Tung tree (Vernicia fordii) is an economically important woody oil plant that produces tung oil rich in eleostearic acid. Here, we report a high-quality chromosome-scale genome sequence of tung tree. The genome sequence was assembled by combining Illumina short reads, Pacific Biosciences single-molecule real-time long reads, and Hi-C sequencing data. The size of tung tree genome is 1.12 Gb, with 28,422 predicted genes and over 73% repeat sequences. The V. fordii underwent an ancient genome triplication event shared by core eudicots but no further whole-genome duplication in the subsequent ca. 34.55 million years of evolutionary history of the tung tree lineage. Insertion time analysis revealed that repeat-driven genome expansion might have arisen as a result of long-standing long terminal repeat retrotransposon bursts and lack of efficient DNA deletion mechanisms. The genome harbors 88 resistance genes encoding nucleotide-binding sites; 17 of these genes may be involved in early-infection stage of Fusarium wilt resistance. Further, 651 oil-related genes were identified, 88 of which are predicted to be directly involved in tung oil biosynthesis. Relatively few phosphoenolpyruvate carboxykinase genes, and synergistic effects between transcription factors and oil biosynthesis-related genes might contribute to the high oil content of tung seed. The tung tree genome constitutes a valuable resource for understanding genome evolution, as well as for molecular breeding and genetic improvements for oil production.

Background: Camellia oleifera is an important oil-yielding woody plant native to China. Tea oil extracted from the seeds is rich in health-beneficial compounds. Huajin is a high-yielding elite variety of C. oleifera, with large fruits and remarkable resilience, widely cultivated in southern China; however, its seedling quality tends to be uneven. At present, techniques such as grafting, and cuttings are primarily adopted to propagate C. oleifera. These approaches are susceptible to environmental constraints owing to the long growth period, resulting in the lack of C. oleifera seedlings. Methods to make the cultivation more economical are warranted; this can be facilitated by tissue culture technology to provide good-quality seedlings in a short time. Methods: In vitro cultured plantlets of C. oleifera Huajin were exposed to red light (RL), blue light (BL), red:blue light at a 4:1 ratio (R4:B1), and red:blue light at a 1:4 ratio (R1:B4); white light (WL) was used as the control treatment. To investigate the influence of light spectral quality on the proliferation coefficient, photosynthetic pigments, soluble proteins, plant height, leaf shape, Rubisco enzyme activity, and stomata and leaf anatomical features. Results: The highest proliferation coefficient was observed under combined red and blue (4:1) light. In addition, this treatment resulted in the second highest chlorophyll content, the thickest palisade and spongy tissues, and consequently, the thickest leaves. The same treatment resulted in the second highest stomatal density, albeit concomitantly with the smallest average stomatal length and width. Discussion: These results indicate that high-quality propagation of Huajin shoots can be achieved by culturing the plants in vitro under a combination of red and blue (4:1) lights. Previous studies have shown that red and blue lights improve rooting and transplanting rates of tissue culture seedlings. Hence, future research should focus on the effect of light quality on rooting and transplanting of tissue culture plantlets of Huajin and its specific molecular mechanisms.

Camellia-oil trees are economically valuable, oil-rich species within the genus Camellia, family Theaceae. Among these species, C. oleifera, a member of Section Oleifera in the genus, is the most extensively cultivated in China. In this study, we assembled the mitochondrial genomes (mitogenomes) of two Camellia species, namely C. oleifera and C. lanceoleosa. These two species are closely related and belong to the same genus and section, with C. oleifera being hexaploid and C. lanceoleosa being diploid. The mitogenome of C. oleifera is comprised of 1,039,838 base pairs (bp), and C. lanceoleosa is comprised of 934,155 bp. Both genomes exhibit a multipartite genome structure, which is supported by our PCR experiments. We conducted codon usage and RNA editing site analysis on these two mitogenomes, which showed highly consistent results. However, analysis of repetitive sequences and mitochondrial plastid sequences (MTPTs) revealed differences between the two mitogenomes. Phylogenetic analysis indicated that these two species clustered together, suggesting a close evolutionary relationship. The collinearity analysis results showed extensive genome rearrangements in the mitogenomes of Camellia species. We successfully assembled the mitogenomes of C. oleifera and C. lanceoleosa, marking a significant advancement in understanding camellia-oil tree mitogenomes. Unlike circular mitogenomes reported before, our research confirms multiple-branched configurations in these two species. This sheds light on mitogenome structural complexities and contributes to our understanding of evolutionary processes. Additionally, these results enrich Camellia genetic resources and expand our knowledge of mitogenome variation.

Squalene synthase (SQS) is the most direct key enzyme regulating squalene synthesis. To better understand the regulatory mechanisms of squalene biosynthesis, a 1423-bp long promoter region of the CoSQS gene was isolated from Camellia oleifera. Plant CARE and PLACE analysis affirmed the existence of the core promoter elements such as TATA and CAAT boxes and transcription factor binding sites like W-box and MYB in the isolated sequence. Exogenous factors regulating the CoSQS promoter were obtained by using Yeast one-hybrid screening, and the key transcription factor CoWRKY15 was found. AOS (Antibody Optimization System) analysis showed that CoWRKY15 had the highest interactions with a confidence level of 0.9026. Bioinformatics analysis showed that CoWRKY15 belonged to class 2 of the WRKY gene family. The results of subcellular localization showed that CoWRKY15 functioned in the nucleus. The results of CoWRKY15 promoter analysis showed that 8 out of 14 cis-elements with annotatable functions were related to the light response. The region of the CoSQS promoter that interacts with CoWRKY15 is −186 bp~−536 bp. The histochemical assay and squalene content suggested that the CoSQS promoter could drive the expression of GUS gene and specific promotion of CoSQS expression. It was found that CoWRKY15 could act on the −186 bp~−536 bp CoSQS promoter to regulate the expression of CoSQS and the content of squalene in C. oleifera seed kernels.

Tea oil obtained fromCamellia oleifera is a high-quality vegetable oil rich in squalene, a triterpenoid bioactive substance with several demonstrated health benefits. WRKY transcription factors in C. oleifera play essential roles in a wide range of physiological processes. To further develop C. oleifera as a functional material with genetic improvement, we constructed a tea oil seed transcriptome database and screened out the C.oleifera WRKY genes (CoWRKY) to determine their relation with squalene anabolism. Forty-one CoWRKY transcription factors were obtained, including five showing a potential association with squalene metabolism and regulating the expression of the squalene synthase gene (CoSQS). Through bioinformatics analysis of the CoSQS promoter, it was predicted that W-box components that may bind to CoWRKY were present at the – 293 bp position. Real-time polymerase chain reaction showed a significant positive correlation between the expression levels of the CoWRKY1 and CoSQS genes, further indicating their interaction, which was confirmed via a yeast one-hybrid assay. Overall, these results suggest that the CoWRKY1 transcription factor may bind to the W-box cis-acting element of the CoSQS promoter to positively regulate the synthesis of squalene in C. oleifera, offering new strategies for engineering strains with production of oil highly enriched in squalene.