Metabolomic and transcriptomic analyses reveal the biosynthetic mechanisms of pigments and main taste compounds in an albino tea cultivar.

Significant variations in leaf colours, pigment contents, and main taste compounds in young shoots from albino tea plants (Camellia sinensis) influence tea flavour. However, the seasonal metabolic pattern and molecular regulatory mechanism of these metabolites remain largely elusive. Herein, we conducted morphological, biochemical, metabolomic and transcriptomic analyses between an albino tea cultivar 'Zhonghuang 3' ('ZH3') and a green strain 'Tai cha 15' ('TC15') at four-time points (April 12, May 31, July 14, and August 17) to elucidate dynamic changes in these compounds and predict the relationships among transcription factors (TFs), target genes (TGs), and metabolite abundance. Generally, leaf colours and pigment contents were significantly lighter and lower, respectively, in 'ZH3' than in 'TC15' from spring to summer, but were subsequently similar. Compared to 'TC15', 'ZH3' had a lower and broader phenol/ammonia ratio as well as stable caffeine content and showed more significantly different metabolites and differentially expressed genes. The relationship between pigments, main taste compounds, and their biosynthetic genes, as well as TFs and their TGs, had genetic specificity. These results suggested that the biosynthesis of these compounds was probably both season- and variety-dependent. In total, 12 models of the TF-TG-metabolite regulatory network were proposed to uncover the biosynthetic and regulatory mechanisms of these metabolites in tea plants. A high correlation was observed between some structural genes and TFs with the accumulation of these metabolites. These findings provide novel insights into the regulatory mechanisms underlying accumulation of pigments and main taste compounds in tea plants.

Small RNA and degradome profiling reveals important roles for microRNAs and their targets in tea plant response to drought stress.

Tea (Camellia sinensis) is a popular beverage worldwide. Drought stress (DS) is a major constraint on the growth, yield and quality of tea plants. MicroRNAs (miRNAs) play important roles in plant responses to DS. We constructed eight small RNA libraries from the drought-tolerant 'Ningzhou 2' (NZ2) and drought-susceptible 'Zhuyeqi' (ZYQ) cultivars during four stages [control (CK), the fourth day of DS, the eighth day of DS and after recovery (RC)]. A total of 268 conserved and 62 novel miRNAs were identified using small RNA sequencing. In total, 139 (52.9%) and 96 (36.0%) conserved miRNAs were differentially expressed during the four stages (P ≤ 0.05) in NZ2 and ZYQ, respectively. A total of 814 predicted target genes were identified as differentially regulated by 199 miRNAs through degradome sequencing. Among them, 201 and 218 genes were specific to the NZ2 and ZYQ cultivars, respectively, and 395 were common to both cultivars. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed the biological roles of these targets and showed that some of the targets responded to DS in a stress- and cultivar-dependent manner. Correlated expression patterns between miRNA and their targets showed that specific miRNAs target the miRNA effector Argonaute 1 (AGO1), drought signaling-related receptors and enzymes, transcription factors, and other structural and functional proteins. The predicted regulatory networks provide insights into a potential miRNA-mediated regulatory mechanism. These results will contribute to the breeding of drought-tolerant tea plants and to elucidating miRNA regulation in response to drought.

Transcriptomic Analysis of Tea Plant Responding to Drought Stress and Recovery.

Tea plant (Camellia sinensis) is an economically important beverage crop. Drought stress (DS) seriously limits the growth and development of tea plant, thus affecting crop yield and quality. To elucidate the molecular mechanisms of tea plant responding to DS, we performed transcriptomic analysis of tea plant during the three stages [control (CK) and during DS, and recovery (RC) after DS] using RNA sequencing (RNA-Seq). Totally 378.08 million high-quality trimmed reads were obtained and assembled into 59,674 unigenes, which were extensively annotated. There were 5,955 differentially expressed genes (DEGs) among the three stages. Among them, 3,948 and 1,673 DEGs were up-regulated under DS and RC, respectively. RNA-Seq data were further confirmed by qRT-PCR analysis. Genes involved in abscisic acid (ABA), ethylene, and jasmonic acid biosynthesis and signaling were generally up-regulated under DS and down-regulated during RC. Tea plant potentially used an exchange pathway for biosynthesis of indole-3-acetic acid (IAA) and salicylic acid under DS. IAA signaling was possibly decreased under DS but increased after RC. Genes encoding enzymes involved in cytokinin synthesis were up-regulated under DS, but down-regulated during RC. It seemed probable that cytokinin signaling was slightly enhanced under DS. In total, 762 and 950 protein kinases belonging to 26 families were differentially expressed during DS and RC, respectively. Overall, 547 and 604 transcription factor (TF) genes belonging to 58 families were induced in the DS vs. CK and RC vs. DS libraries, respectively. Most members of the 12 TF families were up-regulated under DS. Under DS, genes related to starch synthesis were down-regulated, while those related to starch decomposition were up-regulated. Mannitol, trehalose and sucrose synthesis-related genes were up-regulated under DS. Proline was probably mainly biosynthesized from glutamate under DS and RC. The mechanism by which ABA regulated stomatal movement under DS and RC was partly clarified. These results document the global and novel responses of tea plant during DS and RC. These data will serve as a valuable resource for drought-tolerance research and will be useful for breeding drought-resistant tea cultivars.

Identification and expression profiling of the auxin response factors (ARFs) in the tea plant (Camellia sinensis (L.) O. Kuntze) under various abiotic stresses.

Auxin response factor (ARF) proteins are a multigene family of regulators involved in various physiological and developmental processes in plants. However, their modes of action in the tea plant (Camellia sinensis) remain largely unknown. In this study, we identified 15 members of the tea ARF gene family, using the public information about C. sinensis, both in our laboratory, as well as in other laboratories, and analyzed their phylogenetic relationships, conserved domains and the compositions of the amino acids in the middle region. A comprehensive expression analysis in different tissues and organs revealed that many ARF genes were expressed in a tissue-specific manner, suggesting they have different functions in the growth and development processes of the tea plant. The expression analysis under three forms of auxin (indole-3-acetic acid, 2,4-dichlorophenoxyacetic acid, naphthylacetic acid) treatment showed that the majority of the ARF genes were down-regulated in the shoots and up-regulated in the roots, suggesting opposite action mechanisms of the ARF genes in the shoots and roots. The expression levels of most ARF genes were changed under various phytohormone and abiotic stresses, indicating the ARF gene family plays important roles in various phytohormone and abiotic stress signals and may mediate the crosstalk between phytohormones and abiotic stresses. The current study provides basic information for the ARF genes of the tea plant and will pave the way for deciphering the precise role of ARFs in tea developmental processes and breeding stress-tolerant tea varieties.