UGT74AF3 enzymes specifically catalyze the glucosylation of 4-hydroxy-2,5-dimethylfuran-3(2H)-one, an important volatile compound in Camellia sinensis

CsEXL3 regulate mechanical harvest-related droopy leaves under the transcriptional activation of CsBES1.2 in tea plant

Due to a labor shortage, the mechanical harvesting of tea plantations has become a focal point. However, mechanical harvest efficiency was hampered by droopy leaves, leading to a high rate of broken tea shoots and leaves. Here, we dissected the genetic structure of leaf droopiness in tea plants using genome-wide association studies (GWAS) on 146 accessions, combined with transcriptome from two accessions with contrasting droopy leaf phenotypes. A set of 16 quantitative trait loci (QTLs) containing 54 SNPs and 34 corresponding candidate genes associated with droopiness were then identified. Among these, CsEXL3 (EXORDIUM-LIKE 3) from Chromosome 1 emerged as a candidate gene. Further investigations revealed that silencing CsEXL3 in tea plants resulted in weaker vascular cell malformation and brassinosteroid-induced leaf droopiness. Additionally, brassinosteroid signal factor CsBES1.2 was proved to participate in CsEXL3-induced droopiness and vascular cell malformation via using the CsBES1.2-silencing tea plant. Notably, CsBES1.2 bound on the E-box of CsEXL3 promoter to transcriptionally activate CsEXL3 expression as CUT&TAG based ChIP-qPCR and ChIP-seq suggested in vivo as well as EMSA and Y1H indicated in vitro. Furthermore, CsEXL3 instead of CsBES1.2 decreased lignin content and the expressing levels of lignin biosynthesis genes. Overall, our findings suggest that CsEXL3 regulates droopy leaves, partially through the transcriptional activation of CsBES1.2, with the potential to improve mechanical harvest efficiency in tea plantations.

Depicting the genetic and metabolic panorama of chemical diversity in the tea plant

As a frequently consumed beverage worldwide, tea is rich in naturally important bioactive metabolites. Combining genetic, metabolomic and biochemical methodologies, here, we present a comprehensive study to dissect the chemical diversity in tea plant. A total of 2837 metabolites were identified at high-resolution with 1098 of them being structurally annotated and 63 of them were structurally identified. Metabolite-based genome-wide association mapping identified 6199 and 7823 metabolic quantitative trait loci (mQTL) for 971 and 1254 compounds in young leaves (YL) and the third leaves (TL), respectively. The major mQTL (i.e., P < 1.05 × 10-5, and phenotypic variation explained (PVE) > 25%) were further interrogated. Through extensive annotation of the tea metabolome as well as network-based analysis, this study broadens the understanding of tea metabolism and lays a solid foundation for revealing the natural variations in the chemical composition of the tea plant. Interestingly, we found that galloylations, rather than hydroxylations or glycosylations, were the largest class of conversions within the tea metabolome. The prevalence of galloylations in tea is unusual, as hydroxylations and glycosylations are typically the most prominent conversions of plant specialized metabolism. The biosynthetic pathway of flavonoids, which are one of the most featured metabolites in tea plant, was further refined with the identified metabolites. And we demonstrated the further mining and interpretation of our GWAS results by verifying two identified mQTL (including functional candidate genes CsUGTa, CsUGTb, and CsCCoAOMT) and completing the flavonoid biosynthetic pathway of the tea plant.

2,4-Dihydroxybenzoic Acid, a Novel SA Derivative, Controls Plant Immunity via UGT95B17-Mediated Glucosylation: A Case Study in Camellia Sinensis

