Enhanced transcriptome responses in herbivore-infested tea plants by the green leaf volatile (Z)-3-hexenol

Analysis of aroma precursors in Jinmudan fresh tea leaves and dynamic change of fatty acid volatile during black tea processing

Aroma is an important factor affecting the quality of tea. Fatty acids are one of precursors and their derived contributes to tea aroma considerably. In this study, we analyzed the fatty acids of Jinmudan fresh tea leaves in different stalk position. It was found that with shoot maturity increased, the content of PUFAs (Polyunsaturated fatty acids) was increased while the content of SFAs (Saturated fatty acids) and MUFAs (Monounsaturated fatty acids) gradually decreased. During the processing period, totally 704 kinds of compounds were identified, among them, 27 kinds of fatty acid-derived volatile compounds were selected including 6 kinds of aldehydes, 8 kinds of alcohols, 13 kinds of esters and their dynamic change were revealed. Finally, the character of aroma during main processing stages and processed tea was concluded by using a flavor wheel. This study results provide a theoretical basis for the improvement of processing and quality in Jinmudan black tea.

Biosynthetic Pathway and Bioactivity of Vanillin, a Highly Abundant Metabolite Distributed in the Root Cortex of Tea Plants (Camellia sinensis)

Volatiles are important for plant root stress resistance. The diseases in tea root are serious, causing major losses. The volatile composition in tea root and whether it can resist diseases remain unclear. In this study, the volatile composition in different tea tissues was revealed. The vanillin content was higher in the root (mainly in root cortex) than in aerial parts. The antifungal effects of vanillin on pathogenic fungi in tea root were equal to or greater than those of other metabolites. O-methyltransferase (CsOMT), a key enzyme in one of two biosynthetic pathways of vanillin, converted protocatechualdehyde to vanillin in vitro. Furthermore, its characteristics and kinetic parameters were studied. In Arabidopsis thaliana protoplasts, the transiently expressed CsOMT was localized in the cytoplasm and nucleus. These findings have clarified the formation and bioactivities of volatiles in tea roots and provided a theoretical basis for understanding how tea plants resist root diseases.

Changes in volatile profile and related gene expression during senescence of tobacco leaves

BACKGROUND

Volatile organic compounds are critical for food flavor and play important roles in plant-plant interactions and plants' communications with the environment. Tobacco is well-studied for secondary metabolism and most of the typical flavor substances in tobacco leaves are generated at the mature stage of leaf development. However, the changes in volatiles during leaf senescence are rarely studied.

RESULTS

The volatile composition of tobacco leaves at different stages of senescence was characterized for the first time. Comparative volatile profiling of tobacco leaves at different stages was performed using solid-phase microextraction coupled with gas chromatography/mass spectrometry. In total, 45 volatile compounds were identified and quantified, including terpenoids, green leaf volatiles (GLVs), phenylpropanoids, Maillard reaction products, esters, and alkanes. Most of the volatile compounds showed differential accumulation during leaf senescence. Some terpenoids, including neophytadiene, β-springene, and 6-methyl-5-hepten-2-one, increased significantly with the progress of leaf senescence. Hexanal and phenylacetaldehyde also showed increased accumulation in leaves during senescence. The results from gene expression profiling indicated that genes involved in metabolism of terpenoids, phenylpropanoids, and GLVs were differentially expressed during leaf yellowing.

CONCLUSION

Dynamic changes in volatile compounds during tobacco leaf senescence are observed and the integration of gene-metabolites datasets offers important readouts for the genetic control of volatile production during the process of leaf senescence. © 2023 Society of Chemical Industry.

