Superkingdom: Archaeplastida

The Archaeplastida or Primoplantae are a major line of eukaryotes, comprising the land plants, green and red algae, and a small group called the glaucophytes. All of these organisms have plastids surrounded by two membranes, suggesting they developed directly from endosymbiotic cyanobacteria, although the hypothesis that these three groups were the only ones for which this is true remains unproven. In all other groups, plastids are surrounded by three or four membranes, and were acquired secondarily from green or red algae.

The cells typically lack centrioles and have mitochondria with flat cristae. There is usually a cell wall including cellulose, and food is stored in the form of starch. However, these characters are also shared with other eukaryotes. The main evidence the Archaeplastida form a monophyletic group comes from genetic studies, which indicate that plastids probably had a single origin.

The archaeplastids fall in two main evolutionary lines. The red algae are pigmented with chlorophyll a and phycobiliproteins, like most cyanobacteria. The green algae and land plants (together known as Viridiplantae, Latin for "green plants") are pigmented with chlorophylls a and b, but lack phycobiliproteins. The positions of the glaucophytes are uncertain; they have the typical cyanobacterial pigments, and are unusual in retaining a cell wall within the plastids (called cyanelles). In fact, some studies suggest that other groups, like cryptophytes, katablepharids, and haptophytes may be more closely related to red and green algae than glaucophytes are.

Taxonomic history

Some authors have simply referred to the red algae, green algae, and glaucophytes as plants or Plantae. Since the same name has also been applied to less inclusive clades, such as Viridiplantae and embryophytes, this larger group is sometimes known as Plantae sensu lato ("plants in the broad sense").

Because the name Plantae is ambiguous, other names have been proposed. Primoplantae, which appeared in 2004, seems to be the first new name suggested for this group.

Another name that has been applied to this node is Plastida, defined as the clade sharing "plastids of primary (direct prokaryote) origin in Magnolia virginiana Linnaeus 1753".

Most recently, the name Archaeplastida was proposed.

Morphology

All archaeplastids have plastids called chloroplasts that carry out photosynthesis, derived from captured cyanobacteria. In glaucophytes, perhaps the most primitive members of the group, the chloroplast is called a cyanelle and shares several features with cyanobacteria, including a peptidoglycan cell wall, that are not retained in other primoplants. The resemblance of cyanelles to cyanobacteria supports the endosymbiotic theory.

Archaeplastids vary widely in the degree of their cell organization, from isolated cells to filaments to colonies to multi-celled organisms. The earliest primoplants were unicellular, and many groups remain so today. Multicelluarity evolved separately in several groups, including red algae, ulvophyte green algae, and in the green algae that gave rise to stoneworts and land plants. The cells of most archaeplastids have walls, commonly but not always made of cellulose.

Endosymbiosis

Because the ancestral archaeplastid is hypothesized to have acquired its chloroplasts directly by engulfing cyanobacteria, the event is known as a primary endosymbiosis. Evidence for this includes the presence of a double membrane around the chloroplasts; one membrane belonged to the bacterium, and the other to the eukaryote that captured it. Over time, many genes from the chloroplast have been transferred to the nucleus of the host cell. The presence of such genes in the nuclei of eukaryotes without chloroplasts suggests this transfer happened early in the primoplants' evolution.

If archaeplastids were the only ones to undergo primary endosymbiosis by engulfing cyanobacteria, other eukaryotes with chloroplasts gained them by engulfing a single-celled archaeplastid with its own bacterially-derived chloroplasts. The chloroplasts of euglenids and chlorarachniophytes appear to be captured green algae. Other photosynthetic eukaryotes have chloroplasts that are captured (primoplant) red algae, and include heterokont algae, cryptophytes, haptophytes, and dinoflagellates. Because these involve endosymbiosis of cells that have their own endosymbionts, the process is called secondary endosymbiosis. The chloroplasts of these eukaryotes are typically surrounded by more than two membranes, reflecting their history of multiple engulfment.

Fossil record

Perhaps the most ancient remains of Archaeplastida are microfossils from the Roper group in northern Australia. The structure of these single-celled fossils resemble that of modern green algae. These date to the Mesoproterozoic Era, about 1500 to 1300 Ma (million years ago) These fossils are consistent with a molecular clock study that calculated that this clade diverged about 1500 Ma. The oldest fossil that can be assigned to a specific modern group is the red alga Bangiomorpha, from 1200 Ma.

In the late Neoproterozoic Era, algal fossils became more numerous and diverse. Eventually, in the Paleozoic Era, plants emerged onto land, and have continued to flourish up to the present.

References

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