Viridiplantae

Compared to the archaeplastidian sister groups Rhodaria (red algae, rhodelphidians and picozoans) and Glaucophyta ("grey algae"), Viridiplantae are unique in that almost all members (with the exception of parasitic plants and the heterotrophic algae Prototheca) have chloroplasts with both chlorophyll a and chlorophyll b (the latter absent in other archaeplastidians) but lack phycobiliproteins (which are present in other archaeplastidians) — thus typically presenting an untainted green color — and carbohydrates produced by photosynthetic carbon fixation is stored as starch directly inside the chloroplasts or in specialized non-pigmented plastids known as amyloplasts. Their cell walls consist of a network of semicrystalline cellulose microfibrils hydrogen-bonded with minor amounts of hemicellulosic polysaccharides such as xyloglucan and glucuronan, or with the cellulose replaced by mannans as well as sulfated/pyruvylated galactans and arabinans; while "later-diverging charophytes" (charophyceans, coleochaetophyceans, zygnematophyceans and embryophytes) have a framework of cellulose microfibrils embedded in a matrix of pectins, hemicelluloses, arabinogalactans and extensins as well as lignin or lignin-like polymers.

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.

Definition

Viridiplantae (lit. 'green plants') 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. 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 during the Ordivician.

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, thus making Viridiplantae a monophyletic group. Since the realization that the embryophytes emerged from green algae, some authors are starting to include them.

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 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. Together with Rhodophyta (red algae), Glaucophyta (grey algae) and other basal groups such as the phagotrophic Rhodelphidia and the picoplanktonic Picozoa (both considered sister to red algae), Viridiplantae belong to the larger primary algae clade Archaeplastida.