(Z)-3-Hexenol integrates drought and cold stress signaling by activating abscisic acid glucosylation in tea plants

Cold and drought stresses severely limit crop production and can occur simultaneously. Although some transcription factors and hormones have been characterized in plants subjected each stress, the role of metabolites, especially volatiles, in response to cold and drought stress exposure is rarely studied due to lack of suitable models. Here, we established a model for studying the role of volatiles in tea (Camellia sinensis) plants experiencing cold and drought stresses simultaneously. Using this model, we showed that volatiles induced by cold stress promote drought tolerance in tea plants by mediating reactive oxygen species and stomatal conductance. Needle trap microextraction combined with GC-MS identified the volatiles involved in the crosstalk and showed that cold-induced (Z)-3-hexenol improved the drought tolerance of tea plants. In addition, silencing C. sinensis alcohol dehydrogenase 2 (CsADH2) led to reduced (Z)-3-hexenol production and significantly reduced drought tolerance in response to simultaneous cold and drought stress. Transcriptome and metabolite analyses, together with plant hormone comparison and abscisic acid (ABA) biosynthesis pathway inhibition experiments, further confirmed the roles of ABA in (Z)-3-hexenol-induced drought tolerance of tea plants. (Z)-3-Hexenol application and gene silencing results supported the hypothesis that (Z)-3-hexenol plays a role in the integration of cold and drought tolerance by stimulating the dual-function glucosyltransferase UGT85A53, thereby altering ABA homeostasis in tea plants. Overall, we present a model for studying the roles of metabolites in plants under multiple stresses and reveal the roles of volatiles in integrating cold and drought stresses in plants.

Volatile compound-mediated plant-plant interactions under stress with the tea plant as a model

Plants respond to environmental stimuli via the release of volatile organic compounds (VOCs), and neighboring plants constantly monitor and respond to these VOCs with great sensitivity and discrimination. This sensing can trigger increased plant fitness and reduce future plant damage through the priming of their own defenses. The defense mechanism in neighboring plants can either be induced by activation of the regulatory or transcriptional machinery, or it can be delayed by the absorption and storage of VOCs for the generation of an appropriate response later. Despite much research, many key questions remain on the role of VOCs in interplant communication and plant fitness. Here we review recent research on the VOCs induced by biotic (i.e. insects and pathogens) and abiotic (i.e. cold, drought, and salt) stresses, and elucidate the biosynthesis of stress-induced VOCs in tea plants. Our focus is on the role of stress-induced VOCs in complex ecological environments. Particularly, the roles of VOCs under abiotic stress are highlighted. Finally, we discuss pertinent questions and future research directions for advancing our understanding of plant interactions via VOCs.

Application of Multi-Perspectives in Tea Breeding and the Main Directions

Tea plants are an economically important crop and conducting research on tea breeding contributes to enhancing the yield and quality of tea leaves as well as breeding traits that satisfy the requirements of the public. This study reviews the current status of tea plants germplasm resources and their utilization, which has provided genetic material for the application of multi-omics, including genomics and transcriptomics in breeding. Various molecular markers for breeding were designed based on multi-omics, and available approaches in the direction of high yield, quality and resistance in tea plants breeding are proposed. Additionally, future breeding of tea plants based on single-cellomics, pangenomics, plant-microbe interactions and epigenetics are proposed and provided as references. This study aims to provide inspiration and guidance for advancing the development of genetic breeding in tea plants, as well as providing implications for breeding research in other crops.

CML8 and GAD4 function in (Z)-3-hexenol-mediated defense by regulating γ-aminobutyric acid accumulation in Arabidopsis

(Z)-3-hexenol, a small gaseous molecule, is produced in plants under biotic stress and induces defense responses in neighboring plants. However, little is known about how (Z)-3-hexenol induces plant defense-related signaling. In this study, we uncovered how (Z)-3-hexenol treatment enhances plant resistance to insect attacks by increasing γ-aminobutyric acid (GABA) contents in Arabidopsis leaves. First, (Z)-3-hexenol increases the intracellular content of calcium as secondary messenger in Arabidopsis leaf mesophyll cells. Both intracellular and extracellular calcium stores regulate changes in calcium content. Then, CML8 and GAD4 transmit calcium signaling to affect (Z)-3-hexenol induced GABA content and plant resistance. Herein, CML8 interaction with GAD4 was examined via yeast two-hybrid assays, firefly luciferase complementation imaging, and GST pull-down assays. These results indicate that (Z)-3-hexenol treatment increased the GABA contents in Arabidopsis leaves based on CML8 and GAD4, thus increasing plant resistance to the insect Plutella xylostella. This study revealed the mechanism of activating plant insect defense induced by (Z)-3-hexenol, which guides the study of volatiles as biological pest control.

