Tea GOLDEN2-LIKE genes enhance catechin biosynthesis through activating R2R3-MYB transcription factor

Association analysis of BSA-seq, BSR-seq, and RNA-seq reveals key genes involved in purple leaf formation in a tea population (Camellia sinensis)

Purple tea, rich in anthocyanins, has a variety of health benefits and is attracting global interest. However, the regulation mechanism of anthocyanin in purple tea populations has not been extensively studied. In this experiment, RNA-seq, BSA-seq, and BSR-seq were performed using 30 individuals with extreme colors (dark-purple and green) in an F 1 population of 'Zijuan' and 'Jinxuan'. The results show that 459 genes were differentially expressed in purple and green leaves, among which genes involved in the anthocyanin synthesis and transport pathway, such as CHS, F3H, ANS, MYB75, GST, MATE, and ABCC, were highly expressed in purple leaves. Moreover, there were multiple SNP/InDel variation sites on chromosomes 2 and 14 of the tea plant, as identified by BSA-seq. The integrated analysis identified two highly expressed genes (CsANS and CsMYB75) with SNP/InDel site variations in the purple tea plants. By silencing leaves, we proved that CsMYB75 could positively regulate anthocyanin accumulation and expression of related structural genes in tea plants. A 181-bp InDel in the CsMYB75 promoter was also found to be co-segregating with leaf color. The results of this study provide a theoretical reference for the molecular mechanism of anthocyanin accumulation in purple tea plants and contribute to the creation of new tea cultivars with high anthocyanin content.

Analysis of the Spatholobus suberectus full-length transcriptome identified an R2R3-MYB transcription factor-encoding gene SsMYB158 that regulates flavonoid biosynthesis

Spatholobus suberectus Dunn (Leguminosae) has been used for medicinal purposes for a long period. Flavonoids are the major bioactive components of S. suberectus. However, there is still limited knowledge of the exact method via which transcription factors (TFs) regulate flavonoid biosynthesis. The full-length transcriptome of S. suberectus was analyzed using SMRT sequencing; 61,548 transcripts were identified, including 12,311 new gene loci, 53,336 novel transcripts, 44,636 simple sequence repeats, 36,414 complete coding sequences, 871 long non-coding RNAs and 6781 TFs. The SsMYB158 TF, which is associated with flavonoid biosynthesis, belongs to the R2R3-MYB class and is localized subcellularly to the nucleus. The overexpression of SsMYB158 in Nicotiana benthamiana and the transient overexpression of SsMYB158 in S. suberectus resulted in a substantial enhancement in both flavonoids and catechin levels. In addition, there was a remarkable upregulation in the expression of essential enzyme-coding genes associated with the flavonoid biosynthesis pathways. Our study revealed SsMYB158 as a critical regulator of flavonoid biosynthesis in S. suberectus and laying the foundation for its molecular breeding.

The pleiotropic functions of GOLDEN2-LIKE transcription factors in plants

The regulation of gene expression is crucial for biological plant growth and development, with transcription factors (TFs) serving as key switches in this regulatory mechanism. GOLDEN2-LIKE (GLK) TFs are a class of functionally partially redundant nuclear TFs belonging to the GARP superfamily of MYB TFs that play a key role in regulating genes related to photosynthesis and chloroplast biogenesis. Here, we summarized the current knowledge of the pleiotropic roles of GLKs in plants. In addition to their primary functions of controlling chloroplast biogenesis and function maintenance, GLKs have been proven to regulate the photomorphogenesis of seedlings, metabolite synthesis, flowering time, leaf senescence, and response to biotic and abiotic stress, ultimately contributing to crop yield. This review will provide a comprehensive understanding of the biological functions of GLKs and serve as a reference for future theoretical and applied studies of GLKs.

Overexpression of Liriodendron Hybrid LhGLK1 in Arabidopsis Leads to Excessive Chlorophyll Synthesis and Improved Growth

Chloroplasts is the site for photosynthesis, which is the main primary source of energy for plants. Golden2-like (GLK) is a key transcription factor that regulates chloroplast development and chlorophyll synthesis. However, most studies on GLK genes are performed in crops and model plants with less attention to woody plants. In this study, we identified the LhGLK1 and LhGLK2 genes in the woody plant Liriodendron hybrid, and they are specifically expressed in green tissues. We showed that overexpression of the LhGLK1 gene improves rosette leaf chlorophyll content and induces ectopic chlorophyll biogenesis in primary root and petal vascular tissue in Arabidopsis. Although these exhibit a late-flowering phenotype, transgenic lines accumulate more biomass in vegetative growth with improved photochemical quenching (qP) and efficiency of photosystem II. Taken together, we verified a conserved and ancient mechanism for regulating chloroplast biogenesis in Liriodendron hybrid and evaluated its effect on photosynthesis and rosette biomass accumulation in the model plant Arabidopsis.

A flavonoid metabolon: cytochrome b5 enhances B-ring trihydroxylated flavan-3-ols synthesis in tea plants

Flavan-3-ols are prominent phenolic compounds found abundantly in the young leaves of tea plants. The enzymes involved in flavan-3-ol biosynthesis in tea plants have been extensively investigated. However, the localization and associations of these numerous functional enzymes within cells have been largely neglected. In this study, we aimed to investigate the synthesis of flavan-3-ols in tea plants, particularly focusing on epigallocatechin gallate. Our analysis involving the DESI-MSI method to reveal a distinct distribution pattern of B-ring trihydroxylated flavonoids, primarily concentrated in the outer layer of buds. Subcellular localization showed that CsC4H, CsF3'H, and CsF3'5'H localizes endoplasmic reticulum. Protein-protein interaction studies demonstrated direct associations between CsC4H, CsF3'H, and cytoplasmic enzymes (CHS, CHI, F3H, DFR, FLS, and ANR), highlighting their interactions within the biosynthetic pathway. Notably, CsF3'5'H, the enzyme for B-ring trihydroxylation, did not directly interact with other enzymes. We identified cytochrome b5 isoform C serving as an essential redox partner, ensuring the proper functioning of CsF3'5'H. Our findings suggest the existence of distinct modules governing the synthesis of different B-ring hydroxylation compounds. This study provides valuable insights into the mechanisms underlying flavonoid diversity and efficient synthesis and enhances our understanding of the substantial accumulation of B-ring trihydroxylated flavan-3-ols in tea plants.

