Pale green mutant analyses reveal the importance of CsGLKs in chloroplast developmental regulation and their effects on flavonoid biosynthesis in tea plant

Integrative Analysis of Transcriptomic and Metabolomic Profiles Uncovers the Mechanism of Color Variation in the Tea Plant Callus

Tea plants (Camellia sinensis) are among the world's most significant economic tree species. Tissue culture serves as a crucial method in commercial breeding by facilitating the rapid propagation of valuable genotypes and the generation of disease-free clones. However, callus browning represents a prevalent challenge in tea plant tissue culture, and may adversely affect explant growth and development. Our research demonstrates that although anti-browning agents can effectively suppress browning, they induce distinct color changes in the callus. These color variations could significantly influence callus induction and subsequent growth patterns. In this study, callus tissues from C. sinensis var. Assamica cv. Mengku were employed as experimental materials and treated with three commonly used anti-browning agents: ascorbic acid (VC), activated carbon (AC), and polyvinylpyrrolidone (PVP). The results demonstrated that while these three reagents effectively inhibited browning, they also induced distinct color changes in the explants, which appeared red, green, and white, respectively. Furthermore, this study investigated the molecular mechanisms underlying callus color changes using transcriptomic and metabolomic approaches. Based on transcriptome analysis, it was revealed that photosynthesis and flavonoid biosynthesis pathways were significantly enriched. Metabolome analysis identified 14 phenolic acids, which exhibited significant variation in accumulation across calluses of different colors. The differential expression of genes involved in flavonoid biosynthesis pathways, coupled with the distinct accumulation patterns of metabolites, can effectively alleviate photooxidative damage and enhance the resistance of callus to browning. AC activates the photosynthesis of callus by regulating carbon source allocation and upregulating the expression of key genes in the psa, psb, and pet families within the photosynthetic system. This process promotes chlorophyll biosynthesis, thereby enabling the callus to grow green, while VC activates the expression of key genes such as CHS, F3H, C4H, CYP75B1, and ANR in the flavonoid pathway, which are involved in the regulation of pigment synthesis in red callus. This study elucidated the molecular mechanisms underlying the effects of anti-browning agents on color variations in C. sinensis callus, thereby providing a robust theoretical foundation for optimization, the establishment of tea plant tissue culture systems, and enhancing cultivar quality.

Response of photosynthetic capacity to ecological factors and its relationship with EGCG biosynthesis of tea plant (Camellia sinensis)

BACKGROUND

Epigallocatechin gallate (EGCG) imparts unique health benefits and flavour to tea. Photosynthesis plays a crucial role in modulating secondary metabolite production in plants, and this study investigated its impact on the biosynthesis of EGCG in tea plants under different ecological conditions.

RESULTS

Enhanced photosynthetic activity and the increased EGCG content, total esterified catechins (TEC), total catechins (TC) responded synchronously to changes in ecological factors. The photosynthetic capacity of tea plants and the EGCG content fit surface model equations (Extreme 2D and Polynomial 2D) and multiple regression equations (R2 > 70%). Additionally, logistic regression and ROC curves revealed that photosynthetic capacity was related to EGCG accumulation patterns in response to ecological variations. Upon perceiving ecological changes, the response of photosynthesis-related genes (CspsaA from photosystem I, CspsbB, CspsbC from photosystem II, and CsLHCB3 from the antenna protein pathway) was associated to carbon cycle-related genes (CsALDO, CsACOX, CsICDH, Csrbcs), which mediated the expression of CsPAL in the phenylalanine pathway; CsaroDE in the shikimate pathway; and CsCHS, CsF3H, CsF3'H, and CsANS in the flavonoid pathway. Eventually, this influenced the accumulation of EGCG and its precursors (gallic acid and epigallocatechin) in tea plants.

CONCLUSIONS

This study reveals the effects of photosynthesis on EGCG biosynthesis in response to ecological factors, providing insights for optimizing tea cultivation and quality.