The plant hormone salicylic acid (SA) plays critical roles in plant innate immunity. Several SA derivatives and associated modification are identified, whereas the range and modes of action of SA-related metabolites remain elusive. Here, the study discovered 2,4-dihydroxybenzoic acid (2,4-DHBA) and its glycosylated form as native SA derivatives in plants whose accumulation is largely induced by SA application and Ps. camelliae-sinensis (Pcs) infection. CsSH1, a 4/5-hydroxylase, catalyzes the hydroxylation of SA to 2,4-DHBA, and UDP-glucosyltransferase UGT95B17 catalyzes the formation of 2,4-DHBA glucoside. Down-regulation reduced the accumulation of 2,4-DHBA glucosides and enhanced the sensitivity of tea plants to Pcs. Conversely, overexpression of UGT95B17 increased plant disease resistance. The exogenous application of 2,4-DHBA and 2,5-DHBA, as well as the accumulation of DHBA and plant resistance comparison, indicate that 2,4-DHBA functions as a potentially bioactive molecule and is stored mainly as a glucose conjugate in tea plants, differs from the mechanism described in Arabidopsis. When 2,4-DHBA is applied exogenously, UGT95B17-silenced tea plants accumulated more 2,4-DHBA than SA and showed induced resistance to Pcs infection. These results indicate that 2,4-DHBA glucosylation positively regulates disease resistance and highlight the role of 2,4-DHBA as potentially bioactive molecule in the establishment of basal resistance in tea plants.

A geraniol synthase regulates plant defense via alternative splicing in tea plants

Geraniol is an important contributor to the pleasant floral scent of tea products and one of the most abundant aroma compounds in tea plants; however, its biosynthesis and physiological function in response to stress in tea plants remain unclear. The proteins encoded by the full-length terpene synthase (CsTPS1) and its alternative splicing isoform (CsTPS1-AS) could catalyze the formation of geraniol when GPP was used as a substrate in vitro, whereas the expression of CsTPS1-AS was only significantly induced by Colletotrichum gloeosporioides and Neopestalotiopsis sp. infection. Silencing of CsTPS1 and CsTPS1-AS resulted in a significant decrease of geraniol content in tea plants. The geraniol content and disease resistance of tea plants were compared when CsTPS1 and CsTPS1-AS were silenced. Down-regulation of the expression of CsTPS1-AS reduced the accumulation of geraniol, and the silenced tea plants exhibited greater susceptibility to pathogen infection than control plants. However, there was no significant difference observed in the geraniol content and pathogen resistance between CsTPS1-silenced plants and control plants in the tea plants infected with two pathogens. Further analysis showed that silencing of CsTPS1-AS led to a decrease in the expression of the defense-related genes PR1 and PR2 and SA pathway-related genes in tea plants, which increased the susceptibility of tea plants to pathogens infections. Both in vitro and in vivo results indicated that CsTPS1 is involved in the regulation of geraniol formation and plant defense via alternative splicing in tea plants. The results of this study provide new insights into geraniol biosynthesis and highlight the role of monoterpene synthases in modulating plant disease resistance via alternative splicing.

Integrated physiological, metabolite and proteomic analysis reveal the glyphosate stress response mechanism in tea plant (Camellia sinensis)

Glyphosate residues can tremendously impact the physiological mechanisms of tea plants, thus threatening tea security and human health. Herein, integrated physiological, metabolite, and proteomic analyses were performed to reveal the glyphosate stress response mechanism in tea plant. After exposure to glyphosate (≥1.25 kg ae/ha), the leaf ultrastructure was damaged, and chlorophyll content and relative fluorescence intensity decreased significantly. The characteristic metabolites catechins and theanine decreased significantly, and the 18 volatile compounds content varied significantly under glyphosate treatments. Subsequently, tandem mass tags (TMT)-based quantitative proteomics was employed to identify the differentially expressed proteins (DEPs) and to validate their biological functions at the proteome level. A total of 6287 proteins were identified and 326 DEPs were screened. These DEPs were mainly catalytic, binding, transporter and antioxidant active proteins, involved in photosynthesis and chlorophyll biosynthesis, phenylpropanoid and flavonoid biosynthesis, sugar and energy metabolism, amino acid metabolism, and stress/defense/detoxification pathway, etc. A total of 22 DEPs were validated by parallel reaction monitoring (PRM), demonstrating that the protein abundances were consistent between TMT and PRM data. These findings contribute to our understanding of the damage of glyphosate to tea leaves and molecular mechanism underlying the response of tea plants to glyphosate.

