Viridiplantae

Viridiplantae (kingdom Plantae sensu stricto), the green plants, is a natural group or clade of around half a million eukaryotes. They are green because they contain chloroplasts, cell organelles able to produce food by photosynthesis. They are major primary producers of food both in the sea and on land. The group includes both green algae and the land plants (embryophytes) that arose from them.

In 2005, Sina Adl and colleagues proposed the name Chloroplastida for the group. In 2012, Frederik Leliaert and colleagues suggested a revised taxonomy of the Viridiplantae. In 2019, M. Leebens-Mack and colleagues proposed a phylogeny based on analysis of over a thousand plant genomes. It renders the former "chlorophyte algae" and "streptophyte algae" paraphyletic, as the land plants arose from within them.

Viridiplantae (lit. 'green plants')^[6] is a clade of around 450,000–500,000 species of chloroplast-bearing eukaryotes. Most of them are autotrophs that obtain their energy by photosynthesis and play important primary production roles in both terrestrial and aquatic ecosystems.^[7] The clade includes all green algae, which are primarily aquatic; many are microscopic unicellular phytoplankton. It also includes the macroscopic, multicellular, generally complex-structured land plants (embryophytes, i.e. Plantae sensu strictissimo), which emerged from within the freshwater green algae clade Streptophyta^[8][9][10] during the Ordivician.^[11][12]

In traditional taxonomy, the classification of green algae typically exclude the land plants, rendering them a paraphyletic group; however it is cladistically accurate to regard land plants as a specialized clade of green algae that had evolved to thrive on dry land,^[13] thus making Viridiplantae a monophyletic group. Since the realization that the embryophytes emerged from green algae, some authors are starting to include them.^{[13][14][15][16][17]}

Viridiplantae species all have cells with cellulose in their cell walls, and primary chloroplasts derived from endosymbiosis with cyanobacteria that contain chlorophylls a and b and lack phycobilins. In some classification systems, the group has been treated as a kingdom^[18] under various names such as Viridiplantae, Chlorobionta or simply the kingdom Plantae (sensu stricto), the lattermost expanding upon the traditional grouping of (land) plants to include all green algae closely and distantly related to Embryophyta. Adl et al., who produced a classification for all eukaryotes in 2005, introduced the name Chloroplastida for this group, reflecting the group having primary chloroplasts, and they rejected the name Viridiplantae on the grounds that some of the species are not plants as understood traditionally.^[19] Together with Rhodophyta (red algae), Glaucophyta (grey algae) and other basal groups such as the phagotrophic Rhodelphidia^[20] and the picoplanktonic Picozoa (both considered sister to red algae), Viridiplantae belong to the larger primary algae clade Archaeplastida, which in itself is sometimes described as "Plantae sensu lato".

Leliaert et al, 2012 propose the following simplified taxonomy of the Viridiplantae.^[21]

• Viridiplantae
  • Chlorophyta
    • core chlorophytes
      • Ulvophyceae
        • Cladophorales
        • Dasycladales
        • Bryopsidales
        • Trentepohliales
        • Ulvales-Ulotrichales
        • Oltmannsiellopsidales
      • Chlorophyceae
        • Oedogoniales
        • Chaetophorales
        • Chaetopeltidales
        • Chlamydomonadales
        • Sphaeropleales
      • Trebouxiophyceae
        • Chlorellales
        • Oocystaceae
        • Microthamniales
        • Trebouxiales
        • Prasiola clade
      • Chlorodendrophyceae
        • Chlorodendrales
      • Pedinophyceae^[22]
    • prasinophytes (paraphyletic)
      • Pyramimonadales
      • Mamiellophyceae
      • Pycnococcaceae
      • Nephroselmidophyceae
      • Prasinococcales
      • Palmophyllales
  • Streptophyta
    • Charophytes
      • Mesostigmatophyceae
        • Mesostigmatales
        • Chlorokybales
      • Klebsormidiophyceae
      • Phragmoplastophyta
        • Charophyceae
        • Coleochaetophyceae
        • Zygnematophyceae
        • Embryophyta (land plants)

In 2019, a phylogeny based on genomes and transcriptomes from 1,153 plant species was proposed.^[23] The placing of algal groups is supported by phylogenies based on genomes from the Mesostigmatophyceae and Chlorokybophyceae that have since been sequenced. Both the "chlorophyte algae" and the "streptophyte algae" are treated as paraphyletic (vertical bars beside phylogenetic tree diagram) in this analysis.^[24][25] The classification of Bryophyta is supported both by Puttick et al. 2018,^[26] and by phylogenies involving the hornwort genomes that have also since been sequenced.^[27][28]

Ancestrally, the green algae were flagellates.^[21]