Some similarities to :
Witches' Broom which is a gall on
Hornbeam and also on Wild Cherry (Gean) trees trees usually caused by the fungus Taphrina betulina which look rather like bird's nests, but they are usually more dense than is Mistletoe.
Uniquely identifiable characteristics
Distinguishing Feature :
Mistletoe is Dioecious, with male and female flowers on separate plants, the sticky white berries only appearing (between November to December) on female plants. The berries are eaten by Mistle Thrush birds as well as by other birds, but they are so sticky that they also stick to the bird's beak as they try to eat them from where they are ready to be implanted on the next high branch of a tree that the bird visits. Because birds prefer to be high up in the trees, Mistletoes is to be mostly found near the top of the tree. But Mistletoe will only take on first or second year tree growth, and even then can take a whole year to 'root' and establish a connection with the host tree before it can obtain nutrients and fluids. Before that first year is up, it is on its own.
Mistletoe is Hemi-parasitic and grows on, obtaining some nutrients (but not all) from deciduous trees only, especially Malus (
Apple) at 40% of mistletoe occurrences, then in descending order of popularity (by the Mistletoe choice itself): Tilia (Lime), Crataegus (
Hawthorn), Populus (especially
Black Poplar), Salix (Willow), False Acacia and only rarely on Quercus (
Oak). But it can grow on virtually any tree, but these are all of much lower probability. It is grown commercially on mainly Apple trees in orchards for the Christmas trade in mistletoe for kissing beneath.
It grows from a seedling by 'gluing' itself to the bark of a tree by means of the sticky viscous liquid they contain. In the first year it will have grown 4 leaves on one branch. It branches into two once a year, the number of branches doubling each year, until it is a globular mass which grows slightly larger every year. In the last 15 years since 2000AD mistletoe seems to be spreading faster than usual.
It is capable of photosynthesis itself but draws water and minerals from its host vascular system at distinct swellings or gall where the two conjoin. It looks as though it buries itself under the bark of the tree, but this is incorrect; it grows on the surface of the bark, but initiates the tree to grow bark over and around itself, so that eventually it is growing from a large knob of altered bark tissue (aka haustorium). After gluing itself to the bark, the seedling puts tentacles into the tree by which means it can obtain some nutrients, and especially water, from the tree. These tentacles penetrate only a short distance into the tree. It is known that Maple trees produce toxins in some cells in response to this invasion in a plan to stop further invasion. Other trees may do likewise. Mistletoe does have green leaves, so it is able to generate some of its own nutrients by photosynthesis; but those it cannot it steals from the host tree, especially water.
Out of several sub-species found in the World, only the 'parent' grows in the UK, Viscum album. It contains a toxic protein and lectin called
Viscumin which has a high molecular weight. Other sources make mention of
viscotoxin as one of the toxins. They are, like Ricin, a RIP, a Ribosome Inactivating plant Protein which is another poisonous lectin, although the two target and bind to differing sites. Although Mistletoe is poisonous, fatalities are rare. The toxin is concentrated in the white berries but is said to be present throughout the whole plant.
Mistletoe does, of course, contain other chemical compounds, but because Mistletoes can grow on a wide variety of differing trees and is hemi-parasitic on them, obtaining some nutrients from them, the composition and proportions of these secondary metabolites varies. However, any toxicity of these compounds will probably pale into insignificance compared to the toxicity of the viscotoxins. A new acyclic monoterpene diglycoside has been found in Mistletoe, but it lacks a common name and the chemical name is very long.
Its greatest population density in the UK is in Somerset and Devon with a very broad spread around the home counties and a smaller outbreak on the Mid-Wales border. Virtually no presence north of South Yorkshire apart from one or two hectads. It grows mainly in gardens and orchards with a lot of parkland presence also. Roads, hedgerows, fields and woods taken together account for just 20% of the population.
MITOCHONDRIA and ATP IN PLANTS
It was once thought that all plants (multicellular eukaryotes) use
mitochondria to generate energy until early 2018, when scientists were astonished to find an exception to this: Mistletoe! The parasitic character of Mistletoe has somehow enabled it to lose much of the functionality within the mitochondria (which it still possesses, but in abnormal form). 'Use it or lose it', as the saying goes. But in the case of Mistletoe, unlike many other parasitic plants, it has lost it! Mistletoe has much less need to generate energy using ATP in mitochondria because it obtains nearly all of its energy needs from the host tree upon which it is parasitic. And now, it can't, although scientists do not know when it lost most of this ability, but it is likely to be thousands of years ago in the annals of prehistory.
Mitochondria are present in all other multicellular eukaryotes, which includes plants and animals (humans too). [Mitochondria are absent or low in single-celled eukaryotes, such as yeasts]. All normally rely on mitochondria and the
Adenoside TriPhosphate) within them to generate and convey the energy which they require. This is a complex process involving many enzymes and complexes, named Complex I to Complex V.
In Mistletoe it was first thought that they had lost the genes required to synthesize the subunits of Complex I, or that the genes required may have been transferred from the mitochondrial DNA to the nuclear DNA. But it is now thought that (European) Mistletoe completely lacks Complex I and that the genes encoding for the subunits of Complex I are completely absent from Mistletoe. Not only that, but it also has greatly reduced amounts of Complex II and Complex V. Moreover, Complex III and Complex IV have, in Mistletoe, have formed SuperComplexes, which are remarkably stable and involved in respiration. Mistletoes entire repiratory chain has been completely overhauled and remodelled. The levels of Complex IV (and the enzyme which synthesises ATP) are present in amounts 5 times lower than that of another 'commonly studied laboratory plant' (possibly meaning Thale Cress - your Author). But the presence of other essential metabolic enzymes were measured at higher levels. These findings illustrate that the mitochondrial functions within Mistletoe have undergone extreme alterations over their evolutionary timeframe.
Because Mistletoe obtains nutrients from it's host trees it seems that it has now lost the ability to generate much ATP within mitochondria. Mistletoe is now far more reliant on its host to obtain ATP, synthesizing but 25% of what its capacity would be if all were intact, although there is still the possibility it may generate ATP in other cellular components. For instance, cells might pick up part of the shortfall by using sugar to generate ATP by glycolysis; although this pathway is 12-fold less efficient than by the normal way of synthesizing ATP within normal mitochondria. Mistletoe may save energy overall though by stealing the sugars from its host. More research is obviously required.
The green parts are the
TriPhosphate part. The blue is the Adenine base whilst the red is the sugar moiety, in this case
Ribose which is a pentose sugar - the same sugar as occurs in
RNA being short for
RiboNucleic Acid. When giving up the energy stored in this ATP molecule the triphosphate becomes first
Adenosine DiPhosphate) then
Adenosine MonoPhospate). ATP is both the precursor molecule to both
ATP is the energy source for a great many biological processes and chemical reactions in both plants and animals. It is used by cells as a coenzyme. ATP has a great many functions in cells; synthesizing macromolecules such as proteins, RNA and DNA, and in the transportation of macromolecules through cell membranes, both into or out of the cell.
It has been found that normal cells (not those of Mistletoe) contain far more ATP than these processes actually require - 5 times more in fact. This might be because ATP is also used in other ways - it can make proteins soluble and serve as a signalling molecule. As an amphiphilic molecule it has characteristics of both a hydrophilic and hydrophobic substance, but does not assemble itself into films such as micelles.