Genomes of early-diverging streptophyte algae shed light on plant terrestrialization

Abstract

Mounting evidence suggests that terrestrialization of plants started in streptophyte green algae, favoured by their dual existence in freshwater and subaerial/terrestrial environments. Here, we present the genomes of Mesostigma viride and Chlorokybus atmophyticus, two sister taxa in the earliest-diverging clade of streptophyte algae dwelling in freshwater and subaerial/terrestrial environments, respectively. We provide evidence that the common ancestor of M. viride and C. atmophyticus (and thus of streptophytes) had already developed traits associated with a subaerial/terrestrial environment, such as embryophyte-type photorespiration, canonical plant phytochrome, several phytohormones and transcription factors involved in responses to environmental stresses, and evolution of cellulose synthase and cellulose synthase-like genes characteristic of embryophytes. Both genomes differed markedly in genome size and structure, and in gene family composition, revealing their dynamic nature, presumably in response to adaptations to their contrasting environments. The ancestor of M. viride possibly lost several genomic traits associated with a subaerial/terrestrial environment following transition to a freshwater habitat. ; This work is part of the 10KP project led by BGI-Shenzhen and China National GeneBank. ; We thank G. Günther (http://www.mikroskopia.de/index.html), who took microscopic images of M. viride and C. atmophyticus. Financial support was provided by the Shenzhen Municipal Government of China (grant nos. JCYJ20151015162041454 and JCYJ20160531194327655) and the Guangdong Provincial Key Laboratory of Genome Read and Write (grant no. 2017B030301011). This work is part of the 10KP project led by BGI-Shenzhen and China National GeneBank. ; The Shenzhen Municipal Government of China and the Guangdong Provincial Key Laboratory of Genome Read and Write. ; http://www.nature.com/natureplants ; am2021 ; Biochemistry ; Genetics ; Microbiology and Plant Pathology