TheArchaeplastida (or kingdomPlantaesensu lato "in a broad sense"; pronounced/ɑːrkɪˈplæstɪdə/) are a major group ofeukaryotes, comprising thephotoautotrophicred algae (Rhodophyta),green algae,land plants, and the minor groupglaucophytes.[6] It also includes the non-photosynthetic lineageRhodelphidia, a predatorial (eukaryotrophic) flagellate that is sister to the Rhodophyta, and probably the microscopicpicozoans.[7] The Archaeplastida havechloroplasts that are surrounded by two membranes, suggesting that they were acquired directly through a singleendosymbiosis event byphagocytosis of acyanobacterium.[8] All other groups which have chloroplasts, besides the amoeboid genusPaulinella, have chloroplasts surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae.[note 1] Unlike red and green algae, glaucophytes have never been involved in secondary endosymbiosis events.[10]
The cells of the Archaeplastida typically lackcentrioles and havemitochondria with flatcristae. They usually have acell wall that containscellulose, and food is stored in the form ofstarch. However, these characteristics are also shared with other eukaryotes. The main evidence that the Archaeplastida form amonophyletic group comes from genetic studies, which indicate theirplastids probably had a single origin. This evidence is disputed.[11][12] Based on the evidence to date, it is not possible to confirm or refute alternative evolutionary scenarios to a singleprimary endosymbiosis.[13] Photosynthetic organisms with plastids of different origin (such asbrown algae) do not belong to the Archaeplastida.
The archaeplastidans fall into two main evolutionary lines. The red algae are pigmented withchlorophylla andphycobiliproteins, like most cyanobacteria, and accumulate starch outside the chloroplasts. The green algae and land plants – together known asViridiplantae (Latin for "green plants") or Chloroplastida – are pigmented with chlorophyllsa andb, but lack phycobiliproteins, and starch is accumulated inside the chloroplasts.[14] The glaucophytes have typical cyanobacterial pigments, but their plastids (called cyanelles) differ in having a peptidoglycan outer layer.[1]
Archaeplastida should not be confused with the older and obsolete name Archiplastideae, which refers tocyanobacteria and other groups of bacteria.[15][16]
The consensus in 2005, when the group consisting of the glaucophytes and red and green algae and land plants was named 'Archaeplastida',[1] was that it was aclade, i.e. wasmonophyletic. Many studies published since then have provided evidence in agreement.[17][18][19][20] Other studies, though, have suggested that the group isparaphyletic.[21][22][23][12][24] To date, the situation appears unresolved, but a strong signal for Plantae (Archaeplastida) monophyly has been demonstrated in a recent study (with an enrichment of red algal genes).[25] The assumption made here is that Archaeplastida is a valid clade.
Various names have been given to the group. Some authors have simply referred to the group as plants or Plantae.[26][27] However, the name Plantae is ambiguous, since it has also been applied to less inclusiveclades, such asViridiplantae andembryophytes. To distinguish, the larger group is sometimes known as Plantaesensu lato ("plants in the broad sense").
To avoid ambiguity, other names have been proposed. Primoplantae, which appeared in 2004, seems to be the first new name suggested for this group.[5] Another name applied to this node is Plastida, defined as the clade sharing "plastids of primary (direct prokaryote) origin [as] inMagnolia virginiana Linnaeus 1753".[28]
Although many studies have suggested the Archaeplastida form amonophyletic group,[29] a 2009 paper argues that they are in factparaphyletic.[23] The enrichment of novel red algal genes in a recent study demonstrates a strong signal for Plantae (Archaeplastida) monophyly and an equally strong signal of gene sharing history between the red/green algae and other lineages.[25] This study provides insight on how rich mesophilic red algal gene data are crucial for testing controversial issues in eukaryote evolution and for understanding the complex patterns of gene inheritance in protists.
The name Archaeplastida was proposed in 2005 by a large international group of authors (Adlet al.), who aimed to produce a classification for theeukaryotes which took into account morphology, biochemistry, and phylogenetics, and which had "some stability in the near term." They rejected the use of formal taxonomic ranks in favour of a hierarchical arrangement where the clade names do not signify rank. Thus, the phylum name 'Glaucophyta' and the class name 'Rhodophyceae' appear at the same level in their classification. The divisions proposed for the Archaeplastida are shown below in both tabular anddiagrammatic form.[1]
Glaucophytes are a small group of freshwater single-celled algae. Their chloroplasts, calledcyanelles, have apeptidoglycan layer, making them more similar to cyanobacteria than those of the remaining Archaeplastida.
