| Hornwort | |
|---|---|
 | |
| Phaeoceros laevis (L.)Prosk. | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Embryophytes |
| Division: | Anthocerotophyta Stotler & Stotl.-Crand., 1977[1] |
| Classes and orders | |
seeClassification. | |
| Synonyms | |
Anthocerotae | |
Hornworts are a group ofnon-vascularEmbryophytes (land plants) constituting the divisionAnthocerotophyta (/ˌænθoʊˌsɛrəˈtɒfətə,-təˈfaɪtə/). The common name refers to the elongated horn-like structure, which is thesporophyte. As inmosses andliverworts, hornworts have agametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information; the flattened, green plant body of a hornwort is thegametophyte stage of the plant.
Hornworts may be found worldwide, though they tend to grow only in places that are damp or humid. Some species grow in large numbers as tiny weeds in the soil of gardens and cultivated fields. Large tropical andsub-tropical species ofDendroceros may be found growing on the bark of trees.
The total number of species is still uncertain. While there are more than 300 published species names, the actual number could be as low as 100–150 species.[2]
Like all bryophytes, the dominant life phase of a hornwort is thehaploidgametophyte. This stage usually grows as a thinrosette or ribbon-likethallus between one and five centimeters in diameter. Hornworts have lost twoplastid division-associated genes, ARC3 and FtsZ2, and have just a singlechloroplast percell (monoplastidy), with the exception of the genusMegaceros and some species in the generaNothoceros andAnthoceros, which have more than one chloroplast per cell (polyplastidy). In the polyplastidic species, and also some of the monoplastidic species, a cellular structure called apyrenoid is absent.[3][4] Thepyrenoid is a liquid-like organelle which enables a more efficient photosynthesis,[5] has evolved independently five to six times in hornworts and is present in half of the roughly 200 species.[6] It is formed by the fusion of the chloroplast with otherorganelles and is composed predominantly ofRuBisCO, the key enzyme in carbon fixation. By using inorganic carbon transporters and carbonic anhydrases, up to a 50-fold increase in CO2 levels can be achieved.[7] This particular feature is very unusual in landplants, unique to hornworts, but is common amongalgae.[8][9] They are also the only group of land plants whereflavonoids are completely absent.[10]
Many hornworts develop internalmucilage-filled cavities or canals when groups of cells break down. These cavities secrete hormogonium-inducing factors (HIF) that stimulate nearby, free-livingphotosyntheticcyanobacteria, especially species ofNostoc, to invade and colonize these cavities.[11] Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color. Symbioticcyanobacteria have not been reported inMegaceros orFolioceros.[12] There may also be smallslime pores on the underside of thethallus. These pores superficially resemble thestomata of other plants.
The horn-shapedsporophyte grows from anarchegonium embedded deep in the gametophyte. The growth of the hornwort sporophyte happens from a persistent basalmeristem, in contrast to the sporophyte of moss (apical growth) and liverworts (intercalary growth).[13] Unlikeliverworts, hornworts have truestomata on their sporophyte as most mosses do. The exceptions are the speciesFolioceros incurvus, the genusNotothylas and the three closely related generaMegaceros,Nothoceros andDendroceros, which do not have stomata.[14][15]Notothylas also differ from other hornworts in having a reduced sporophyte only a few millimeters tall. The sporophyte in hornworts is unique among bryophytes in being long-lived with a persistent photosynthetic capacity.[16] The sporophyte lacks anapical meristem, anauxin-sensitive point of divergence with other land plants some time in theLate Silurian/Early Devonian.[17][18]
When the sporophyte is mature, it has a multicellular outer layer, a central rod-likecolumella running up the center, and a layer oftissue in between that produces spores andpseudo-elaters. The pseudo-elaters are multi-cellular, unlike the elaters ofliverworts. They havehelical thickenings that change shape in response to drying out; they twist and thereby help to disperse the spores. Hornwort spores are relatively large forbryophytes, measuring between 30 and 80μm in diameter or more. The spores are polar, usually with a distinctive Y-shaped tri-radiate ridge on theproximal surface, and with adistal surface ornamented with bumps or spines.
The life of a hornwort starts from ahaploid spore. The spores can be yellow, brown or green. Yellow and brown spores have a thicker wall and contain oils that both protect against desiccation and function as a nutrient storage, allowing them to survive for years. The speciesFolioceros fuciformis and the generaMegaceros,Nothoceros andDendroceros have short-lived spores with thin and colorless walls that appear green due to the presence of a chloroplast.[19][20] In most species, there is a single cell inside the spore, and a slender extension of this cell called thegerm tube germinates from the proximal side of the spore.[21] The tip of the germ tube divides to form anoctant (solid geometry) of cells, and the firstrhizoid grows as an extension of the original germ cell.[clarification needed] The tip continues to divide new cells, which produces a thalloidprotonema. By contrast, species of the familyDendrocerotaceae may begin dividing within the spore, becomingmulticellular and evenphotosynthetic before the spore germinates.[21] In either case, the protonema is a transitory stage in the life of a hornwort.
