thorough review. Transcription factors function as activators or suppressors independently or in response to environmental stimuli, fine-tuning terpenoid accumulation through regulating rate-limiting enzyme expression. This study investigates the advancements in transcription factors related to terpenoid biosynthesis and systematically summarizes previous works on the specific mechanisms of transcription factors that regulate terpenoid biosynthesis via hormone signal-transcription regulatory networks in plants. This will help us to better comprehend the regulatory network of terpenoid biosynthesis and build the groundwork for terpenoid development and effective utilization.

Transcription factors TgbHLH95 and TgbZIP44 cotarget terpene biosynthesis gene TgGPPS in Torreya grandis nuts

Terpenes are volatile compounds responsible for aroma and the postharvest quality of commercially important xiangfei (Torreya grandis) nuts, and there is interest in understanding the regulation of their biosynthesis. Here, a transcriptomics analysis of xiangfei nuts after harvest identified 156 genes associated with the terpenoid metabolic pathway. A geranyl diphosphate (GPP) synthase (TgGPPS) involved in production of the monoterpene precursor GPP was targeted for functional characterization, and its transcript levels positively correlated with terpene levels. Furthermore, transient overexpression of TgGPPS in tobacco (Nicotiana tabacum) leaves or tomato (Solanum lycopersicum) fruit led to monoterpene accumulation. Analysis of differentially expressed transcription factors identified one basic helix-loop-helix protein (TgbHLH95) and one basic leucine zipper protein (TgbZIP44) as potential TgGPPS regulators. TgbHLH95 showed significant transactivation of the TgGPPS promoter, and its transient overexpression in tobacco leaves led to monoterpene accumulation, whereas TgbZIP44 directly bound to an ACGT-containing element in the TgGPPS promoter, as determined by yeast 1-hybrid test and electrophoretic mobility shift assay. Bimolecular fluorescence complementation, firefly luciferase complementation imaging, co-immunoprecipitation, and GST pull-down assays confirmed a direct protein-protein interaction between TgbHLH95 and TgbZIP44 in vivo and in vitro, and in combination these proteins induced the TgGPPS promoter up to 4.7-fold in transactivation assays. These results indicate that a TgbHLH95/TgbZIP44 complex activates the TgGPPS promoter and upregulates terpene biosynthesis in xiangfei nuts after harvest, thereby contributing to its aroma.

A simple and efficient protocol for transient transformation of sliced grape berries

Grape is an economically important crop but recalcitrant to Agrobacterium-mediated genetic transformation and in vitro regeneration. Here, we have developed a protocol for transient transformation of grapes by investigating the effects of explant pre-culture and duration of vacuum infiltration on transformation efficiency. Using sliced grape berries of "Shine-Muscat" (Vitis labrusca × Vitis vinifera) between the end of fruit expansion phase and the mature stage as explants, we firstly compared the effect of pre-culture explants into a susceptible state (incubation on Murashige and Skoog (MS) agar plate in the dark at 25 ± 1 °C for 48 h) with no pre-culture and then tested different vacuum infiltration times on transformation efficiency using β-glucuronidase (GUS) reporter system. Pre-culture increased the susceptibility of explants to the agrobacteria infection and increased transient transformation efficiency as assessed by histochemical GUS activity, with intense blue coloration compared with the faint staining observed in the non-susceptible explants. Using a Circulating Water Vacuum Pump system to facilitate agrobacteria entry into berry cells, we tested vacuum durations of 5, 10, and 15 min and observed that transformation efficiency increased with vacuum duration of infiltration. These results were confirmed by relative gene expression of GUS transgene as assessed by RT-qPCR and GUS activity assay. To further confirm the usefulness of our protocol, we transiently transformed grape berries with the hydrogen peroxide sensor gene VvHPCA3, and this was confirmed by gene expression analysis as well as increased sensitivity of the explants to hydrogen peroxide treatment. Overall, this study has resulted in a simple but efficient transient transformation protocol for grape berries and would be a valuable tool for the rapid testing of gene function and the study of key regulatory networks in this important crop.

