Validation of reference genes for gene expression studies in post-harvest leaves of tea plant (Camellia sinensis)

The Dynamic Change in Aromatic Compounds and Their Relationship with CsAAAT Genes during the Post-Harvest Process of Oolong Tea

Formed by L-phenylalanine (L-phe) ammonia under the action of aromatic amino acid aminotransferases (AAATs), volatile benzenoids (VBs) and volatile phenylpropanoids (VPs) are essential aromatic components in oolong tea (Camellia sinensis). However, the key VB/VP components responsible for the aromatic quality of oolong tea need to be revealed, and the formation mechanism of VBs/VPs based on AAAT branches during the post-harvest process of oolong tea remains unclear. Therefore, in this study, raw oolong tea and manufacturing samples were used as the test materials, and targeted metabolomics combined with transcriptome analysis was also conducted. The results showed that thirteen types of VBs/VPs were identified, including nine types of VPs and four types of VBs. Based on the OAV calculation, in raw oolong tea, 2-hydroxy benzoic acid methyl ester and phenylethyl alcohol were identified as key components of the aromatic quality of oolong tea. As for the results from the selection of related genes, firstly, a total of sixteen candidate CsAAAT genes were selected and divided into two sub-families (CsAAAT1 and CsAAAT2); then, six key CsAAAT genes closely related to VB/VP formation were screened. The upregulation of the expression level of CsAAAT2-type genes may respond to light stress during solar-withering as well as the mechanical force of turnover. This study can help to understand the formation mechanism of aromatic compounds during oolong tea processing and provide a theoretical reference for future research on the formation of naturally floral and fruity aromas in oolong tea.

Integrated metabolomic and transcriptomic analysis reveal the effect of mechanical stress on sugar metabolism in tea leaves (Camellia sinensis) post-harvest

Sugar metabolites not only act as the key compounds in tea plant response to stress but are also critical for tea quality formation during the post-harvest processing of tea leaves. However, the mechanisms by which sugar metabolites in post-harvest tea leaves respond to mechanical stress are unclear. In this study, we aimed to investigate the effects of mechanical stress on saccharide metabolites and related post-harvest tea genes. Withered (C15) and mechanically-stressed (V15) for 15 min Oolong tea leaves were used for metabolome and transcriptome sequencing analyses. We identified a total of 19 sugar metabolites, most of which increased in C15 and V15. A total of 69 genes related to sugar metabolism were identified using transcriptome analysis, most of which were down-regulated in C15 and V15. To further understand the relationship between the down-regulated genes and sugar metabolites, we analyzed the sucrose and starch, galactose, and glycolysis metabolic pathways, and found that several key genes of invertase (INV), α-amylase (AMY), β-amylase (BMY), aldose 1-epimerase (AEP), and α-galactosidase (AGAL) were down-regulated. This inhibited the hydrolysis of sugars and might have contributed to the enrichment of galactose and D-mannose in V15. Additionally, galactinol synthase (Gols), raffinose synthase (RS), hexokinase (HXK), 6-phosphofructokinase 1 (PFK-1), and pyruvate kinase (PK) genes were significantly upregulated in V15, promoting the accumulation of D-fructose-6-phosphate (D-Fru-6P), D-glucose-6-phosphate (D-glu-6P), and D-glucose. Transcriptome and metabolome association analysis showed that the glycolysis pathway was enhanced and the hydrolysis rate of sugars related to hemicellulose synthesis slowed in response to mechanical stress. In this study, we explored the role of sugar in the response of post-harvest tea leaves to mechanical stress by analyzing differences in the expression of sugar metabolites and related genes. Our results improve the understanding of post-harvest tea's resistance to mechanical stress and the associated mechanism of sugar metabolism. The resulting treatment may be used to control the quality of Oolong tea.