Genome-wide identification of UDP-glycosyltransferases in the tea plant (Camellia sinensis) and their biochemical and physiological functions

Tea (Camellia sinensis) has been an immensely important commercially grown crop for decades. This is due to the presence of essential nutrients and plant secondary metabolites that exhibit beneficial health effects. UDP-glycosyltransferases (UGTs) play an important role in the diversity of such secondary metabolites by catalysing the transfer of an activated sugar donor to acceptor molecules, and thereby creating a huge variety of glycoconjugates. Only in recent years, thanks to the sequencing of the tea plant genome, have there been increased efforts to characterise the UGTs in C. sinensis to gain an understanding of their physiological role and biotechnological potential. Based on the conserved plant secondary product glycosyltransferase (PSPG) motif and the catalytically active histidine in the active site, UGTs of family 1 in C. sinensis are identified here, and shown to cluster into 21 groups in a phylogenetic tree. Building on this, our current understanding of recently characterised C. sinensis UGTs (CsUGTs) is highlighted and a discussion on future perspectives made.

Single-cell transcriptome atlas reveals developmental trajectories and a novel metabolic pathway of catechin esters in tea leaves

The tea plant is an economically important woody beverage crop. The unique taste of tea is evoked by certain metabolites, especially catechin esters, whereas their precise formation mechanism in different cell types remains unclear. Here, a fast protoplast isolation method was established and the transcriptional profiles of 16 977 single cells from 1st and 3rd leaves were investigated. We first identified 79 marker genes based on six isolated tissues and constructed a transcriptome atlas, mapped developmental trajectories and further delineated the distribution of different cell types during leaf differentiation and genes associated with cell fate transformation. Interestingly, eight differently expressed genes were found to co-exist at four branch points. Genes involved in the biosynthesis of certain metabolites showed cell- and development-specific characteristics. An unexpected catechin ester glycosyltransferase was characterized for the first time in plants by a gene co-expression network in mesophyll cells. Thus, the first single-cell transcriptional landscape in woody crop leave was reported and a novel metabolism pathway of catechin esters in plants was discovered.

Feasible strategies for studying the involvement of DNA methylation and histone acetylation in the stress-induced formation of quality-related metabolites in tea (Camellia sinensis)

Tea plants are subjected to multiple stresses during growth, development, and postharvest processing, which affects levels of secondary metabolites in leaves and influences tea functional properties and quality. Most studies on secondary metabolism in tea have focused on gene, protein, and metabolite levels, whereas upstream regulatory mechanisms remain unclear. In this review, we exemplify DNA methylation and histone acetylation, summarize the important regulatory effects that epigenetic modifications have on plant secondary metabolism, and discuss feasible research strategies to elucidate the underlying specific epigenetic mechanisms of secondary metabolism regulation in tea. This information will help researchers investigate the epigenetic regulation of secondary metabolism in tea, providing key epigenetic data that can be used for future tea genetic breeding.

Amplification of early drought responses caused by volatile cues emitted from neighboring plants

Plants have developed sophisticated mechanisms to survive in dynamic environments. Plants can communicate via volatile organic compounds (VOCs) to warn neighboring plants of threats. In most cases, VOCs act as positive regulators of plant defense. However, the communication and role of volatiles in response to drought stress are poorly understood. Here, we showed that tea plants release numerous VOCs. Among them, methyl salicylate (MeSA), benzyl alcohol, and phenethyl alcohol markedly increased under drought stress. Interestingly, further experiments revealed that drought-induced MeSA lowered the abscisic acid (ABA) content in neighboring plants by reducing 9-cis-epoxycarotenoid dioxygenase (NCED) gene expression, resulting in inhibition of stomatal closure and ultimately decreasing early drought tolerance in neighboring plants. Exogenous application of ABA reduced the wilting of tea plants caused by MeSA exposure. Exposure of Nicotiana benthamiana to MeSA also led to severe wilting, indicating that the ability of drought-induced MeSA to reduce early drought tolerance in neighboring plants may be conserved in other plant species. Taken together, these results provide evidence that drought-induced volatiles can reduce early drought tolerance in neighboring plants and lay a novel theoretical foundation for optimizing plant density and spacing.

