INDOLES and INDOLE ACETIC ACIDS - AUXINS
There are not many naturally occurring
organochlorides in the plant world, but there are some. Perhaps the most abundant organochloride in the plant world is that contained in one of the four indole plant growth hormones,
4-chloro-IndoleAcetic acid (4-Cl-IAA), which is bio-synthesized in members of the Pea Family such as green peas, lentils, vetch, various beans and other peas. Alongside it is the non-chlorinated version, another auxin. The potency of the growth hormone function of the chlorinated version is 100-fold stronger than that of the normal version, Indole-3-Acetic Acid (IAA). However, it has been found to be far too strong to be of much use as a growth promoter in the plants that synthesize it: pea cuttings treated with it at first rooted profusely, but for seven days started producing large amounts of the gas ethylene, a senescence inducer and almost died, whereas normal non-chlorinated auxins only initiate production of Ethylene for one day. It is the ethylene which also stimulates growth, but it can also over-stimulate growth, resulting in death. It is theorised that the plant may produce chlorinated auxins which they then use as the often observed (but so far un-identified) 'death hormone' produced by the seeds during their development which eventually kills the mother plant thereby giving the seedling a much better survival chance.
Control of the shape of leaves is accomplished by the concentration of auxins in certain areas encouraging higher growth whilst the concentration from locality to locality of anti-auxins reduces or inhibiting growth in other areas. By this means leaves can be made flat, cusped, crinkled, cup-shaped or have pointed ends, rounded ends, spines or teeth on the edge, etc.
Sunflowers (and several other flowers) are heliotropic (pointing towards the sun and following its progress across the sky) only when in the bud stage and not when in flower. When they set flower they affix themselves to point in one direction only, usually towards the morning Sun. The seeds are thus protected from the full blaze of the midday sun. The heliotropic response is mediated by one of the plant hormones, Indole-3-Acetic Acid (aka 3-Indolylacetic Acid or IAA) which is a heteroauxin or plant growth regulator. Indole-3-Acetic Acid is a light sensitive hormone which causes the cell to grow in an asymmetric fashion, elongating it. It is this aligned elongation which causes the flower to turn, nominally towards the source of the strong light, the sun. This is a case of Photonasty (light stimulated movements of plants) It seems that another auxin, Indole-3-acetonitrile (aka 3-Indolylacetonitrile or IAN), which has an inhibitory effect on asymmetrical cell growth may also be involved.
Two other native Auxins produced by plants are (Indole-3-Butyric Acid IBA) and
2-PhenylAcetic Acid (PAA), the latter not being an Indole but it behaves similarly so is classed as an auxin.
Indole-3-Butyric Acid (IBA) is present in Maize and also the non-native
Tea Plant (Camellia sinensis) and in all
Willow Trees (Salix) but it is not widely found elsewhere. It is a phytohormone belonging to the Auxin family which includes such members as Indole Acetic Acid (IAA) and the far more potent (by a 100-fold)
4-Chloro-IndoleAcetic Acid (4-Cl-IAA). It is used commercially as a plant hormone and was once thought to be non-natural until it was discovered in several species of plants. Its modus operandi is contentious, with some scientists suggesting Indole-3-Butyric Acid is converted into IndoleAcetic Acid before being and effective hormone whilst other researches suggest that it acts as an auxin in its own right.
PhenylAcetic Acid (PAA) is a non-indole Auxin which possesses hormonal effects in plants similar to those induced by Indole-3-Acetic Acid (IAA) but is several times less potent. It was once thought not to be found in plants but it has been found in the most-studied of plants, Thale Cress (Arabidopsis thaliana) and has since been found in Tobacco - at several times the concentration that the more potent IAA. It is now known to be widely distributed in both vascular and non-vascular plants (those with and without vessels for transporting fluids). Synthetically-derived PAA has been used since 1975 as a phytohormone.
There are many other indole-based hormones which are produced synthetically.
MELATONIN & OTHER INDOLES & DERIVATIVES
Melatonin (not to be confused with
Melanin, a black pigment made in the skin of dark-skin people and which is also responsible for sun-tan caused by exposing the skin to sunlight or UV radiation) is a hormone produced by the pineal gland in mammals which regulates sleep/wake patterns in response to the bright light of daylight. It synchronises their circadian rhythms, which otherwise free-run at a period slightly longer than a day (about 25 hours, rather than the 24 hours of a day). Melatonin is also involved in the mechanism by which certain amphibians and reptiles can change the colour of their skin, such as can chameleons. It was discovered in 1958.
Up until 2005 no one ever suspected that Melatonin would also be found in a great many plants (in all plants that have so far been screened). It occurs in all parts of the plants, roots, stems, leaves, flowers, fruits and seeds, in varying proportions. Some species of plant have very high amounts of it, varying from several µg/g to just pico g/g. The highest amounts are found in tea, coffee, beer, barley, corn, rice, oats and wheat. It is synthesized in response to stress, the higher the stress the plant is under, the more it generates. The stress could be a fungal infection, insect damage, water deficiency, extremes of temperature, toxins, heavy metals, or soil salinity, for example. It performs so many diverse functions within the plants that it is difficult to classify. It co-opts a plethora of co-enzymes and other substances to help it perform these tasks. Researchers still do not fully understand all of the interactions of melatonins within plants; study is still on-going. Its effects are fairly widespread including auxin-like effects. It is also capable of chelating heavy metal elements forming complexes, such as cadmium and many others.