Red algae form one of the largest groups of algae. Most are seaweeds, being multicellular and marine. Their red colour comes fromphycobiliproteins, used asaccessory pigments in light capture for photosynthesis.
ChloroplastidaAdlet al., 2005 (ViridiplantaeCavalier-Smith 1981; ChlorobiontaJeffrey 1982, emend. Bremer 1985, emend. Lewis and McCourt 2004; ChlorobiotaKendrick and Crane 1997)
Chloroplastida is the term chosen by Adlet al. for the group made up of the green algae and land plants (embryophytes). Except where lost secondarily, all have chloroplasts without a peptidoglycan layer and lack phycobiliproteins.
ChlorophytaPascher, 1914, emend. Lewis & McCourt, 2004 – green algae (part)
Adl et al. employ a narrow definition of the Chlorophyta; other sources include the Chlorodendrales and Prasinophytae, which may themselves be combined.
CharophytaKarolet al., 2001, emend. Lewis & McCourt, 2004 (CharophyceaeSmith 1938, emend. Mattox and Stewart 1984) – green algae (part) and land plants
Charophytasensu lato, as used by Adlet al., is a monophyletic group which is made up of some green algae, including the stoneworts (Charophytasensu stricto), as well as the land plants (embryophytes).
Sub-divisions other than Streptophytina (below) were not given by Adl et al.
Below is a consensus reconstruction of the relationships of Archaeplastida with its nearest neighbours, mainly based on molecular data.[31][32][33][34]
There has been disagreement near the Archaeplastida root, e.g. whether Cryptista emerged within the Archaeplastida. In 2014 a thorough review was published on these inconsistencies.[36] The position ofTelonemia andPicozoa are not clear. Also Hacrobia (Haptista + Cryptista) may be completely associated with the SAR clade. The SAR are often seen as eukaryote-eukaryote hybrids, contributing to the confusion in the genetic analyses. A sister ofGloeomargarita lithophora has been engulfed by an ancestor of the Archaeplastida, leading to theplastids which are living in permanent endosymbiosis in most of the descendant lineages. Because both Gloeomargarita and related cyanobacteria, in addition to the most primitive archaeplastids, all live in freshwater, it seems the Archaeplastida originated in freshwater, and only colonized the oceans in the late Proterozoic.[37][38]
In 2019, a phylogeny of the Archaeplastida based on genomes and transcriptomes from 1,153 plant species was proposed.[39] 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.[40][41] The classification of Bryophyta is supported both by Putticket al. 2018,[42] and by phylogenies involving the hornwort genomes that have also since been sequenced.[43][44] Recent work on non-photosynthetic algae placesRhodelphidia as sister to Rhodophyta or to Glaucophyta and Viridiplantae;[45][46] andPicozoa sister to that pair of groups.[47]
All archaeplastidans have plastids (chloroplasts) that carry out photosynthesis and are believed to be derived from endosymbiotic cyanobacteria. In glaucophytes, perhaps the most primitive members of the group, the chloroplast is called acyanelle and shares several features with cyanobacteria, including a peptidoglycan cell wall, that are not retained in other members of the group. The resemblance of cyanelles to cyanobacteria supports theendosymbiotic theory.
The cells of most archaeplastidans have walls, commonly but not always made of cellulose.[citation needed]
The Archaeplastida vary widely in the degree of their cell organization, from isolated cells to filaments to colonies to multi-celled organisms. The earliest were unicellular, and many groups remain so today. Multicellularity evolved separately in several groups, including red algae,ulvophyte green algae, and in the green algae that gave rise tostoneworts and land plants.
