Dendrobium nestor is a famous orchid species in the Orchidaceae family. There is a diversity of flower colorations in the Dendrobium species, but knowledge of the genes involved and molecular mechanism underlying the flower color formation in D. nestor is less studied. Therefore, we performed transcriptome profiling using Illumina sequencing to facilitate thorough studies of the purple color formation in petal samples collected at three developmental stages, namely—flower bud stage (F), half bloom stage (H), and full bloom stage (B) in D. nestor. In addition, we identified key genes and their biosynthetic pathways as well as the transcription factors (TFs) associated with purple flower color formation. We found that the phenylpropanoid–flavonoid–anthocyanin biosynthesis genes such as phenylalanine ammonia lyase, chalcone synthase, anthocyanidin synthase, and UDP-flavonoid glucosyl transferase, were largely up-regulated in the H and B samples as compared to the F samples. This upregulation might partly account for the accumulation of anthocyanins, which confer the purple coloration in these samples. We further identified several differentially expressed genes related to phytohormones such as auxin, ethylene, cytokinins, salicylic acid, brassinosteroid, and abscisic acid, as well as TFs such as MYB and bHLH, which might play important roles in color formation in D. nestor flower. Sturdy upregulation of anthocyanin biosynthetic structural genes might be a potential regulatory mechanism in purple color formation in D. nestor flowers. Several TFs were predicted to regulate the anthocyanin genes through a K-mean clustering analysis. Our study provides valuable resource for future studies to expand our understanding of flower color development mechanisms in D. nestor.

Most orchids have high ornamental value with long-lived flower. However, the mechanisms by which orchids maintain floral longevity are poorly understood. Here, we hypothesized that long floral longevity in Dendrobium is maintained by high resource investment and complementary water and nutrient utilization in different structural units of the perianth. To test this hypothesis, we determined which water- and nutrient-related traits are correlated with flower longevity in twenty-three Dendrobium species or cultivars and examined the variations of the related traits during flower development of one long-lived cultivar. We found that floral longevity was correlated with dry mas per unit area of perianths and total flower biomass, which indicates that maintaining long floral longevity requires increased resource investment. During development of long-lived flowers, labella showed high capacity for water storage and nutrient reutilization, which can partly remedy high water demand and biomass investment. Sepals and petals, in contrast, had stronger desiccation avoidance and higher metabolic activity with lower biomass investment. These findings indicate that Dendrobium flowers maintain longevity by complementary water and utilization strategies in the sepals, petals and labella, with labella consuming more water and nutrients to extend flower display, while sepals and petals use a more conservative strategy.

Background Paphiopedilum hirsutissimum is a member of Orchidaceae family that is famous for its ornamental value around the globe, it is vulnerable due to over-exploitation and was listed in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora, which prevents its trade across borders. Variation in flower color that gives rise to different flower patterns is a major trait contributing to its high ornamental value. However, the molecular mechanism underlying color formation in P. hirsutissimum still remains unexplored. In the present study, we exploited natural variation in petal and labellum color of Paphiopedilum plants and used comparative transcriptome analysis as well as pigment measurements to explore the important genes, metabolites and regulatory pathways linked to flower color variation in P. hirsutissimum. Result We observed that reduced anthocyanin and flavonoid contents along with slightly higher carotenoids are responsible for albino flower phenotype. Comparative transcriptome analysis identified 3287 differentially expressed genes (DEGs) among normal and albino labellum, and 3634 DEGs between normal and albino petals. Two genes encoding for flavanone 3-hydroxylase (F3H) and one gene encoding for chalcone synthase (CHS) were strongly downregulated in albino labellum and petals compared to normal flowers. As both F3H and CHS catalyze essentially important steps in anthocyanin biosynthesis pathway, downregulation of these genes is probably leading to albino flower phenotype via down-accumulation of anthocyanins. However, we observed the downregulation of major carotenoid biosynthesis genes including VDE, NCED and ABA2 which was inconsistent with the increased carotenoid accumulation in albino flowers, suggesting that carotenoid accumulation was probably controlled at post-transcriptional or translational level. In addition, we identified several key transcription factors (MYB73, MYB61, bHLH14, bHLH106, MADS-SOC1, AP2/ERF1, ERF26 and ERF87) that may regulate structural genes involved in flower color formation in P. hirsutissimum. Importantly, over-expression of some of these candidate TFs increased anthocyanin accumulation in tobacco leaves which provided important evidence for the role of these TFs in flower color formation probably via regulating key structural genes of the anthocyanin pathway. Conclusion The genes identified here could be potential targets for breeding P. hirsutissimum with different flower color patterns by manipulating the anthocyanin and carotenoid biosynthesis pathways.

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In order to make reasonable use of collected and preserved Dendrobium germplasm resources, ISSR molecular marker method was used to analyze the genetic relationship and genetic diversity of 22 Dendrobium species. A total of 6 primers with clear amplification bands, high polymorphism and good repeatability were chosen from 100 ISSR primers. The selected primers were used to amplify the genomic DNA of 22 Dendrobium species by PCR, and a total of 241 bands were generated, of which 241 were polymorphic bands. Polymorphic ratio was 100%. By GenAlEx 6.5 software, average value of observed allele number, effective number of alleles, Nei's gene diversity and Shannon's information index was 1.983, 1.167, 0.133 and 0.247, respectively, indicating that a high level of genetic diversity among 22 Dendrobium species. The genetic similarity coefficient among 22 Dendrobium species ranged from 0.698 4 to 0.878 7 by NTSYS-pc 2.1 software. UPGMA clustering based on genetic similarity coefficient, 22 Dendrobium species could be divided into 10 groups when genetic similarity was 0.795. UPGMA clustering results are consistent with traditional morphological classification results. The DNA fingerprint map constructed with 3 pairs of primers can separately identify 22 Dendrobium species. This research laid a theoretical foundation for the identification of Dendrobium germplasm resources and the selection of parents for cross breeding.