The complete chloroplast genome sequence of an Endangered orchid species Dendrobium bellatulum (Orchidaceae)

In-depth analysis of genomes and functional genomics of orchid using cutting-edge high-throughput sequencing

High-throughput sequencing technology has been facilitated the development of new methodologies and approaches for studying the origin and evolution of plant genomes and subgenomes, population domestication, and functional genomics. Orchids have tens of thousands of members in nature. Many of them have promising application potential in the extension and conservation of the ecological chain, the horticultural use of ornamental blossoms, and the utilization of botanical medicines. However, a large-scale gene knockout mutant library and a sophisticated genetic transformation system are still lacking in the improvement of orchid germplasm resources. New gene editing tools, such as the favored CRISPR-Cas9 or some base editors, have not yet been widely applied in orchids. In addition to a large variety of orchid cultivars, the high-precision, high-throughput genome sequencing technology is also required for the mining of trait-related functional genes. Nowadays, the focus of orchid genomics research has been directed to the origin and classification of species, genome evolution and deletion, gene duplication and chromosomal polyploidy, and flower morphogenesis-related regulation. Here, the progressing achieved in orchid molecular biology and genomics over the past few decades have been discussed, including the evolution of genome size and polyploidization. The frequent incorporation of LTR retrotransposons play important role in the expansion and structural variation of the orchid genome. The large-scale gene duplication event of the nuclear genome generated plenty of recently tandem duplicated genes, which drove the evolution and functional divergency of new genes. The evolution and loss of the plastid genome, which mostly affected genes related to photosynthesis and autotrophy, demonstrated that orchids have experienced more separate transitions to heterotrophy than any other terrestrial plant. Moreover, large-scale resequencing provide useful SNP markers for constructing genetic maps, which will facilitate the breeding of novel orchid varieties. The significance of high-throughput sequencing and gene editing technologies in the identification and molecular breeding of the trait-related genes in orchids provides us with a representative trait-improving gene as well as some mechanisms worthy of further investigation. In addition, gene editing has promise for the improvement of orchid genetic transformation and the investigation of gene function. This knowledge may provide a scientific reference and theoretical basis for orchid genome studies.

The complete chloroplast genome sequence of Dendrobium thyrsiflorum (Orchidaceae) and its phylogenetic analysis

Dendrobium thyrsiflorum Rchb.f., a native species to China, is widely used as an important garden flower and a traditional Chinese medicine. Herein, the complete chloroplast (cp) genome of D. thyrsiflorum was deciphered by high-throughput sequencing. The cp genome exhibited a typical quadripartite cycle of 151,686 bp in length, comprising of a pair of inverted repeats (IRa and IRb) of 26,293 bp which were intersected by a large single copy (LSC) region of 84,749 bp and a small single copy (SSC) region of 14,351 bp. A total of 126 genes were de novo assembled in this cp genome, including 78 protein genes, 40 tRNA genes and 8 rRNA genes. Among these genes, 86 genes (22 tRNAs and 64 coding genes) were single copy, the rest were two-copy genes, and the average of GC content of the whole genome is 37.55%. Phylogenetic trees showed that the D. thyrsiflorum was closely related to D. devonianum. This study provides molecular information for future evolution, genetic and molecular biology studies of Dendrobium.