From the protonema grows the adultgametophyte, which is the persistent and independent stage in the life cycle. This stage usually grows as a thinrosette or ribbon-likethallus between one and five centimeters in diameter, and several layers of cells in thickness. It is green or yellow-green from thechlorophyll in its cells, or bluish-green when colonies of cyanobacteria grow inside the plant.
When the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants aremonoecious, with both sex organs on the same plant, but some plants (even within the same species) aredioecious, with separate male and female gametophytes. The female organs are known asarchegonia (singular archegonium) and the male organs are known asantheridia (singular antheridium). Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells.
The biflagellatesperm must swim from the antheridia, or else be splashed to the archegonia. When this happens, the sperm and egg cell fuse to form azygote, the cell from which the sporophyte stage of the life cycle will develop. Unlike all other bryophytes, the first cell division of the zygote islongitudinal. Further divisions produce three basic regions of the sporophyte.
At the bottom of thesporophyte (closest to the interior of the gametophyte), is a foot. This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence. In the middle of the sporophyte (just above the foot), is ameristem that will continue to divide and produce new cells for the third region. This third region is thecapsule. Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to producepseudo-elaters andspores. These are released from the capsule when it splits lengthwise from the tip.
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While the fossil record ofcrown group hornworts only begins in the upperCretaceous, the lower DevonianHorneophyton may represent a stem group to the clade, as it possesses asporangium with central columella not attached at the roof.[22] However, the same form of columella is also characteristic of basal moss groups, such as theSphagnopsida andAndreaeopsida, and has been interpreted as a character common to all early land plants withstomata.[23] The divergence between hornworts andSetaphyta (mosses and liverworts) is estimated to have occurred 479–450 million years ago,[24] and the last common ancestor of present-day hornworts lived in middle Permian about 275 million years ago.[25] Chromosome-scale genome sequencing of three hornwort species corroborates that stomata evolved only once during land plant evolution. It also shows that the three groups ofbryophytes share a common ancestor that branched off from the other landplants early in evolution, and thatliverworts andmosses are more closely related to each other than to hornworts.[26] Unlike other land plants, the hornwort genome has the low-CO2 inducible B gene (LCIB), which is also found in some species of algae. Because the diffusion rate of carbon dioxide is 10,000-fold higher in air than in water, aquatic algae require a mechanism to concentrate CO2 in chloroplasts so as to allow the photosyntheticRuBisCo protein to function efficiently. LCIB is one component of this CO2-concentrating mechanism.[27]
Hornworts were traditionally considered a class within the division Bryophyta (bryophytes). Later on, the bryophytes were consideredparaphyletic, and hence the hornworts were given their own division,Anthocerotophyta (sometimes misspelledAnthocerophyta). However, the most recent phylogenetic evidence leans strongly towards bryophyte monophyly,[28] and it has been proposed that hornworts are de-ranked to the original classAnthocerotopsida.[29]
Traditionally, there was a single class of hornworts, called Anthocerotopsida, or olderAnthocerotae. More recently, a second class Leiosporocertotopsida has been segregated for the singularly unusual speciesLeiosporoceros dussii. All other hornworts remain in the class Anthocerotopsida. These two classes are divided further into fiveorders, each containing a singlefamily.
Among land plants, hornworts are one of the earliest-diverging lineages of the early land plant ancestors;[26] cladistic analysis implies that the group originated prior to theDevonian, around the same time as the mosses and liverworts. There are about 200species known, but new species are still being discovered. The number and names ofgenera are a current matter of investigation, and several competing classification schemes have been published since 1988.
Structural features that have been used in the classification of hornworts include: the anatomy of chloroplasts and their numbers within cells, the presence of apyrenoid, the numbers ofantheridia within androecia, and the arrangement of jacket cells of the antheridia.[30]
Recent studies of molecular, ultrastructural, and morphological data have yielded a new classification of hornworts.[31][32]
Class Leiosporocerotopsida
Class Anthocerotopsida
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| The currentphylogeny and composition of the Anthocerotophyta.[31][33][34][35] | ||||||||||||||||||||||||||||||||||||||||||||||
Classification ofArchaeplastida orPlantaes.l. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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