With the development of Next Generation Sequencing (NGS), we could get more biological information for the species identification, molecular genetic markers and evolution within and between different species (W. et al., 1995; Grassi et al., 2002; Leonie et al., 2011; Straub et al., 2011). Therefore, the complete chloroplast genome also could furnish abundant genetic information and molecular markers that are valuable tools to solve obscure phylogenetic relationships among land plants (Luo et al., 2014; Alzahrani et al., 2021).
Here, using the Illumina sequencing platform and GENEIOUS program, we firstly sequenced and assembled the complete chloroplast genome of D. huoshanense. Reported Dendrobium complete chloroplast genomes range in size from 156,612 to 156,781 bp (Wang et al., 2017; Yan-xia et al., 2018; Zhang et al., 2018), and the size of assembled chloroplast genome of D.huoshanense is 153,188 bp, which is consistent with those reported previously in plants of the same species. The average GC content of the D. huoshanense chloroplast genome is 40.0%, similar to other Dendrobium species (Gao et al.; Wang et al., 2017; Zhang et al., 2018). It suggested that this method which was used for assembling the plant complete chloroplast genome is feasible. Furthermore this method is very more convenient and efficient than other methods.
In this newly determined chloroplast genome, 129 predicted genes were found. There are 42 genes are involved in photosynthesis, of which 6 encode different subunits of ATP synthase, 9 for the subunits of the NADH-oxidoreductase, 6 for the cytochrome b6/f complex, 5 for photosystem I, 15 for photosystem II, and 1 for the Rubisco. In addition, there are 38 tRNA genes, 36 ribosomal subunit genes (15 small subunits and 11 large subunits), 8 rRNA genes and 4 RNA polymerase genes. Furthermore, 5 genes encode miscellaneous proteins which were involved in different functions, and 6 genes were of unknown function (Table 5). Those results also showed that the genome organization appeared to be more conserved with unique gene sequence as discovered previously in Dendrobium species (Wang et al., 2017; Yan-xia et al., 2018; Zhang et al., 2018).
Table 5
Genes present in the D. huoshanense chloroplast genome.
Group | Name of genes | Numbers |
Photosystem I | psaA, psaB, psaC, psaI, psaJ | 5 |
Photosystem II | psbA, psbB, psbC, psbD, psbE, psbF, psbH, psbI, psbJ, psbK, psbL, psbM, psbN, psbT, psbZ | 15 |
Cytochrome b6/f | petA, petB, petD, petG, petL, petN | 6 |
ATP synthase | atpA, atpB, atpE, atpF, atpH, atpI | 6 |
NADH dehydrogenase | ndhA, ndhB (×2), ndhD, ndhE, ndhF, ndhG, ndhH, ndhJ | 9 |
Rubisco | rbcL | 1 |
RNA polymerase | rpoA, rpoB, rpoC1, rpoC2 | 4 |
Small subunits of ribosome | rps2, rps3, rps4, rps7(×2), rps8, rps11, rps12(×2), rps14, rps15, rps16, rps18,rps19(×2) | 15 |
Large subunits of ribosome | rpl2(×2), rpl14, rpl16, rpl20, rpl22, rpl23(×2), rpl32, rpl33, rpl36 | 11 |
Other genes | accD, ccsA, cemA, clpP, infA, matK | 6 |
Miscellaneous proteins coding gene | ycf1, ycf2(×2), ycf3, ycf4 | 5 |
tRNAs | trnA-UGC(x2), trnC-GCA, trnD-GUC, trnE-UUC, trnF-GAA, trnfM-CAU, trnG-GCC, trnG-UCC, trnH-GUG(x2), trnI-CAU(x2), trnI-GAU(x2), trnK-UUU, trnL-CAA(x2), trnL-UAA, trnL-UAG, trnM-CAU, trnN-GUU(x2), trnP-UGG trnQ-UUG, trnR-ACG(x2), trnR-UCU, trnS-GCU, trnS-GGA, trnS-UGA, trnT-GGU, trnT-UGU, trnV-GAC, trnV-GAC, trnV-UAC, trnW-CCA, trnY-GUA | 38 |
rRNAs | rrn4.5(×2), rrn5(×2), rrn16(×2), rrn23(×2) | 8 |
Additionally, we also found that there are 23,448 codons encoding the genes in the D. huoshanense chloroplast genome, and the most universal codons are the coding for the amino acids Leucine, which has been previously reported in some complete chloroplast genomes of plants (Liu et al., 2018; Alzahrani et al., 2021). Furthermore, there are 165 simple sequence repeats (SSRs) markers found in the 153,188 bp sequence of the D. huoshanense chloroplast genome. In other words, the observed frequency of SSRs was approximately 1/928.4 bp of chloroplast genome. As previously study, the SSRs were also only observed in the non-coding region of the chloroplast genome (Raveendar et al., 2015).
With the mVISTA program, we examined the degree of DNA sequence divergence in the four-chloroplast genome. In addition to the genes of ycf1, rps19, rpoC2, and atpF, many different non-coding regions among the four chloroplast genomes. We also found that the gene-coding regions are more highly conserved than those of their non-coding districts, which was similar to the other Dendrobium chloroplast genome. Otherwise, the IR-LCS and IR-SSC boundaries of four chloroplast genome plants were compared. The results displayed that N. tabacum has the greatest chloroplast genome from the four plants, while, D. catenatum has the smallest chloroplast genome in size. The largest LSC region in N. tabacum and the smallest in A. thaliana, The largest SSC region in N. tabacum and the smallest in D. catenatum, the largest IR region in D. huoshanense and the smallest in N. tabacum.
Phylogenetic relationships based on the chloroplast genomes protein-coding genes placed all samples into five main clades, where every family is in a separate clade (Fig. 4). The results advised that all species in the same clade could be clustered into the same genus, which indicated that the relationships and phylogenetic positions of this family could be resolved by the chloroplast genome. Nevertheless, it should use different methods to analyze the phylogeny for accurately clarify the evolution of the Orchidaceae family (Fig. 4). This study could offer a reference for plant classification among Dendrobium, and other genus with using the chloroplast genome.