A comprehensive review of nano-delivery system for tea polyphenols: Construction, applications, and challenges

Tea polyphenols (TPs) are important bioactive compounds in tea and have excellent physiological regulation functions. However, the extraction and purification of TPs are key technologies affecting their further application, and the chemical instability, poor bioavailability of TPs are major challenges for researchers. In the past decade, therefore, research and development of advanced carrier systems for the delivery of TPs has been greatly promoted to improve their poor stability and poor bioavailability. In this review, the properties and function of TPs are introduced, and the recent advances in the extraction and purification technologies are systematically summarized. Particularly, the intelligent delivery of TPs via novel nano-carriers is critically reviewed, and the application of TPs nano-delivery system in medical field and food industry is also described. Finally, the main limitations, current challenges and future perspectives are highlighted in order to provide research ideas for exploiting nano-delivery carriers and their application in TPs.

Effects of turning over intensity on fatty acid metabolites in postharvest leaves of Tieguanyin oolong tea (Camellia sinensis)

Fatty acid derived volatiles (FADVs) are major contributors to the aroma quality of oolong tea (Camellia sinensis). Most of the processing time for oolong tea is taken up by turning over treatments, but the full profile of fatty acid metabolic changes during this process remains unclear. In this study, we detected fatty acids, their derived volatiles, and related genes of Tieguanyin oolong tea using biochemical and molecular biology methods. The results showed that with an increase in turning over intensities, the content of total unsaturated fatty acids continuously dropped and the content of characteristic FADVs, such as hexanoic acid (Z)-3-Hexenly ester and 2-exenal, continued to increase. Lipoxygenase (LOX), a key gene family in the fatty acid metabolic pathway, showed different patterns, and CsLOX1 (TEA025499.1) was considered to be a key gene during the turning over processes. We found that fruit-like aroma (Z)-3-Hexen-1-ol acetate had a strong correlation with the expression levels of eight Camelia sinensis LOX family genes. Tieguanyin had relatively rich pleasant volatile compounds with moderate turning over intensity (five times turning over treatments). This study provides an overall view of how fatty acid metabolites change and affect the quality of oolong tea with different turning over intensities during processing.

Tea (Camellia sinensis): A Review of Nutritional Composition, Potential Applications, and Omics Research

Tea (Camelliasinensis) is the world’s most widely consumed non-alcoholic beverage with essential economic and health benefits since it is an excellent source of polyphenols, catechins, amino acids, flavonoids, carotenoids, vitamins, and polysaccharides. The aim of this review is to summarize the main secondary metabolites in tea plants, and the content and distribution of these compounds in six different types of tea and different organs of tea plant were further investigated. The application of these secondary metabolites on food processing, cosmetics industry, and pharmaceutical industry was reviewed in this study. With the rapid advancements in biotechnology and sequencing technology, omics analyses, including genome, transcriptome, and metabolome, were widely used to detect the main secondary metabolites and their molecular regulatory mechanisms in tea plants. Numerous functional genes and regulatory factors have been discovered, studied, and applied to improve tea plants. Research advances, including secondary metabolites, applications, omics research, and functional gene mining, are comprehensively reviewed here. Further exploration and application trends are briefly described. This review provides a reference for basic and applied research on tea plants.

Influence of reduced N-fertilizer application on foliar chemicals and functional qualities of tea plants under Toxoptera aurantii infestation

BACKGROUND

The tea aphid, Toxoptera aurantii (Boyer de Fonscolombe) is a polyphagous pest predominant in tea orchards and has become the most pernicious pest deteriorating tea quality. Nitrogen (N) is essential to plant growth improvement, and it can significantly impact plant defensive ability against aphid infestation. This study was designed to quantify the influence of reduced N-fertilizer application on foliar chemicals and functional quality parameters of tea plants against the infestation of T. aurantii. In this study, the tea seedlings (cv. Longjing43) were applied with normal level (NL) of N-fertilizer (240 kg N ha^-1) along with reduced N-fertilizer levels (70%NL and 50%NL), and with and without T. aurantii infestation.

RESULTS

The results showed that N-fertilizer application significantly affected plant biomass and photosynthetic indexes, foliar soluble nutrients and polyphenols, tea catechins, caffeine, essential amino acids, volatile organic compounds of tea seedlings, and the population dynamics of T. aurantii. Compared with the normal N-fertilizer level, the reduced N-fertilizer application (70%NL and 50%NL) significantly decreased all the foliar functional quality components of tea seedlings without aphid infestation, while these components were increased in tea seedlings with aphid infestation. Moreover, the transcript expression levels of foliar functional genes (including CsTCS, CsTs1, and CsGT1) were significantly higher in the NL, and significantly lower in the 50%NL for tea seedlings without aphid infestation, while the transcript expression levels were significantly higher in 50%NL in aphid inoculated tea seedlings.