Identification of the CsFtsH genes from Camellia sinensis reveals its potential role in leaf color phenotype

The filamentation temperature-sensitive H (FtsH) metalloprotease participates in the chloroplast photosystem II (PSII) repair cycle, playing a crucial role in regulating leaf coloration. However, the evolutionary history and biological function of the FtsH family in albino tea plants are still unknown. In this study, 35 CsFtsH members, including 7 CsFtsH-like (CsFtsHi1-CsFtsHi7) proteins, mapping onto 11 chromosomes in 6 subgroups, were identified in the 'Shuchazao2' tea genome, and their exon/intron structure, domain characteristics, collinearity, protein interaction network, and secondary structure were comprehensively analyzed. Furthermore, real-time fluorescence quantitative PCR (RT-qPCR) analysis revealed that the expression levels of CsFtsH1/2/5/8 were significantly positively correlated with the leaf color of tea plants. The subcellular localization revealed that they were located in the chloroplast. The transgenic Arabidopsis has demonstrated that CsFtsH2 and CsFtsH5 could restore the chlorophyll content and chlorophyll fluorescence intensity in var1 and var2 mutants, respectively. Moreover, protein-protein interactions have confirmed that CsFtsH1 with CsFtsH5, and CsFtsH2 with CsFtsH8 could form a hetero-comples and function in chloroplasts. In summary, this study aims to not only increase the understanding of the underlying molecular mechanisms of CsFtsH but also to provide a solid and detailed theoretical foundation for the breeding of albino tea plant varieties.

Molecular Mechanisms of Chlorophyll Deficiency in Ilex × attenuata 'Sunny Foster' Mutant

Ilex × attenuata 'Sunny Foster' represents a yellow leaf mutant originating from I. × attenuata 'Foster#2', a popular ornamental woody cultivar. However, the molecular mechanisms underlying this leaf color mutation remain unclear. Using a comprehensive approach encompassing cytological, physiological, and transcriptomic methodologies, notable distinctions were discerned between the mutant specimen and its wild type. The mutant phenotype displayed aberrant chloroplast morphology, diminished chlorophyll content, heightened carotenoid/chlorophyll ratios, and a decelerated rate of plant development. Transcriptome analysis identified differentially expressed genes (DEGs) related to chlorophyll metabolism, carotenoid biosynthesis and photosynthesis. The up-regulation of CHLD and CHLI subunits leads to decreased magnesium chelatase activity, while the up-regulation of COX10 increases heme biosynthesis-both impair chlorophyll synthesis. Conversely, the down-regulation of HEMD hindered chlorophyll synthesis, and the up-regulation of SGR enhanced chlorophyll degradation, resulting in reduced chlorophyll content. Additionally, genes linked to carotenoid biosynthesis, flavonoid metabolism, and photosynthesis were significantly down-regulated. We also identified 311 putative differentially expressed transcription factors, including bHLHs and GLKs. These findings shed light on the molecular mechanisms underlying leaf color mutation in I. × attenuata 'Sunny Foster' and provide a substantial gene reservoir for enhancing leaf color through breeding techniques.

Integrated Analysis of Metabolome and Transcriptome Revealed Different Regulatory Networks of Metabolic Flux in Tea Plants [Camellia sinensis (L.) O. Kuntze] with Varied Leaf Colors