Functional characterization of a terpene synthase responsible for (E)-β-ocimene biosynthesis identified in Pyrus betuleafolia transcriptome after herbivory

(E)-β-ocimene, a ubiquitous monoterpene volatile in plants, is emitted from flowers to attract pollinators and/or from vegetative tissues as part of inducible defenses mediated by complex signaling networks when plants are attacked by insect herbivores. Wild pear species Pyrus betuleafolia used worldwide as rootstock generally displays valuable pest-resistant traits and is a promising genetic resource for pear breeding. In the current study, transcriptional changes in this wild pear species infested with a polyphagous herbivore Spodoptera litura and the underlying molecular mechanisms were fully investigated. A total of 3,118 differentially expressed genes (DEGs) were identified in damaged pear leaf samples. Spodoptera litura larvae infestation activated complex phytohormonal signaling networks in which jasmonic acid, ethylene, brassinosteroids, cytokinin, gibberellic acid and auxin pathways were induced, whereas salicylic acid and abscisic acid pathways were suppressed. All DEGs associated with growth-related photosynthesis were significantly downregulated, whereas most DEGs involved in defense-related early signaling events, transcription factors, green leaf volatiles and volatile terpenes were significantly upregulated. The PbeOCS (GWHGAAYT028729), a putative (E)-β-ocimene synthase gene, was newly identified in P. betuleafolia transcriptome. The upregulation of PbeOCS in S. litura-infested pear leaves supports a potential role for PbeOCS in herbivore-induced plant defenses. In enzyme-catalyzed reaction, recombinant PbeOCS utilized only geranyl pyrophosphate but not neryl diphosphate, farnesyl pyrophosphate or geranylgeranyl diphosphate as a substrate, producing (E)-β-ocimene as the major product and a trace amount of (Z)-β-ocimene. Moreover, as a catalytic product of PbeOCS, (E)-β-ocimene showed repellent effects on larvae of S. litura in dual-choice bioassays. What is more, (E)-β-ocimene increased mortalities of larvae in no-choice bioassays. These findings provide an overview of transcriptomic changes in wild pears in response to chewing herbivores and insights into (E)-β-ocimene biosynthesis in pear plants, which will help elucidate the molecular mechanisms underlying pear-insect interactions.

Transcription factor MdLSD1 negatively regulates α-farnesene biosynthesis in apple-fruit skin tissue

α-Farnesene is a sesquiterpene present in plants. It was first discovered in apples. It plays an important role in the plant defence response and is considered a key factor in the occurrence of superficial scald. The gene encoding α-farnesene synthase, which is the last key enzyme in the biosynthetic pathway of α-farnesene in apple fruit, has become the primary target enzyme for controlling the genetic manipulation of α-farnesene biosynthesis. In this study, the yeast one-hybrid assay and the dual luciferase assay were used to ascertain the relationship between MdLSD1 and MdAFS. Real-time PCR was used to analyse the molecular mechanism underlying the regulation of MdAFS by MdLSD1. Our results revealed that transcription factor MdLSD1, which is closely related to programmed cell death in apple fruit tissues, binds to MdAFS. Transient transformation of apple skin with vectors overexpressing MdLSD1 showed that the gene negatively regulates MdAFS. Overall, we suggest that MdLSD1 negatively regulates MdAFS. Our results are of great significance for future research on the transcriptional regulation of the α-farnesene synthase gene and provide a new direction for exploring the specific mechanism of programmed cell death involved in superficial-scald incidence.

ERF4 interacts with and antagonizes TCP15 in regulating endoreduplication and cell growth in Arabidopsis

Endoreduplication is prevalent during plant growth and development, and is often correlated with large cell and organ size. Despite its prevalence, the transcriptional regulatory mechanisms underlying the transition from mitotic cell division to endoreduplication remain elusive. Here, we characterize ETHYLENE-RESPONSIVE ELEMENT BINDING FACTOR 4 (ERF4) as a positive regulator of endoreduplication through its function as a transcriptional repressor. ERF4 was specifically expressed in mature tissues in which the cells were undergoing expansion, but was rarely expressed in young organs. Plants overexpressing ERF4 exhibited much larger cells and organs, while plants that lacked functional ERF4 displayed smaller organs than the wild-type. ERF4 was further shown to regulate cell size by controlling the endopolyploidy level in the nuclei. Moreover, ERF4 physically associates with the class I TEOSINTE BRANCHED 1/CYCLOIDEA/PCF (TCP) protein TCP15, a transcription factor that inhibits endoreduplication by activating the expression of a key cell-cycle gene, CYCLIN A2;3 (CYCA2;3). A molecular and genetic analysis revealed that ERF4 promotes endoreduplication by directly suppressing the expression of CYCA2;3. Together, this study demonstrates that ERF4 and TCP15 function as a module to antagonistically regulate each other's activity in regulating downstream genes, thereby controlling the switch from the mitotic cell cycle to endoreduplication during leaf development. These findings expand our understanding of how the control of the cell cycle is fine-tuned by an ERF4-TCP15 transcriptional complex.

