Report on the complete organelle genomes of Orobanche Filicicola Nakai ex Hyun, Y. S. Lim & H. C. Shin (Orobanchaceae): insights from comparison with Orobanchaceae plant genomes.

Abstract

Background: Orobanche is a parasitic plant distributed in the temperate zone of Northern Hemisphere, with approximately 200 species found worldwide. In the Republic of Korea, two species of Orobanche, namely O. coerulescens Stephan ex Willd. and O. filicicola Nakai ex Hyun, Y. S. Lim & H. C. Shin, are present, with O. filicicola being endemic. Genome analysis of this species has not yet been performed, and characterizing its complete organelle genome will provide valuable insights into the phylogeny and genome evolution of parasitic plants.

Results: The chloroplast and mitochondrial genomes were analyzed, revealing distinct characteristics. The chloroplast genome is 91,529 bp long with a GC content of 33.6%, containing 33 protein-coding, 30 tRNA, and 4 rRNA genes. In contrast, the mitochondrial genome is 1,058,991 bp long with a GC content of 45.5%, featuring 31 protein-coding, 16 tRNA, and 3 rRNA genes. The mitochondrial genome has over three times more simple sequence repeats and longer long repeats than the chloroplast genome. Analysis of synonymous codon usage in protein-coding genes from nine Orobanchaceae species revealed significant differences between chloroplasts and mitochondria, with codons ending in A or T exhibiting higher coding rates. Ka/Ks ratio calculations indicated that psbI and atpB had the smallest and largest ratios in chloroplasts, respectively, while ccmFC was identified as the only gene under positive selection in mitochondria genomes. Sequence alignment identified 30 homologous fragments between the two genomes, totaling 7,247 bp. Comparison of O. filicicola's chloroplast genome with related species showed gene loss and conserved inverted repeat sequences. Numerous homologous collinear blocks were found in mitochondrial genomes of related species, but some regions lacked homology. Phylogenetic analysis indicated identical topologies for chloroplasts and mitochondria, with Orobanchaceae forming a strong monophyletic group.

Conclusions: Characterizing the complete organelle genome of O. filicicola enabled a comprehensive analysis of the Orobanchaceae organelle genome, providing important baseline data for its structure and evolution.