Leaf color variations in tea plants were widely considered due to their attractive phenotypes and characteristic flavors. The molecular mechanism of color formation was extensively investigated. But few studies focused on the transformation process of leaf color change. In this study, four strains of 'Baijiguan' F1 half-sib generation with similar genetic backgrounds but different colors were used as materials, including Green (G), Yellow-Green (Y-G), Yellow (Y), and Yellow-Red (Y-R). The results of broadly targeted metabolomics showed that 47 metabolites were differentially accumulated in etiolated leaves (Y-G, Y, and Y-R) as compared with G. Among them, lipids were the main downregulated primary metabolites in etiolated leaves, which were closely linked with the thylakoid membrane and chloroplast structure. Flavones and flavonols were the dominant upregulated secondary metabolites in etiolated leaves, which might be a repair strategy for reducing the negative effects of dysfunctional chloroplasts. Further integrated analysis with the transcriptome indicated different variation mechanisms of leaf phenotype in Y-G, Y, and Y-R. The leaf color formation of Y-G and Y was largely determined by the increased content of eriodictyol-7-O-neohesperidoside and the enhanced activities of its modification process, while the color formation of Y-R depended on the increased contents of apigenin derivates and the vigorous processes of their transportation and transcription factor regulation. The key candidate genes, including UDPG, HCT, CsGSTF1, AN1/CsMYB75, and bHLH62, might play important roles in the flavonoid pathway.

Genome-Wide Identification and Characterization of GARP Transcription Factor Gene Family Members Reveal Their Diverse Functions in Tea Plant (Camellia sinensis)

Golden2, ARR-B, Psr1 (GARP) proteins are plant-specific transcription factors that play vital and diverse roles in plants. However, systematic research on the GARP gene family in plants, including tea plant (Camellia sinensis), is scarce. In this study, a total of 69 GARP genes were identified and characterized from the tea plant genome based on the B-motif sequence signature. The CsGARP genes were clustered into five subfamilies: PHR1/PHL1, KAN, NIGT1/HRS1/HHO, GLK and ARR-B subfamilies. The phylogenetic relationships, gene structures, chromosomal locations, conserved motifs and regulatory cis-acting elements of the CsGARP family members were comprehensively analyzed. The expansion of CsGARP genes occurred via whole-genome duplication/segmental duplication, proximal duplication, and dispersed duplication under purifying selective pressure. The expression patterns of the CsGARP genes were systematically explored from various perspectives: in different tissues during different seasons; in different leaf color stages of tea plant; under aluminum treatment and nitrogen treatment; and in response to abiotic stresses such as cold, drought and salt and to biotic stress caused by Acaphylla theae. The results demonstrate that CsGARP family genes are ubiquitously expressed and play crucial roles in the regulation of growth and development of tea plant and the responses to environmental stimuli. Collectively, these results not only provide valuable information for further functional investigations of CsGARPs in tea plant but also contribute to broadening our knowledge of the functional diversity of GARP family genes in plants.

A Comparative Metabolomic Analysis Reveals Difference Manufacture Suitability in "Yinghong 9" and "Huangyu" Teas (Camellia sinensis)

"Yinghong 9" is a widely cultivated large-leaf variety in South China, and the black tea made from it has a high aroma and strong sweet flavor. "Huangyu" is a light-sensitive tea variety with yellow leaves. It was cultivated from the bud-mutation of "Yinghong 9" and has a very low level of chlorophyll during young shoot development. Due to chlorophyll being involved in carbon fixation and assimilation, the changes in photosynthesis might potentially affect the accumulation of flavor metabolites, as well as the quality of "Huangyu" tea. Although "Huangyu" has a golden yellow color and high amino acid content, the mechanism underlying the formation of leaf color and drinking value remains unclear. The widely targeted metabolomics and GC-MS analysis were performed to reveal the differences of key metabolites in fresh and fermented leaves between "Yinghong 9" and "Huangyu." The results showed that tea polyphenols, total chlorophyll, and carotenoids were more abundant in "Yinghong 9." Targeted metabolomics analysis indicated that kaempferol-3-glycoside was more abundant in "Yinghong 9," while "Huangyu" had a higher ratio of kaempferol-3-glucoside to kaempferol-3-galactoside. Compared with "Yinghong 9" fresh leaves, the contents of zeaxanthin and zeaxanthin palmitate were significantly higher in "Huangyu." The contents of α-farnesene, β-cyclocitral, nerolidol, and trans-geranylacetone, which were from carotenoid degradation and involved in flowery-fruity-like flavor in "Huangyu" fermented leaves, were higher than those of "Yinghong 9." Our results indicated that "Huangyu" was suitable for manufacturing non-fermented tea because of its yellow leaf and flowery-fruity-like compounds from carotenoid degradation.