CONCLUSION

The results demonstrated that the reduced N-fertilizer application could enhance foliar chemicals and functional quality parameters of tea plants especially with T. aurantii infestation, which can relieve soil nitrogen pressure and reduce pesticide use for control of tea aphid infestation in tea plantations.

Effects of Long-Term Non-Pruning on Main Quality Constituents in 'Dancong' Tea (Camellia sinensis) Leaves Based on Proteomics and Metabolomics Analysis

'Dancong' tea is a famous traditional Oolong tea. In order to keep the original taste of "ancient tea trees", most of the 'Dancong' tea plants are planted in a single plant pattern without pruning. The objective of this study was to explore the effects of long-term non-pruning on main quality constituents in 'Dancong' tea. The results showed that the contents of free amino acids, chlorophylls, and floral-honey aromatic substances in tea leaves of unpruned tea plants were higher than those in every year pruned tea plants, while the catechin content in leaves of pruned tea plants was higher than that in leaves of unpruned tea plants. Quantitative proteomics analysis showed that most enzymes involved in biosynthesis of catechins were downregulated in leaves of unpruned tea plants. Five proteins involved in chlorophyll metabolism and 12 proteins related to photosynthesis were upregulated, and the results suggested that higher chlorophyll content and more efficient photosynthetic energy conversion may be important for the higher accumulation of special quality components in leaves of unpruned tea plants. The findings of this study will advance our understanding of the mechanism of formation of different metabolites in leaves of unpruned and pruned tea plants.

Metabolic Flow of C6 Volatile Compounds From LOX-HPL Pathway Based on Airflow During the Post-harvest Process of Oolong Tea

Aroma is an essential quality indicator of oolong tea, a tea derived from the Camellia sinensis L. plant. Carboxylic 6 (C6) acids and their derivative esters are important components of fatty acid (FA)-derived volatiles in oolong tea. However, the formation and regulation mechanism of C6 acid during postharvest processing of oolong tea remains unclear. To gain better insight into the molecular and biochemical mechanisms of C6 compounds in oolong tea, a combined analysis of alcohol dehydrogenase (ADH) activity, CsADH2 key gene expression, and the FA-derived metabolome during postharvest processing of oolong tea was performed for the first time, complemented by CsHIP (hypoxia-induced protein conserved region) gene expression analysis. Volatile fatty acid derivative (VFAD)-targeted metabolomics analysis using headspace solid-phase microextraction-gas chromatography time-of-flight mass spectrometry (HS-SPEM-GC-TOF-MS) showed that the (Z)-3-hexen-1-ol content increased after each turnover, while the hexanoic acid content showed the opposite trend. The results further showed that both the ADH activity and CsADH gene expression level in oxygen-deficit-turnover tea leaves (ODT) were higher than those of oxygen-turnover tea leaves (OT). The C6-alcohol-derived ester content of OT was significantly higher than that of ODT, while C6-acid-derived ester content showed the opposite trend. Furthermore, the HIP gene family was screened and analyzed, showing that ODT treatment significantly promoted the upregulation of CsHIG4 and CsHIG6 gene expression. These results showed that the formation mechanism of oolong tea aroma quality is mediated by airflow in the lipoxygenase-hydroperoxide lyase (LOX-HPL) pathway, which provided a theoretical reference for future quality control in the postharvest processing of oolong tea.