GhMYC2 activates cytochrome P450 gene CYP71BE79 to regulate gossypol biosynthesis in cotton

GhMYC2 regulates the gossypol biosynthesis pathway in cotton through activation of the expression of gossypol synthesis gene CYP71BE79, CDNC, CYP706B1, DH1, and CYP82D113. Cotton is one of the main cash crops globally. Cottonseed contains fiber, fat, protein, and starch, and has important economic value. However, gossypol in cottonseed seriously affects the development and utilization of cottonseed. Nonetheless, gossypol has great application potential in agriculture, medicine, and industry. Therefore, it is very important to study gossypol biosynthesis and its upstream regulatory pathways. It has been reported that the content of gossypol in hairy roots of cotton is regulated through jasmonic acid signaling; however, the specific molecular mechanism has not been revealed yet. We found that the expression of basic helix-loop-helix family transcription factor GhMYC2 was significantly upregulated after exogenous administration of methyl jasmonate to cotton seedlings, and the content of gossypol changed significantly with the variation of GhMYC2 expression. Further studies revealed that GhMYC2 could specifically bind to the G-Box in the promoter region of CDNC, CYP706B1, DH1, CYP82D113, CYP71BE79 to activate its expression and regulate gossypol synthesis, and its activation of CYP71BE79 promoter was inhibited by GhJAZ2. Not only that GhMYC2 could also interact with GoPGF. In this work, the molecular mechanisms of gossypol biosynthesis regulated by GhMYC2 were analyzed. The results provide a theoretical basis for cultivating new varieties of low-gossypol or high-gossypol cotton and creating excellent germplasm resources.

Characterization of a Novel Insect-Induced Sesquiterpene Synthase GbTPS1 Based on the Transcriptome of Gossypium barbadense Feeding by Cotton Bollworm

When attacked by insect herbivores, plants initiate sophisticated defenses mediated by complex signaling networks and usually release a blend of functional volatiles such as terpenes against infestation. The extra-long staple cotton Gossypium barbadense cultivated worldwide as natural textile fiber crop is frequently exposed to a variety of herbivores, such as cotton bollworm Helicoverpa armigera. However, little is known about insect-induced transcriptional changes and molecular mechanisms underlying subsequent defense responses in G. barbadense. In the current study, transcriptome changes in G. barbadense infested with chewing H. armigera larvae were investigated, and we identified 5,629 differentially expressed genes (DEGs) in the infested cotton leaves compared with non-infested controls. H. armigera feeding triggered complex signaling networks in which almost all (88 out of 90) DEGs associated with the jasmonic acid (JA) pathway were upregulated, highlighting a central role for JA in the defense responses of G. barbadense against target insects. All DEGs involved in growth-related photosynthesis were downregulated, whereas most DEGs associated with defense-related transcript factors and volatile secondary metabolism were upregulated. It was noteworthy that a terpene synthase gene in the transcriptome data, GbTPS1, was strongly expressed in H. armigera-infested G. barbadense leaves. The upregulation of GbTPS1 in qPCR analysis also suggested an important role for GbTPS1 in herbivore-induced cotton defense. In vitro assays showed that recombinant GbTPS1 catalyzed farnesyl pyrophosphate and neryl diphosphate to produce three sesquiterpenes (selinene, α-gurjunene, and β-elemene) and one monoterpene (limonene), respectively. Moreover, these catalytic products of GbTPS1 were significantly elevated in G. barbadense leaves after H. armigera infestation, and elemene and limonene had repellent effects on H. armigera larvae in a dual-choice bioassay and increased larval mortality in a no-choice bioassay. These findings provide a valuable insight into understanding the transcriptional changes reprogramming herbivore-induced sesquiterpene biosynthesis in G. barbadense infested by H. armigera, which help elucidate the molecular mechanisms underlying plant defense against insect pests.