Induced mutations in SlE8 and SlACO1 control tomato fruit maturation and shelf-life

Fruit maturation and softening are critical traits that control fruit shelf-life. In the climacteric tomato (Solanum lycopersicum L.) fruit, ethylene plays a key role in fruit ripening and softening. We characterized two related proteins with contrasting impact on ethylene production, ACC oxidase 1 (SlACO1) and SlE8. We found SlACO1 and SlE8 to be highly expressed during fruit ripening. To identify loss-of-function alleles, we analysed the tomato genetic diversity but we did not find any natural mutations impairing the function of these proteins. We also found the two loci evolving under purifying selection. To engineer hypomorphic alleles, we used TILLING (target-induced local lesions in genomes) to screen a tomato ethylmethane sulfonate-mutagenized population. We found 13 mutants that we phenotyped for ethylene production, shelf-life, firmness, conductivity, and soluble solid content in tomato fruits. The data demonstrated that slaco1-1 and slaco1-2 alleles could be used to improve fruit shelf-life, and that sle8-1 and sle8-2 alleles could be used to accelerate ripening. This study highlights further the importance of SlACO1 and SlE8 in ethylene production in tomato fruit and how they might be used for post-harvest fruit preservation or speeding up fruit maturation.

Herbivore-induced DMNT catalyzed by CYP82D47 plays an important role in the induction of JA-dependent herbivore resistance of neighboring tea plants

Herbivore-induced plant volatiles play important ecological roles in defense against stresses. However, if and which volatile(s) are involved in the plant-plant communication in response to herbivorous insects in tea plants remains unknown. Here, plant-plant communication experiments confirm that volatiles emitted from insects-attacked tea plants can trigger plant resistance and reduce the risk of herbivore damage by inducing jasmonic acid (JA) accumulation in neighboring plants. The emission of six compounds was significantly induced by geometrid Ectropis obliqua, one of the most common pests of the tea plant in China. Among them, (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) could induce the accumulation of JA and thus promotes the resistance of neighboring intact plants to herbivorous insects. CsCYP82D47 was identified for the first time as a P450 enzyme, which catalyzes the final step in the biosynthesis of DMNT from (E)-nerolidol. Down-regulation of CsCYP82D47 in tea plants resulted in a reduced accumulation of DMNT and significantly reduced the release of DMNT in response to the feeding of herbivorous insects. The first evidence for plant-plant communication in response to herbivores in tea plants will help to understand how plants respond to volatile cues in response to herbivores and provide new insight into the role(s) of DMNT in tea plants.

UGT85A53 promotes flowering via mediating abscisic acid glucosylation and FLC transcription in Camellia sinensis

Uridine diphosphate (UDP)-dependent glycosyltransferases catalyse the glycosylation of small molecules and play important roles in maintaining cell homeostasis and regulating plant development. Glycosyltransferases are widely distributed, but their detailed roles in regulating plant growth and development are largely unknown. In this study, we identified a UDP-glycosyltransferase, UGT85A53, from Camellia sinensis, the expression of which was strongly induced by various abiotic stress factors and its protein product was distributed in both the cytoplasm and nucleus. Ectopic overexpression of CsUGT85A53 in Arabidopsis resulted in an early-flowering phenotype under both long- and short-day conditions. The transcript accumulation of the flowering repressor genes FLC and ABI5, an activator of FLC in ABA-regulated flowering signaling, were both significantly decreased in transgenic Arabidopsis compared with wild-type plants. The decreased expression level of FLC might be associated with an increased level of DNA methylation that was observed in CsUGT85A53-overexpressing (OE) plants. Biochemical analyses showed that CsUGT85A53 could glucosylate ABA to form inactive ABA-glycoside in vitro and in planta. Overexpression of CsUGT85A53 in Arabidopsis resulted in a decreased concentration of free ABA and increased concentration of ABA-glucoside. The early-flowering phenotype in the CsUGT85A53-OE transgenic lines was restored by ABA application. Furthermore, CsUGT85A53-OE plants displayed an ABA-insensitive phenotype with higher germination rates compared with controls in the presence of low concentrations of exogenous ABA. Our findings are the first to identify a UGT in tea plants that catalyses ABA glucosylation and enhance flowering transition as a positive regulator.