Review: the effect of light on the key pigment compounds of photosensitive etiolated tea plant

BACKGROUND

Light is the ultimate energy source of plant photosynthesis, which has an important impact on the growth, development, physiology and biochemistry of tea plant. Photosensitive etiolated tea plant belongs to a kind of colored leaf plant, which is a physiological response to light intensity. Compared with conventional green bud and leaf of tea plant, the accumulation of pigment compounds (chlorophyll and carotenoids, etc.) closely related to a series of reactions of photosynthesis in photosensitive etiolated tea plant is reduced, resulting in the difference of leaf color of tea. This specific tea resource has high application value, among which high amino acid is one of its advantages. It can be used to process high-quality green tea with delicious taste and attractive aroma, which has been widely attention. The mechanism of the color presentation of the etiolated mutant tea leaves has been given a high topic and attention, especially, what changes have taken place in the pigment compounds of tea leaves caused by light, which makes the leaves so yellow. At present, there have been a lot of research and reports.

PURPOSE OF THE REVIEW

We describe the metabolism and differential accumulation of key pigment compounds affecting the leaf color of photosensitive etiolated tea that are triggered by light, and discuss the different metabolism and key regulatory sites of these pigments in different light environments in order to understand the "discoloration" matrix and mechanism of etiolated tea resources, answer the scientific question between leaf color and light. It provides an important strategy for artificial intervention of discoloration of colored tea plant.

CONCLUSION

The differential accumulation of pigment compounds in tea plant can be induced phytochrome in response to the change of light signal. The synthesis of chlorophyll in photoetiolated tea plants is hindered by strong light, among which, the sites regulated by coproporphyrinogen III oxidase and chlorophyllide a oxidase is sensitive to light and can be inhibited by strong light, resulting in the aggravation of leaf etiolation. The phenomenon can be disappeared or weakened by shading or reducing light intensity, and the leaf color is greenish, but the increase of chlorophyll-b accumulation is more than that of chlorophyll-a. The synthesis of carotenoids is inhibited strong light, and high the accumulation of carotenoids is reduced by shading. Most of the genes regulating carotenoids are up-regulated by moderate shading and down-regulated by excessive shading. Therefore, the accumulation of these two types of pigments in photosensitive etiolated tea plants is closely related to the light environment, and the leaf color phenotype shape of photosensitive etiolated tea plants can be changed by different light conditions, which provides an important strategy for the production and management of tea plant.

Validation of Reference Genes for Quantitative Real-Time PCR Normalization in Ananas comosus var. bracteatus During Chimeric Leaf Development and Response to Hormone Stimuli

Reverse transcription quantitative real-time PCR (RT-qPCR) is a common way to study gene regulation at the transcriptional level due to its sensibility and specificity, but it needs appropriate reference genes to normalize data. Ananas comosus var. bracteatus, with white-green chimeric leaves, is an important pantropical ornamental plant. Up to date, no reference genes have been evaluated in Ananas comosus var. bracteatus. In this work, we used five common statistics tools (geNorm, NormFinder, BestKeeper, ΔCt method, RefFinder) to evaluate 10 candidate reference genes. The results showed that Unigene.16454 and Unigene.16459 were the optimal reference genes for different tissues, Unigene.16454 and zinc finger ran-binding domain-containing protein 2 (ZRANB2) for chimeric leaf at different developmental stages, isocitrate dehydrogenase [NADP] (IDH) and triacylglycerol lipase SDP1-like (SDP) for seedlings under different hormone treatments. The comprehensive results showed IDH, pentatricopeptide repeat-containing protein (PPRC), Unigene.16454, and caffeoyl-CoA O methyltransferase 5-like (CCOAOMT) are the top-ranked stable genes across all the samples. The stability of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was the least during all experiments. Furthermore, the reliability of recommended reference gene was validated by the detection of porphobilinogen deaminase (HEMC) expression levels in chimeric leaves. Overall, this study provides appropriate reference genes under three specific experimental conditions and will be useful for future research on spatial and temporal regulation of gene expression and multiple hormone regulation pathways in Ananas comosus var. bracteatus.