Integration of multiple volatile cues into plant defense responses

The ability to predict future risks is essential for many organisms, including plants. Plants can gather information about potential future herbivory by detecting volatiles that are emitted by herbivore-attacked neighbors. Several individual volatiles have been identified as active danger cues. Recent work has also shown that plants may integrate multiple volatiles into their defense responses. Here, I discuss how the integration of multiple volatiles can increase the capacity of plants to predict future herbivore attack. I propose that integration of multiple volatile cues does not occur at the perception stage, but may through downstream early defense signaling and then be further consolidated by hormonal crosstalk. Exploring plant volatile cue integration can facilitate our understanding and utilization of chemical information transfer.

Processing of Airborne Green Leaf Volatiles for Their Glycosylation in the Exposed Plants

Green leaf volatiles (GLVs), the common constituents of herbivore-infested plant volatiles (HIPVs), play an important role in plant defense and function as chemical cues to communicate with other individuals in nature. Reportedly, in addition to endogenous GLVs, the absorbance of airborne GLVs emitted by infested neighboring plants also play a major role in plant defense. For example, the exclusive accumulation of (Z)-3-hexenyl vicianoside in the HIPV-exposed tomato plants occurs by the glycosylation of airborne (Z)-3-hexenol (Z3HOL); however, it is unclear how plants process the other absorbed GLVs. This study demonstrates that tomato plants dominantly accumulated GLV-glycosides after exposure to green leaf alcohols [Z3HOL, (E)-2-hexenol, and n-hexanol] using non-targeted LC-MS analysis. Three types of green leaf alcohols were independently glycosylated without isomerization or saturation/desaturation. Airborne green leaf aldehydes and esters were also glycosylated, probably through converting aldehydes and esters into alcohols. Further, we validated these findings in Arabidopsis mutants- (Z)-3-hexenal (Z3HAL) reductase (chr) mutant that inhibits the conversion of Z3HAL to Z3HOL and the acetyl-CoA:(Z)-3-hexen-1-ol acetyltransferase (chat) mutant that impairs the conversion of Z3HOL to (Z)-3-hexenyl acetate. Exposure of the chr and chat mutants to Z3HAL accumulated lower and higher amounts of glycosides than their corresponding wild types (Col-0 and Ler), respectively. These findings suggest that plants process the exogenous GLVs by the reductase(s) and the esterase(s), and a part of the processed GLVs contribute to glycoside accumulation. Overall, the study provides insights into the understanding of the communication of the plants within their ecosystem, which could help develop strategies to protect the crops and maintain a balanced ecosystem.

Herbivore-Induced (Z)-3-Hexen-1-ol is an Airborne Signal That Promotes Direct and Indirect Defenses in Tea (Camellia sinensis) under Light

Tea (Camellia sinensis) is the most popular nonalcoholic beverage worldwide. During cultivation, tea plants are susceptible to herbivores and pathogens, which can seriously affect tea yield and quality. A previous report showed that (Z)-3-hexenol is a potentially efficient defensive substance. However, the molecular mechanism mediating (Z)-3-hexenol signaling in tea plants and the resulting effects on plant defenses remain uncharacterized. To clarify the signaling mechanisms in which (Z)-3-hexenol and light are involved, the gene transcription and metabolite levels were assessed, respectively. This study demonstrated that tea plants rapidly and continuously release (Z)-3-hexen-1-ol in response to an insect infestation. (Z)-3-Hexen-1-ol absorbed by adjacent healthy plants would be converted into three insect defensive compounds: (Z)-3-hexenyl-glucoside, (Z)-3-hexenyl-primeveroside, and (Z)-3-hexenyl-vicianoside identified with laboratory-synthesized standards. Moreover, (Z)-3-hexen-1-ol also activates the synthesis of jasmonic acid to enhance the insect resistance of tea plants. Additionally, a continuous light treatment induces the accumulation of (Z)-3-hexenyl-glycosides. Hence, (Z)-3-hexenol serves as a light-regulated signaling molecule that activates the systemic defenses of adjacent plants. Our study reveals the molecular mechanisms by which biotic and abiotic factors synergistically regulate the signaling functions of herbivore-induced plant volatiles in plants, providing valuable information for future comprehensive analyses of the systemic defense mechanisms in plants.