CsPAT1, a GRAS transcription factor, promotes lignin accumulation by antagonistic interacting with CsWRKY13 in tea plants

Lignin is an important component of plant cell walls and plays crucial roles in the essential agronomic traits of tea quality and tenderness. However, the molecular mechanisms underlying the regulation of lignin biosynthesis in tea plants remain unclear. CsWRKY13 acts as a negative regulator of lignin biosynthesis in tea plants. In this study, we identified a GRAS transcription factor, phytochrome A signal transduction 1 (CsPAT1), that interacts with CsWRKY13. Silencing CsPAT1 expression in tea plants and heterologous overexpression in Arabidopsis demonstrated that CsPAT1 positively regulates lignin accumulation. Further investigation revealed that CsWRKY13 directly binds to the promoters of CsPAL and CsC4H and suppresses transcription of CsPAL and CsC4H. CsPAT1 indirectly affects the promoter activities of CsPAL and CsC4H by interacting with CsWRKY13, thereby facilitating lignin biosynthesis in tea plants. Compared with the expression of CsWRKY13 alone, the co-expression of CsPAT1 and CsWRKY13 in Oryza sativa significantly increased lignin biosynthesis. Conversely, compared with the expression of CsPAT1 alone, the co-expression of CsPAT1 and CsWRKY13 in O. sativa significantly reduced lignin accumulation. These results demonstrated the antagonistic regulation of the lignin biosynthesis pathway by CsPAT1 and CsWRKY13. These findings improve our understanding of lignin biosynthesis mechanisms in tea plants and provide insights into the role of the GRAS transcription factor family in lignin accumulation.

RsGLK2.1-RsNF-YA9a module positively regulates the chlorophyll biosynthesis by activating RsHEMA2 in green taproot of radish

Radish (Raphanus sativus L.) is an economically important and widely cultivated root vegetable crop. The coloration of the green skin and green flesh is an important trait influencing the nutrition and flavor quality in fruit radish. GOLDEN2-LIKEs (GLKs) play critically important roles in plastid development and chlorophyll biosynthesis in plants. However, the molecular mechanism underlying chlorophyll biosynthesis still remain elusive in green fruit radish taproot. Herein, the RsGLK2.1 gene exhibited higher expression level in taproot with a green skin (GS) and green flesh (GF) than that in taproot of the white or red radish genotypes. RsGLK2.1 is a nuclear transcription factor that has intrinsic transcriptional activation activity. Overexpression of RsGLK2.1 increased the total chlorophyll content of 20.68%-45.84% in radish leaves. Knockout of the RsGLK2.1 gene via CRISPR/Cas9 technology resulted in a significant decrease in the chlorophyll content. Overexpression of the RsGLK2.1 gene could restore the phenotype of the glk1glk2 mutant Arabidopsis. RsGLK2.1 was participated in regulating the chlorophyll biosynthesis by directly binding to the promoter of RsHEMA2 and activating its transcription. The interaction of RsNF-YA9a with RsGLK2.1 increased the transcriptional activity of the downstream gene RsHEMA2 under the light condition rather than the dark condition, indicating that both of them regulate the chlorophyll biosynthesis in a light-dependent manner of radish. Overall, these results provided insights into the molecular framework of the RsGLK2.1-RsNF-YA9a module, and could facilitate dissecting the regulatory mechanism underlying chlorophyll biosynthesis in green taproot of radish, and genetic improvement of quality traits in fruit radish breeding programs.

Comparative Transcriptomic Analysis Revealed the Suppression and Alternative Splicing of Kiwifruit (Actinidia latifolia) NAP1 Gene Mediating Trichome Development

Kiwifruit (Actinidia chinensis) is commonly covered by fruit hairs (trichomes) that affect kiwifruit popularity in the commercial market. However, it remains largely unknown which gene mediates trichome development in kiwifruit. In this study, we analyzed two kiwifruit species, A. eriantha (Ae) with long, straight, and bushy trichomes and A. latifolia (Al) with short, distorted, and spare trichomes, by second- and third-generation RNA sequencing. Transcriptomic analysis indicated that the expression of the NAP1 gene, a positive regulator of trichome development, was suppressed in Al compared with that in Ae. Additionally, the alternative splicing of AlNAP1 produced two short transcripts (AlNAP1-AS1 and AlNAP1-AS2) lacking multiple exons, in addition to a full-length transcript of AlNAP1-FL. The defects of trichome development (short and distorted trichome) in Arabidopsis nap1 mutant were rescued by AlNAP1-FL but not by AlNAP1-AS1. AlNAP1-FL gene does not affect trichome density in nap1 mutant. The qRT-PCR analysis indicated that the alternative splicing further reduces the level of functional transcripts. These results indicated that the short and distorted trichomes in Al might be caused by the suppression and alternative splicing of AlNAP1. Together, we revealed that AlNAP1 mediates trichome development and is a good candidate target for genetic modification of trichome length in kiwifruit.