The Chara braunii Genome Sheds Light on the Origin of Land Plants

Land plants and charophycean green algae comprise a monophyletic group called Streptophytes, which diverged 1210-730 MYA from other green plants. Among the six orders of charophyceans, Charales, Coleochaetales, and Zygnematales are considered closer to land plants than the other three. Charales shows especially complex organ differentiation, namely, green thali that facilitates photosynthesis, rhizoids that anchor the alga to the basement and presumably transfer nutrients, and reproductive organs including oogonia (female) and antheredia (male). Chara braunii is a relatively small monoeicous alga in Charales, which we can culture in laboratory condition and induce reproductive organs in several weeks. Thus, C. braunii is a potential model for evolutionary studies. We established unialgal cultures of C. braunii and sequenced their genomes. The assembly we obtained consisted of 1.75 Gbp in 11,808 scaffolds, with N50 of 234 and L50 of 2.26 Mb long. Gene prediction using RNA-seq data and homology to plant proteomes resulted in 36,887 gene models in 35,883 loci. When we searched for “plant” gene homologs, we found many of those including NAC and LFY transcription factors, auxin related genes, photoreceptors, and the four subclasses of HD-zip genes. Notably, we found several class of genes in C. braunii that we didn’t find in Physcomitrella patens, namely ASYMMETRIC LEAVES1/ROUGH SHEATH2/PHANTASTICA (ARP)-type Myb transcription factor, AP2-type transcription factor, MAX1 P450 involved in strigolactone synthesis, VIN3, and GIGANTEA. This genome information extend our understanding which gene predate landing of plants and which gene could have played important role in the colonization on lands.