Editing Coleoptile

{{Short description|Protective sheath in certain plants}}
[[Image:Coleoptile.png|thumb|Schematic image of wheat coleoptile (above) and flag leaf (below)]]
[[File:Mais kiemplant.jpg|thumb|Young seedling breaks through the tip of the coleoptile (left). The majority of the tissue remains ungreening throughout the lifecycle (right).]]
'''Coleoptile''' is the pointed protective sheath covering the emerging shoot in [[monocotyledons]] such as grasses in which few leaf primordia and [[shoot apex]] of monocot embryo remain enclosed. The coleoptile protects the first leaf as well as the growing stem in seedlings and eventually, allows the first leaf to emerge.<ref name=":0">{{Cite web|title=Coleoptiles - an overview {{!}} ScienceDirect Topics|url=https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/coleoptiles|access-date=2021-05-04|website=www.sciencedirect.com}}</ref> Coleoptiles have two [[vascular bundle]]s, one on either side. Unlike the flag leaves rolled up within, the pre-emergent coleoptile does not accumulate significant protochlorophyll or carotenoids, and so it is generally very pale. Some preemergent coleoptiles do, however, accumulate purple anthocyanin pigments.

Coleoptiles consist of very similar cells that are all specialised to fast stretch growth. They do not divide, but increase in size as they accumulate more water. Coleoptiles also have water vessels (frequently two) along the axis to provide a water supply.

When a coleoptile reaches the surface, it stops growing and the flag leaves penetrate its top, continuing to grow along. The wheat coleoptile is most developed in the third day of the germination (if in the darkness).

==Tropisms==
Early experiments on [[phototropism]] using coleoptiles suggested that plants grow towards light because [[plant cell]]s on the darker side elongate more than those on the lighter side. In 1880 [[Charles Darwin]] and his son [[Francis Darwin|Francis]] found that coleoptiles only bend towards the light when their tips are exposed.<ref>Darwin, C. R. (1880). ''The Power of Movement in Plants''. London: Murray.</ref> Therefore, the tips must contain the [[photoreceptor cell]]s although the bending takes place lower down on the shoot. A chemical messenger or [[hormone]] called [[auxin]] moves down the dark side of the shoot and stimulates growth on that side. The natural [[plant hormone]] responsible for phototropism is now known to be [[indoleacetic acid]] (IAA).

The [[Cholodny–Went model]] is named after [[Frits Warmolt Went]] of the [[California Institute of Technology]] and the Ukrainian scientist [[Nikolai Cholodny]], who reached the same conclusion independently in 1927. It describes the [[phototropism|phototropic]] and [[gravitropism|gravitropic]] properties of emerging shoots of [[monocotyledon]]s. The model proposes that auxin, a plant growth hormone, is synthesized in the [[Meristem|coleoptile tip]], which senses light or gravity and will send the auxin down the appropriate side of the shoot. This causes [[symmetry|asymmetric]] growth of one side of the plant. As a result, the plant shoot will begin to bend toward a light source or toward the surface.<ref name="basipetal">{{cite journal| journal=Plant Physiology| title=Basipetal Auxin Transport Is Required for Gravitropism in Roots of Arabidopsis| author=Rashotte|date=February 2000| volume=122| number=2| pages=481–490| doi=10.1104/pp.122.2.481|display-authors=etal| pmc=58885| pmid=10677441}}</ref>

Coleoptiles also exhibit strong [[geotropism|geotropic]] reaction, always growing upward and correcting direction after reorientation. Geotropic reaction is regulated by light (more exactly by [[phytochrome]] action).