Identification and characterization of early Fusarium wilt responsive mRNAs and long non-coding RNAs in banana root using high-throughput sequencing

Fusarium wilt disease, caused by Fusarium oxysporum f.sp. cubense (Foc), has been recognized as the most devastating disease to banana. The regulatory role of long non-coding RNAs (lncRNAs) in plant defense has been verified in many plant species. However, the understanding of their role during early FocTR4 (Foc tropical race 4) infection stage is very limited. In this study, lncRNA sequencing was used to reveal banana root transcriptome profile changes during early FocTR4 infection stages. Quantitative real time PCR (qRT-PCR) was performed to confirm the expression of eight differentially expressed (DE) lncRNAs (DELs) and their predicted target genes (DETs), and three DE genes (DEGs). Totally, 12,109 lncRNAs, 36,519 mRNAs and 2642 novel genes were obtained, of which 1398 (including 78 DELs, 1220 DE known genes and 100 DE novel genes) were identified as FocTR4 responsive DE transcripts. Gene function analysis revealed that most DEGs were involved in biosynthesis of secondary metabolites, plant–pathogen interaction, plant hormone signal transduction, phenylalanine metabolism, phenylpropanoid biosynthesis, alpha-linolenic acid metabolism and so on. Coincidently, many DETs have been identified as DEGs in previous transcriptome studies. Moreover, many DETs were found to be involved in ribosome, oxidative phosphorylation, lipoic acid metabolism, ubiquitin mediated proteolysis, N-glycan biosynthesis, protein processing in endoplasmic reticulum and DNA damage response pathways. QRT-PCR result showed the expression patterns of the selected transcripts were mostly consistent with our lncRNA sequencing data. Our present study showed the regulatory role of lncRNAs on known biotic and abiotic stress responsive genes and some new-found FocTR4 responsive genes, which can provide new insights into FocTR4-induced changes in the banana root transcriptome during the early pathogen infection stage.

Transcriptome analysis reveals the effect of short-term sunlight on aroma metabolism in postharvest leaves of oolong tea(Camellia sinensis)

Tea (Camellia sinensis (L.)) is an important economic plant. Light is the earliest external signal factor during the postharvest processing of oolong tea, and the solar withering is an indispensable process for aroma formation. In this study, Tieguanyin was used to analyze the effect of sunlight on aroma metabolism, which indicated that the main aroma compounds were significantly increased during solar withering for 15 min compared to the indoor withering. In addition, differentially expressed genes related to aroma metabolism were identified and quantified using the high-throughput Illumina RNA-Seq technology. The expression levels of key regulatory genes were consistent with the results from the gas chromatography-time of flight mass spectrometry (GC-TOF-MS) analysis, especially in terpenoid metabolic pathway, which showed that aroma metabolism could significantly respond to the short-term light, while its expression level was easily inhibited by the up-regulation of heat shock protein. Taken together, those data provides further insights into the mechanisms, contributing to aroma metabolism of tea plant.

Identification of Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) responsive miRNAs in banana root

The fungus, Fusarium oxysporum f. sp. cubense (Foc), is the causal agent of Fusarium wilt disease, which is the most serious disease affecting the whole banana industry. Although extensive studies have characterized many Foc-responsive genes in banana, the molecular mechanisms on microRNA level underlying both banana defense and Foc pathogenesis are not yet fully understood. In this study, we aimed to reveal the role of miRNA during banana-Foc TR4 interactions. Illumina sequencing was used to reveal the changes in small RNAome profiles in roots of Foc TR4-inoculated ‘Tianbaojiao’ banana (Musa acuminata cv. Tianbaojiao) in the early stages (i.e. 5 h, 10 h and 25 h post Foc TR4 inoculation, respectively). The expression of some differentially expressed (DE) miRNAs and their predicted target genes was studied by using quantitative real time PCR (qRT-PCR). Totally, 254 known miRNAs from 31 miRNA families and 28 novel miRNAs were identified. Differential expression analysis identified 84, 77 and 74 DE miRNAs at the three respective Foc TR4 infection time points compared with control healthy banana (CK). GO and KEGG analysis revealed that most of the predicted target genes of DE miRNAs (DET) were implicated in peroxisome, fatty acid metabolism, auxin-activated signaling pathway, sulfur metabolism, lignin metabolism and so on, and many known stress responsive genes were identified to be DETs. Moreover, expected inverse correlations were confirmed between some miRNA and their corresponding target genes by using qRT-PCR analysis. Our study revealed that miRNA play important regulatory roles during the banana-Foc TR4 interaction by regulating peroxidase, fatty acid metabolism, auxin signaling, sulfur metabolism, lignin metabolism related genes and many known stress responsive genes.