Elucidation of the Regular Emission Mechanism of Volatile β-Ocimene with Anti-insect Function from Tea Plants (Camellia sinensis) Exposed to Herbivore Attack

Herbivore-induced plant volatiles (HIPVs) play an important role in insect resistance. As a common HIPV in tea plants (Camellia sinensis), β-ocimene has shown anti-insect function in other plants. However, whether β-ocimene in tea plants also provides insect resistance, and its mechanism of synthesis and emission are unknown. In this study, β-ocimene was confirmed to interfere with tea geometrid growth via signaling. Light was identified as the key factor controlling regular emission of β-ocimene induced by the wounding from tea geometrids. β-Ocimene synthase (CsBOS1) was located in plastids and catalyzed β-ocimene formation in overexpressed tobacco. CsBOS1 expression in tea leaves attacked by tea geometrids showed a day-low and night-high variation pattern, while CsABCG expression involved in volatile emission showed the opposite pattern. These two genes might regulate the regular β-ocimene emission from tea plants induced by tea geometrid attack. This study advances the understanding on HIPV emission and signaling in tea plants.

Elicitation of biomolecules as host defense arsenals during insect attacks on tea plants (Camellia sinensis (L.) Kuntze)

The most consumed and economically important beverage plant, tea (Camellia sinensis), and its pests have coevolved so as to maintain the plant-insect interaction. In this review, findings of different research groups on pest responsive tolerance mechanisms that exist in tea manifested through the production of secondary metabolites and their inducers are presented. The phytochemicals of C. sinensis have been categorized into volatiles, nonvolatiles, enzymes, and phytohormones for convenience. Two types of pests, namely the piercing-sucking pests and chewing pests, are associated with tea. Both the insect groups can trigger the production of those metabolites and inducers through several primary and secondary biosynthetic pathways. These induced biomolecules can act as insect repellents and most of them are associated with lowering the nutrient quality of plant tissue and increasing the indigestibility in the pest's gut. Moreover, some of them also act as predator attractants of particular pests. The herbivore-induced plant volatiles secreted from tea plants during pest infestation were (E)-nerolidol, α-farnesene, (Z)-3-hexenol, (E)-4,8-dimethyl-1,3,7-nonatriene, indole, benzyl nitrile (BN), linalool, and ocimenes. The nonvolatiles like theaflavin and L-theanine were increased in response to the herbivore attack. Simultaneously, S-adenosyl-L-methionine synthase, caffeine synthase activities were affected, whereas flavonoid synthesis and wax formation were elevated. Defense responsive enzymes like peroxidase, polyphenol oxidase, phenylalanine ammonia-lyase, ascorbate peroxidase, and catalase are involved in pest prevention mechanisms. Phytohormones like jasmonic acid, salicylic acid, abscisic acid, and ethylene act as the modulator of the defense system. The objective of this review is to discuss the defensive roles of these metabolites and their inducers against pest infestation in tea with an aim to develop environmentally sustainable pesticides in the future.Key points• Herbivore-induced volatile signals and their effects on neighboring tea plant protection• Stereochemical conversion of volatiles, effects of nonvolatiles, expression of defense-responsive enzymes, and phytohormones due to pest attack• Improved understanding of metabolites for bio-sustainable pesticide development.

