**Taxonomy and Classification:**
– Archaeplastida is a major group of eukaryotes consisting of red algae, green algae, land plants, and glaucophytes.
– The group has chloroplasts surrounded by two membranes, likely acquired through a single endosymbiosis event.
– The name Archaeplastida was proposed in 2005 based on morphology, biochemistry, and phylogenetics.
– Various names like Plantae, Plantae sensu lato, Primoplantae, and Plastida have been used for Archaeplastida.
– Recent research supports the monophyletic nature of Archaeplastida, with an enrichment of red algal genes.
**Evolutionary Scenarios and Phylogeny:**
– Studies have shown conflicting evidence on whether Archaeplastida is monophyletic or paraphyletic.
– Recent data supports the monophyly of Archaeplastida, particularly with an enrichment of red algal genes.
– The group has been referred to by various names like Plantae, Plantae sensu lato, Primoplantae, and Plastida.
– A 2009 study argued that Archaeplastida may be paraphyletic, but recent data supports monophyly.
– The classification of Archaeplastida was proposed without formal taxonomic ranks to ensure stability and clarity in the classification system.
**Subgroups and Endosymbiosis Events:**
– Glaucophytes, red algae, and Chloroplastida (green algae and land plants) are major subgroups within Archaeplastida.
– Archaeplastida acquired chloroplasts through a primary endosymbiosis event, likely from cyanobacteria.
– Red algae and green algae have distinct pigmentation and starch storage locations within chloroplasts.
– Glaucophytes have cyanobacterial pigments but unique plastids with a peptidoglycan layer.
– Genetic studies support the monophyletic grouping of Archaeplastida with a single origin of their plastids.
**Synonyms and Morphology:**
– Synonyms for Archaeplastida include Plantae, Primoplastobiota, and Primoplantae.
– The name Plastida has been used to describe the clade sharing plastids of primary origin.
– Most archaeplastidans have cellulose cell walls and vary in cell organization from isolated cells to multicellular organisms.
– Glaucophytes have cyanelles resembling cyanobacteria.
– Multicellularity evolved independently in several groups within Archaeplastida.
**Fossil Record and Evolutionary Significance:**
– Oldest remains of Archaeplastida found in stromatolites date back to around 1600 million years ago.
– Molecular clock studies suggest clade divergence around 1500 million years ago.
– Archaeplastida played a crucial role in the evolution of photosynthesis.
– Recent studies provide evidence for the monophyly of Archaeplastida.
– The origin of primary plastids is linked to freshwater cyanobacteria.
The Archaeplastida (or kingdom Plantae sensu lato "in a broad sense"; pronounced /ɑːrkɪˈplæstɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group glaucophytes. It also includes the non-photosynthetic lineage Rhodelphidia, a predatorial (eukaryotrophic) flagellate that is sister to the Rhodophyta, and probably the microscopic picozoans. The Archaeplastida have chloroplasts that are surrounded by two membranes, suggesting that they were acquired directly through a single endosymbiosis event by phagocytosis of a cyanobacterium. All other groups which have chloroplasts, besides the amoeboid genus Paulinella, have chloroplasts surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae. Unlike red and green algae, glaucophytes have never been involved in secondary endosymbiosis events.
| Archaeplastida Temporal range:
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| Conifer trees, grasses, algae, and shrubs in and around Sprague River, Oregon | |
Scientific classification 
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| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| (unranked): | Archaeplastida Adl et al., 2005 |
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The cells of the Archaeplastida typically lack centrioles and have mitochondria with flat cristae. They usually have a cell wall that contains cellulose, and food is stored in the form of starch. However, these characteristics are also shared with other eukaryotes. The main evidence that the Archaeplastida form a monophyletic group comes from genetic studies, which indicate their plastids probably had a single origin. This evidence is disputed. Based on the evidence to date, it is not possible to confirm or refute alternative evolutionary scenarios to a single primary endosymbiosis. Photosynthetic organisms with plastids of different origin (such as brown algae) do not belong to the Archaeplastida.
The archaeplastidans fall into two main evolutionary lines. The red algae are pigmented with chlorophyll a and phycobiliproteins, like most cyanobacteria, and accumulate starch outside the chloroplasts. The green algae and land plants – together known as Viridiplantae (Latin for "green plants") or Chloroplastida – are pigmented with chlorophylls a and b, but lack phycobiliproteins, and starch is accumulated inside the chloroplasts. The glaucophytes have typical cyanobacterial pigments, but their plastids (called cyanelles) differ in having a peptidoglycan outer layer.
Archaeplastida should not be confused with the older and obsolete name Archiplastideae, which refers to cyanobacteria and other groups of bacteria.