Comparison of the Complete Chloroplast Genomes in the Astragalus

Background: Astragalus is one of the largest angiosperms and has important economic value. The chloroplast genomes (Cp) of most plants in its family have been sequenced and annotated, but there are fewer studies on the characteristics of the chloroplast genomes and codon usage bias of Astragalus. In this study, we sequenced and annotated the complete chloroplast genomes of three Astragalus species, that systematically compared the chloroplast genome and codon usage characteristics of three Astragalus. Methods: In this study, we used three Astragalus species as materials. Firstly, we sequenced and assembled their chloroplast genomes. Subsequently, we analyzed the codon usage bias of the chloroplast genomes of the three medicinal Astragalus species using software such as CodonW, CUSP and SPSS. Result: The results illustrated that the length of Astragalus chloroplast genome ranges from 122,815 bp (A. dahuricus) to 123,729 bp (A. melilotoides Pall.). There were 107-110genes, including 75-76 protein-coding genes (PCGs), four ribosomal RNA genes (rRNAs) and 28-30 transfer RNA genes (tRNAs). In addition, the results illustrated that codons in three Cp genomes from Astragalus had the same 11 Optimal codons and ending with A/U. The codon usage frequency of five model creatures were compared and the results revealed that the codon preferences of Astragalus and five model creatures were pretty significant, the natural selection was the main factor in codon preference. Our research provides some value for the researches of chloroplast genetic engineering and molecular breeding in Astragalus.