Formation of volatiles in response to tea green leafhopper (Empoasca onukii Matsuda) herbivory in tea plants: a multi-omics study

Combined transcriptome and metabolome analysis of fresh leaf infestation by tea green leafhoppers (Empoasca (Matsumurasca) onukii Matsuda) suggests roles for alternative pre-mRNA splicing and mRNAs in the regulation of aroma formation in tea plants. Oriental Beauty is a high-grade, oolong tea with a pronounced honey-like aroma and rich ripe fruit flavor that develops primarily as a result of the infestation of the fresh leaves by tea green leafhoppers (Empoasca (Matsumurasca) onukii Matsuda). Here, we used PacBio Iso-Seq and RNA-seq analyses to determine the full-length transcripts and gene expression profiles of fresh tea leaves in response to E. (M.) onukii herbivory. We investigated the relationship between RNA-seq, tea metabolites, and aroma response mechanisms in leaves infested by leafhoppers. We found 3644 differentially expressed genes, of which 2552 were up- and 1092 were down-regulated. A total of 49,913 alternative splicing events were predicted, including 324 differential AS events. Moreover, 3105 differentially expressed transcripts were also identified, of which 2295 were up- and 810 were down-regulated. The characterization of expression patterns of the key gene transcript isoforms involved in the aroma formation pathways identified 130 differentially expressed metabolites, 97 of which were up- and 33 were down-regulated. Two key aroma compounds (phenylacetaldehyde and 4-hydroxybenzaldehyde) were highly correlated with genes of the aroma formation pathways. Our results revealed that pre-mRNA AS plays a crucial role in the metabolic regulation surrounding aroma formation under leafhopper herbivory in tea plants.

Roles of specialized metabolites in biological function and environmental adaptability of tea plant (Camellia sinensis) as a metabolite studying model

Background

Aim of review

Key scientific concepts of review

Foliage Intensity is an Important Cue of Habitat Location for Empoasca onukii

For many herbivorous insects, vision is more important than olfaction in the prealighting stage of host habitat location. Tea leafhoppers, Empoasca onukii (Hemiptera, Cicadellidae), are serious pests that preferentially inhabit the tender leaves of tea plants across China. Here, we investigated whether tea leafhoppers could distinguish foliage colors associated with different leaf ages and use this visual cue to guide suitable habitat location from short distances. Similar to honeybees, the adult E. onukii has an apposition type of compound eye, and each ommatidium has eight retinular cells, in which three spectral types of photoreceptors are distributed, with peak sensitivities at 356 nm (ultraviolet), 435 nm (blue), and 542 nm (green). Both changes in spectral intensity and hue of reflectance light of the host foliage were correlated with varying leaf age, and the intensity linearly decreased with increasing leaf age. Behavioral responses also showed that adult E. onukii could discriminate between the simulated colors of host foliage at different leaf ages without olfactory stimuli and selected the bright colors that strongly corresponded to those of tender leaves. The results suggest that, compared with the spectral composition (hue), the intensity of light reflectance from leaves at different ages is more important for adult leafhoppers when discriminating host foliage and could guide them to tender leaves at the top of tea shoots.

Identification and evaluation of reliable reference genes for quantitative real-time PCR analysis in tea plants under differential biotic stresses

The selection of reliable reference genes (RGs) for normalization under given experimental conditions is necessary to develop an accurate qRT-PCR assay. To the best of our knowledge, only a small number of RGs have been rigorously identified and used in tea plants (Camellia sinensis (L.) O. Kuntze) under abiotic stresses, but no critical RG identification has been performed for tea plants under any biotic stresses till now. In the present study, we measured the mRNA transcriptional levels of ten candidate RGs under five experimental conditions; these genes have been identified as stable RGs in tea plants. By using the ΔCt method, geNorm, NormFinder and BestKeeper, CLATHRIN1 and UBC1, TUA1 and SAND1, or SAND1 and UBC1 were identified as the best combination for normalizing diurnal gene expression in leaves, stems and roots individually; CLATHRIN1 and GAPDH1 were identified as the best combination for jasmonic acid treatment; ACTIN1 and UBC1 were identified as the best combination for Toxoptera aurantii-infested leaves; UBC1 and GAPDH1 were identified as the best combination for Empoasca onukii-infested leaves; and SAND1 and TBP1 were identified as the best combination for Ectropis obliqua regurgitant-treated leaves. Furthermore, our results suggest that if the processing time of the treatment was long, the best RGs for normalization should be recommended according to the stability of the proposed RGs in different time intervals when intragroup differences were compared, which would strongly increase the accuracy and sensitivity of target gene expression in tea plants under biotic stresses. However, when the differences of intergroup were compared, the RGs for normalization should keep consistent across different time points. The results of this study provide a technical guidance for further study of the molecular mechanisms of tea plants under different biotic stresses.