== Physiology ==
The coleoptile acts as a hollow organ with stiff walls, surrounding the young plantlet and the primary source of the gravitropic response.<ref>{{Cite journal|last=Edelmann|first=Hans G.|date=1996-10-01|title=Coleoptiles are gravi-guiding systems vital for gravi-insensitive shoots of germinating grass seedlings|url=https://doi.org/10.1007/BF00208320|journal=Planta|language=en|volume=200|issue=2|pages=281–282|doi=10.1007/BF00208320|pmid=11541944 |s2cid=20664660 |issn=1432-2048|url-access=subscription}}</ref> It is ephemeral, resulting in rapid [[Plant senescence|senescence]] after the shoot emerges. This process resembles the creation of [[aerenchyma]] in roots and other parts of the plant.<ref>{{cite book |last1=Inada |first1=Noriko |last2=Sakai |first2=Atsushi |last3=Kuroiwa |first3=Haruko |last4=Kuroiwa |first4=Tsuneyoshi |year=2002 |chapter=Three-Dimensional Progression of Programmed Death in the Rice Coleoptile |editor-last=Jeon |editor-first=Kwang W.|title=A Survey of Cell Biology |series=International Review of Cytology |volume=218 |pages=221–260e |doi=10.1016/S0074-7696(02)18014-4 |pmid=12199518 |isbn=9780123646224 }}</ref> The coleoptile will emerge first appearing yellowish-white from an imbibed seed before developing chlorophyll on the next day. By the seventh day, it will have withered following [[programmed cell death]]. The coleoptile grows and produces chlorophyll only for the first day, followed by degradation and water potential caused growth. The two vascular bundles are organized parallel longitudinally to one another with a crack forming perpendicularly. Greening mesophyll cells with chlorophyll are present 2 to 3 cell layers from epidermis on the outer region of the crack, while non-greening cells are present everywhere else. The inner region contains cells with large amyloplasts supporting germination as well as the most interior cells dying to form aerenchyma.

The length of the coleoptile can be divided into an irreversible fraction, length at [[Turgor pressure|turgor]] pressure 0, and reversible fraction, or elastic shrinking.<ref>{{Cite journal|last=Kutschera|first=U.|date=2004|title=The Biophysical Basis of Cell Elongation and Organ Maturation in Coleoptiles of Rye Seedlings: Implications for Shoot Development1|url=https://onlinelibrary.wiley.com/doi/abs/10.1055/s-2004-815734|journal=Plant Biology|language=en|volume=6|issue=2|pages=158–164|doi=10.1055/s-2004-815734|pmid=15045666 |issn=1438-8677|url-access=subscription}}</ref> Changes induced by [[White|white light]] increase [[water potential]] in epidermal cells and decrease osmotic pressure, which resulted in an increase in the length of the coleoptile. The presence of the expanding coleoptile has also been shown to support developing tissues in the seedling as a [[Hydrostatics|hydrostatic tube]] prior to its emergence through the coleoptile tip.

[[Adventitious Root|Adventitious]] roots initially derive from the coleoptile [[Plant node|node]], which quickly overtake the [[Radicle|seminal root]] by volume.<ref>{{cite book |editor-last1=Khan |editor-first1=Khalil |editor-last2=Shewry |editor-first2=Peter R.|title=Wheat |date=2009 |publisher=Elsevier |isbn=9781891127557 |edition=4th}}</ref> In addition to being more numerous, these roots will be thicker (0.3–0.7{{nbsp}}mm) than the seminal root (0.2–0.4{{nbsp}}mm). These roots will grow faster than the shoots at low temperatures and slower at high temperatures.

== Anaerobic germination ==
In a small number of plants, such as [[Oryza sativa|rice]], anaerobic germination can occur in waterlogged conditions. The seed uses the coleoptile as a 'snorkel', providing the seed with access to oxygen.<ref>{{cite journal |last1=Magneschi |first1=Leonardo |last2=Perata |first2=Pierdomenico |title=Rice germination and seedling growth in the absence of oxygen |journal= Annals of Botany|date=25 July 2008 |volume=103 |issue=2 |pages=181–196 |doi=10.1093/aob/mcn121 |pmid=18660495 |pmc=2707302 |url=https://academic.oup.com/aob/article/103/2/181/187259 |access-date=27 March 2022}}</ref>

==References==
{{Reflist}}

==External links==
*{{Commons-inline|Category:Coleoptiles|Coleoptiles}}

[[Category:Plant anatomy]]