Light control of catechin accumulation is mediated by photosynthetic capacity in tea plant (Camellia sinensis)

BACKGROUND

Catechins are crucial in determining the flavour and health benefits of tea, but it remains unclear that how the light intensity regulates catechins biosynthesis. Therefore, we cultivated tea plants in a phytotron to elucidate the response mechanism of catechins biosynthesis to light intensity changes.

RESULTS

In the 250 μmol·m^- 2·s^- 1 treatment, the contents of epigallocatechin, epigallocatechin gallate and total catechins were increased by 98.94, 14.5 and 13.0% respectively, compared with those in the 550 μmol·m^- 2·s^- 1 treatment. Meanwhile, the photosynthetic capacity was enhanced in the 250 μmol·m^- 2·s^- 1 treatment, including the electron transport rate, net photosynthetic rate, transpiration rate and expression of related genes (such as CspsbA, CspsbB, CspsbC, CspsbD, CsPsbR and CsGLK1). In contrast, the extremely low or high light intensity decreased the catechins accumulation and photosynthetic capacity of the tea plants. The comprehensive analysis revealed that the response of catechins biosynthesis to the light intensity was mediated by the photosynthetic capacity of the tea plants. Appropriately high light upregulated the expression of genes related to photosynthetic capacity to improve the net photosynthetic rate (Pn), transpiration rate (Tr), and electron transfer rate (ETR), which enhanced the contents of substrates for non-esterified catechins biosynthesis (such as EGC). Meanwhile, these photosynthetic capacity-related genes and gallic acid (GA) biosynthesis-related genes (CsaroB, CsaroDE1, CsaroDE2 and CsaroDE3) co-regulated the response of GA accumulation to light intensity. Eventually, the epigallocatechin gallate content was enhanced by the increased contents of its precursors (EGC and GA) and the upregulation of the CsSCPL gene.

CONCLUSIONS

In this study, the catechin content and photosynthetic capacity of tea plants increased under appropriately high light intensities (250 μmol·m^- 2·s^- 1 and 350 μmol·m^- 2·s^- 1) but decreased under extremely low or high light intensities (150 μmol·m^- 2·s^- 1 or 550 μmol·m^- 2·s^- 1). We found that the control of catechin accumulation by light intensity in tea plants is mediated by the plant photosynthetic capacity. The research provided useful information for improving catechins content and its light-intensity regulation mechanism in tea plant.

Identification, Molecular Characteristic, and Expression Analysis of PIFs Related to Chlorophyll Metabolism in Tea Plant (Camellia sinensis)