Identification and Functional Analysis of SabHLHs in Santalum album L

Santalum album L., a semi-parasitic evergreen tree, contains economically important essential oil, rich in sesquiterpenoids, such as (Z) α- and (Z) β-santalol. However, their transcriptional regulations are not clear. Several studies of other plants have shown that basic-helix-loop-helix (bHLH) transcription factors (TFs) were involved in participating in the biosynthesis of sesquiterpene synthase genes. Herein, bHLH TF genes with similar expression patterns and high expression levels were screened by co-expression analysis, and their full-length ORFs were obtained. These bHLH TFs were named SaMYC1, SaMYC3, SaMYC4, SaMYC5, SabHLH1, SabHLH2, SabHLH3, and SabHLH4. All eight TFs had highly conserved bHLH domains and SaMYC1, SaMYC3, SaMYC4, and SaMYC5, also had highly conserved MYC domains. It was indicated that the eight genes belonged to six subfamilies of the bHLH TF family. Among them, SaMYC1 was found in both the nucleus and the cytoplasm, while SaMYC4 was only localized in the cytoplasm and the remaining six TFs were localized in nucleus. In a yeast one-hybrid experiment, we constructed decoy vectors pAbAi-SSy1G-box, pAbAi-CYP2G-box, pAbAi-CYP3G-box, and pAbAi-CYP4G-box, which had been transformed into yeast. We also constructed pGADT7-SaMYC1 and pGADT7-SabHLH1 capture vectors and transformed them into bait strains. Our results showed that SaMYC1 could bind to the G-box of SaSSy, and the SaCYP736A167 promoter, which SaSSy proved has acted as a key enzyme in the synthesis of santalol sesquiterpenes and SaCYP450 catalyzed the ligation of santalol sesquiterpenes into terpene. We have also constructed pGreenII 62-SK-SaMYC1, pGreenII 0800-LUC-SaSSy and pGreenII 0800-LUC-SaCYP736A167 via dual-luciferase fusion expression vectors and transformed them into Nicotiana benthamiana using an Agrobacterium-mediated method. The results showed that SaMYC1 was successfully combined with SaSSy or SaCYP736A167 promoter and the LUC/REN value was 1.85- or 1.55-fold higher, respectively, than that of the control group. Therefore, we inferred that SaMYC1 could activate both SaSSy and SaCYP736A167 promoters.

ß-Farnesene Exogenous Application as a Novel Damage Induction Model to Fast Explore the Effectiveness of Postharvest Strategies: The Case Study of the ‘Rocha’ Pear DOP

Since the prohibition of diphenylamine, replacement strategies have been needed for long-term disorder prevention, namely superficial scald (SC), in fruit. However, as this disorder only appears after months under cold storage, the assessment of effective strategies to prevent this disorder requires long periods. To tackle this challenge, we report in this paper a rapid and reliable system to induce symptoms, such as SC, based on storage under a β-farnesene-enriched atmosphere. Using this model, SC symptoms in ‘Rocha’ pear were induced after 15 d at 20 °C. As proof of concept, this model system allowed the study of the efficiency of antioxidant natural-based coatings on ‘Rocha’ pear quality maintenance. Pears treated with the coatings were submitted to 4 months of commercial storage under normal atmosphere conditions and the results were compared with those obtained using the induction model system. A PCA of chemical data allowed us to conclude that the model developed simulates the potential of certain strategies to prevent disorders.

APETALA2/ethylene responsive factor in fruit ripening: Roles, interactions and expression regulation

Insects and animals are attracted to, and feed on ripe fruit, thereby promoting seed dispersal. As a vital vitamin and nutrient source, fruit make up an indispensable and enjoyable component of the human diet. Fruit ripening involves a series of physiological and biochemical changes in, among others, pigmentation, chlorophyll (Chl) degradation, texture, sugar accumulation, and flavor. Growing evidence indicates that the coordinated and ordered trait changes during fruit ripening depend on a complex regulatory network consisting of transcription factors, co-regulators, hormonal signals, and epigenetic modifications. As one of the predominant transcription factor families in plants and a downstream component of ethylene signaling, more and more studies are showing that APETALA2/ethylene responsive factor (AP2/ERF) family transcription factors act as critical regulators in fruit ripening. In this review, we focus on the regulatory mechanisms of AP2/ERFs in fruit ripening, and in particular the recent results on their target genes and co-regulators. We summarize and discuss the role of AP2/ERFs in the formation of key fruit-ripening attributes, the enactment of their regulatory mechanisms by interaction with other proteins, their role in the orchestration of phytohormone-signaling networks, and the epigenetic modifications associated with their gene expression. Our aim is to provide a multidimensional perspective on the regulatory mechanisms of AP2/ERFs in fruit ripening, and a reference for understanding and furthering research on the roles of AP2/ERF in fruit ripening.