Wound- and pathogen-activated de novo JA synthesis using different ACX isozymes in tea plant (Camellia sinensis)

Acyl-CoA oxidase (ACX; EC 1.3.3.6) plays a vital role in the biosynthesis of jasmonic acid (JA) in plant peroxisomes. We previously identified an herbivore-induced gene CsACX1 in tea plant (Camellia sinensis) and showed CsACX1 was involved in the wound-induced synthesis of jasmonic acid (JA). Here, another ACX gene CsACX3 was isolated from tea plant. CsACX3 was predicted to consist of 684 amino acid residues. CsACX3 can be induced by mechanical wounding, JA application, and infestation by the tea geometrid Ectropis obliqua Prout and the tea green leafhopper Empoasca (Matsumurasca) onukii Matsuda. These expression patterns are consistent with the previously reported expression pattern of CsACX1 under such treatments. Recombinant CsACX3 showed preference for medium-chain acyl-coA oxidase substrates (C₈- to C₁₄-CoA). CsACX3 expression could also be induced by the infection of a pathogen Colletotrichum gloeosporioides (Cgl), and the increased ACX activities in tea plants were correlated with the Cgl-induced CsACX3 expression. Cgl could not induce the expression of CsACX1, which showed preference for C₁₂- to C₁₆-CoA substrates. The constitutive expression of CsACX3 rescued wound-induced JA biosynthesis and enhanced the Cgl-induced JA biosynthesis in Arabidopsis mutant atacx1. However, constitutive expression of CsACX1 could not enhance the Cgl-induced JA biosynthesis in atacx1 plant. These results indicate that CsACX1 and CsACX3 functions overlap and have distinct roles in the wound- and pathogen-activated de novo JA synthesis via enzymatic routes that utilize different ACX isozymes in tea plant.

Establishing a System for Functional Characterization of Full-Length cDNAs of Camellia sinensis

Tea (Camellia sinensis) is enriched with bioactive secondary metabolites, and is one of the most popular nonalcoholic beverages globally. Two tea reference genomes have been reported; however, the functional analysis of tea genes has lagged, mainly due to tea’s recalcitrance to genetic transformation and the absence of alternative high throughput heterologous expression systems. A full-length cDNA collection with a streamlined cloning system is needed in this economically important woody crop species. RNAs were isolated from nine different vegetative tea tissues, pooled, then used to construct a normalized full-length cDNA library. The titer of unamplified and amplified cDNA library was 6.89 × 10⁶ and 1.8 × 10¹⁰ cfu/mL, respectively; the library recombinant rate was 87.2%. Preliminary characterization demonstrated that this collection can complement existing tea reference genomes and facilitate rare gene discovery. In addition, to streamline tea cDNA cloning and functional analysis, a binary vector (pBIG2113SF) was reengineered, seven tea cDNAs isolated from this library were successfully cloned into this vector, then transformed into Arabidopsis. One FL-cDNA, which encodes a putative P_1B-type ATPase 5 (CsHMA5), was characterized further as a proof of concept. We demonstrated that overexpression of CsHMA5 in Arabidopsis resulted in copper hyposensitivity. Thus, our data demonstrated that this represents an efficient system for rare gene discovery and functional characterization of tea genes. The integration of a tea FL-cDNA collection with efficient cloning and a heterologous expression system would facilitate functional annotation and characterization of tea genes.