Because the ancestral archaeplastidan is hypothesized to have acquired its chloroplasts directly by engulfing cyanobacteria, the event is known as aprimary endosymbiosis (as reflected in the name chosen for the group 'Archaeplastida' i.e. 'ancient plastid'). In 2013 it was discovered that one species of green algae,Cymbomonas tetramitiformis in the orderPyramimonadales, is amixotroph and able to support itself through bothphagotrophy andphototrophy. It is not yet known if this is a primitive trait and therefore defines the last common ancestor of Archaeplastida, which could explain how it obtained its chloroplasts, or if it is a trait regained byhorizontal gene transfer.[48] Since then more species of mixotrophic green algae, such asPyramimonas tychotreta andMantoniella antarctica, has been found.[49]
Evidence for primary endosymbiosis 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 through endosymbiotic gene transfer (EGT). It is estimated that 6–20% of the archaeplastidan genome consist of genes transferred from the endosymbiont.[50] The presence of such genes in the nuclei of eukaryotes without chloroplasts suggests this transfer happened early in the evolution of the group.[51]
Other eukaryotes with chloroplasts appear to have gained them by engulfing a single-celled archaeplastidan with its own bacterially-derived chloroplasts. Because these events involve endosymbiosis of cells that have their own endosymbionts, the process is calledsecondary endosymbiosis. The chloroplasts of such eukaryotes are typically surrounded by more than two membranes, reflecting a history of multiple engulfment. The chloroplasts ofeuglenids,chlorarachniophytes and a small group ofdinoflagellates appear to be captured green algae,[52] whereas those of the remaining photosynthetic eukaryotes, such asheterokont algae,cryptophytes,haptophytes, and dinoflagellates, appear to be captured red algae.[53]
Perhaps the most ancient remains of Archaeplastida are putative red algae (Rafatazmia) withinstromatolites in 1600 Ma (million years ago) rocks in India,[54] as well as possible alga fossils (Tuanshanzia) from China's Gaoyuzhuang Biota of a similar age.[55] Somewhat more recent aremicrofossils from the Roper group in northern Australia. The structure of these single-celled fossils resembles that of modern green algae. They date to theMesoproterozoic Era, about 1500 to 1300 Ma.[56] These fossils are consistent with amolecular clock study that calculated that this clade diverged about 1500 Ma.[57] The oldest fossil that can be assigned to a specific modern group is the red algaBangiomorpha, from 1200 Ma.[58]
In the lateNeoproterozoic Era, algal fossils became more numerous and diverse. Eventually, in thePaleozoic Era, plants emerged onto land, and have continued to flourish up to the present.
^Wetherbee, Richard; Jackson, Christopher J.; Repetti, Sonja I.; Clementson, Lesley A.; Costa, Joana F.; van de Meene, Allison; Crawford, Simon; Verbruggen, Heroen (9 December 2018). "The golden paradox – a new heterokont lineage with chloroplasts surrounded by two membranes".Journal of Phycology.22 (2):257–278.doi:10.1111/jpy.12822.hdl:11343/233613.PMID30536815.S2CID54477112.
^Lipscomb, Diana. 1991. Broad classification: the kingdoms and the protozoa. In: Parasitic Protozoa, Vol. 1, 2nd ed., J.P. Kreier, J.R. Baker (eds.), pp. 81-136. Academic Press, San Diego.
^abNozaki, H.; Maruyama, S.; Matsuzaki, M.; Nakada, T.; Kato, S.; Misawa, K. (December 2009). "Phylogenetic positions of Glaucophyta, green plants (Archaeplastida) and Haptophyta (Chromalveolata) as deduced from slowly evolving nuclear genes".Molecular Phylogenetics and Evolution.53 (3):872–80.Bibcode:2009MolPE..53..872N.doi:10.1016/j.ympev.2009.08.015.PMID19698794.
^Cook, Martha E.; Graham, Linda E. (2017). "Chlorokybophyceae, Klebsormidiophyceae, Coleochaetophyceae". In Archibald, John M.; Simpson, Alastair G. B.; Slamovits, Claudio H. (eds.).Handbook of the Protists. Springer International Publishing. pp. 185–204.doi:10.1007/978-3-319-28149-0_36.ISBN978-3-319-28147-6.
^Novak, Lukas V. F.; Muñoz-Gómez, Sergio A.; Beveren, Fabian van; Ciobanu, Maria; Eme, Laura; López-García, Purificación; Moreira, David (2024-03-11). "Nucleomorph phylogenomics suggests a deep and ancient origin of cryptophyte plastids within Rhodophyta".bioRxiv10.1101/2024.03.10.584144.
^Chen, K.; Miao, L.; Zhao, F.; Zhu, M. (2023). "Carbonaceous macrofossils from the early Mesoproterozoic Gaoyuzhuang Formation in the Yanshan Range, North China".Precambrian Research.392 107074.Bibcode:2023PreR..39207074C.doi:10.1016/j.precamres.2023.107074.
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