The phytochrome-interacting factors (PIFs) proteins belong to the subfamily of basic helix–loop–helix (bHLH) transcription factors and play important roles in chloroplast development and chlorophyll biosynthesis. Currently, knowledge about the PIF gene family in Camellia sinensis remains very limited. In this study, seven PIF members were identified in the C. sinensis genome and named based on homology with AtPIF genes in Arabidopsis thaliana. All C. sinensis PIF (CsPIF) proteins have both the conserved active PHYB binding (APB) and bHLH domains. Phylogenetic analysis revealed that CsPIFs were clustered into four groups—PIF1, PIF3, PIF7, and PIF8—and most CsPIFs were clustered in pairs with their corresponding orthologs in Populus tremula. CsPIF members in the same group tended to display uniform or similar exon–intron distribution patterns and motif compositions. CsPIF genes were differentially expressed in C. sinensis with various leaf colors and strongly correlated with the expression of genes involved in the chlorophyll metabolism pathway. Promoter analysis of structural genes related to chlorophyll metabolism found DNA-binding sites of PIFs were abundant in the promoter regions. Protein–protein interaction networks of CsPIFs demonstrated a close association with phytochrome, PIF4, HY5, TOC1, COP1, and PTAC12 proteins. Additionally, subcellular localization and transcriptional activity analysis suggested that CsPIF3b was nuclear localized protein and possessed transcriptional activity. We also found that CsPIF3b could activate the transcription of CsHEMA and CsPOR in Nicotiana benthamiana leaves. This work provides comprehensive research of CsPIFs and would be helpful to further promote the regulation mechanism of PIF on chlorophyll metabolism in C. sinensis.

Effects of Long-Term Nitrogen Fertilization on the Formation of Metabolites Related to Tea Quality in Subtropical China

As a main agronomic intervention in tea cultivation, nitrogen (N) application is useful to improve tea yield and quality. However, the effects of N application on the formation of tea quality-related metabolites have not been fully studied, especially in long-term field trials. In this study, a 10-year field experiment was conducted to investigate the effect of long-term N application treatments on tea quality-related metabolites, their precursors, and related gene expression. Long-term N application up-regulated the expression of key genes for chlorophyll synthesis and promoted its synthesis, thus increasing tea yield. It also significantly increased the contents of total free amino acids, especially l-theanine, in fresh tea leaves, while decreasing the catechin content, which is conducive to enhancing tea liquor freshness. However, long-term N application significantly reduced the contents of benzyl alcohol and 2-phenylethanol in fresh tea leaves, and also reduced (E)-nerolidol and indole in withered leaves, which were not conducive to the formation of floral and fruity aroma compounds. In general, an appropriate amount of N fertilizer (225 kg/hm²) balanced tea yield and quality. These results not only provide essential information on how N application affects tea quality, but also provide detailed experimental data for field fertilization.

Repressed Gene Expression of Photosynthetic Antenna Proteins Associated with Yellow Leaf Variation as Revealed by Bulked Segregant RNA-seq in Tea Plant Camellia sinensis

The young leaves and shoots of albino tea cultivars are usually characterized as having a yellow or pale color, high amino acid, and low catechin. Increasing attention has been paid to albino tea cultivars in recent years because their tea generally shows high umami and reduced astringency. However, the genetic mechanism of yellow-leaf variation in albino tea cultivar has not been elucidated clearly. In this study, bulked segregant RNA-seq (BSR-seq) was performed on bulked yellow- and green-leaf hybrid progenies from a leaf color variation population. A total of 359 and 1134 differentially expressed genes (DEGs) were identified in the yellow and green hybrid bulked groups (Y_f vs G_f) and parent plants (Y_p vs G_p), respectively. The significantly smaller number of DEGs in Y_f versus G_f than in Y_p versus G_p indicated that individual differences could be reduced within the same hybrid progeny. Analysis of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes revealed that the photosynthetic antenna protein was most significantly enriched in either the bulked groups or their parents. Interaction was found among light-harvesting chlorophyll a/b -binding proteins (LHC), heat shock proteins (HSPs), and enzymes involved in cuticle formation. Combined with the transcriptomic expression profile, results showed that the repressed genes encoding LHC were closely linked to aberrant chloroplast development in yellow-leaf tea plants. Furthermore, the photoprotection and light stress response possessed by genes involved in HSP protein interaction and cuticle formation were discussed. The expression profile of DEGs was verified via quantitative real-time PCR analysis of the bulked samples and other F₁ individuals. In summary, using BSR-seq on a hybrid population eliminated certain disturbing effects of genetic background and individual discrepancy, thereby helping this study to intensively focus on the key genes controlling leaf color variation in yellow-leaf tea plants.