Melatonin is involved during the following plant conditions: seed ripening, fruit ripening, flower buds and other development stages, leaf senescence, intense sunlight (especially ultraviolet), heat, cold, drought, high salinity, bacterial pathogens, chemicals which cause stress in plants. It is also involved in growth regulation and in photosynthesis, but so far no one has been able to establish with certainty any hormonal activity. Only limited evidence (in certain plants only) has any light-induced circadium rhythm generation been found.
The melatonin (and the plethora of other co-enzymes, etc involved in response to the stimuli) initiate the following responses to those plant conditions mentioned above: Increases in resistance to osmotic stress and to oxidative stress; increases in cold resilience - increases in the cold and drought responsive genes; increased thermotolerance - increases in production of heat-shock proteins and other heat-shock substances; increases in pathogen resistance; increases in seed viability and seed germination capability; preservation of chlorophyll which delays cell senescence; increases in anti-oxidant enzymes, increases in low-molecular weight anti-oxidants leading to more scavenging of free radicals and singlet oxygen moieties; increases in photo-protection and strong-sunlight tolerance. This is quite some list, especially since it was only recently discovered in plants.
It is a powerful free-radical scavenger and anti-oxidant. In order to conduct and orchestrate all these diverse effects the Melatonin can also be hydroxylated non-enzymatically by its eager interaction with free radicals, for example with hydroxy radicals. By these means and others the following related substances are also created from Melatonin within plants:
ACTIVE MELATONIN DERIVATIVES CREATED WITHIN PLANTS
ASCORBIC and DEHYDROASCORBIC ACIDS
Butenolide of great importance to both mammals and plants is Ascorbic Acid, better known as Vitamin C (which is not a single compound, but refers to a number of compounds having Vitamin C activity - but to work well, they are converted to the most active form of the vitamer, DehydroAscorbic Acid). Between them, Gibberellic Acid and its antagonist Ascorbic Acid act in anti-concert regarding their roles in seed germination, the first encouraging germination, the second discouraging it. When the relative concentrations of the two tip towards Gibberellic Acid, then the seed sets about germinating, but otherwise if the proportion is tipped towards Ascorbic Acid, then germination is delayed until conditions are just right. A differential mechanism, a balancing act which has far more refined response than that of a trigger point set to the concentration of a single substance. Temperature, rainfall and sunlight-hours could all have their say on the balance between the two mutually antagonistic compounds in ways not yet fully understood.
Abscisic Acid is an ubiquitous and multi-functional plant hormone discovered in 1961 to 1963 in Cotton plants and
Lupin by several researchers. It has since been found to be vital for the growth of most (if not all?) plants. It is a stereoisomeric molecule which can exist in two forms, the (S)-cis-Abscisic Acid and (R)-cis Abscisic Acid forms, of which only the former exhibits hormonal activity in plants.
Since Cotton is non-native to the UK, your Author has put it under Lupin, but it occurs in most (all?) plants. It was named Abscisic Acid because it was first thought to be only involved with abscission in plants (the shedding of parts of a plant, such as seeds, leaves, fruit, flower etc) but it is now known to be involved in a great many other ways in which plants grow and senesce. ABA is also produced by some plant pathogenic fungi for nefarious reasons.
It is now known that Abscisic Acid promotes seed dormancy, assists tolerance to desiccation, and inhibits precocious germination during seed development. It also enhances root growth unless they are stressed by shortage of water in which case it inhibits root growth. It also boosts closure of the stomata to help preserve internal water and accelerates leaf senescence.
ABA is located everywhere in the plant; but concentrations of it vary from 1 to 15nM in the xylem, or up to 3000nM within water-stressed leaves.
Abscisic Acid is synthesized within plants from Zeaxanthin via trans-
ViolaXanthene to produce first
Xanthoxin (not to be confused with Xanthotoxin) which is eventually transformed into Abscisic Acid.
Abscisic Acid is 'in-activated' by being transformed into either Phaseic Acid (itself a hormone found in plants) or to its -β-D-Glucose-Ester. But Phaseic Acid is also a plant hormone which is associated with arresting of photosynthesis and abscission (the shedding of parts of a plant, such as seeds, leaves, fruit, flower etc). High levels of Phaseic Acid impede the closure of stomata (the opposite effect to Abscisic Acid) and to reduce photosynthesis (at least in Thale Cress). Therefore the two hormones Abscisic Acid and its decomposition product Phaseic Acid act partly in opposition to each other, providing a finer level of control that is possible from differential systems, such as in the Ascorbic Acid/Gibberellic Acid (which are both hormones) differential ratio as described on the Danish Scurvygrass page.
See the Violaxanthin Cycle where the balance of Violaxanthin, Antheraxanthin and Zeaxanthin helps protect plants from excess sunlight, which is another balanced cycle.
Gibberellic Acid (aka Gibberellin A3, GA or GA3) is not a Karrikin but a potent plant hormone which acts in synergy with the Karrikins (when they are around, which is not often - only after a hot forest fire) to help germinate seeds. They accomplish this far more effectively than can Gibberellic Acid acting alone (see text above). It is a pentacyclic diterpene which lacks the heterocyclic oxygen atoms within the rings of the Karrikins.
Gibberellic Acid is based on the skeletal structure of ent-Gibberellane, which is similar to that of ent-Kaurane (but that exchanges the 5 membered ring for one of six members with a few other rearrangements or omissions of side-groups). Both are diterpenoids found in plants.
There are a large number of other Gibberellins, 126 as of 2003, found in various living organisms such as plants, fungi and bacteria. All those with 19 carbon atoms are, in general, bio-active, whereas those with 20 carbon atoms are not. Gibberellic Acid is dihydroxylated
Gibberellin A1 (not shown). They are based upon the skeleton of ent-
Gibberellane but synthesized from ent-
Kaurene which possesses a 6-membered ring in place of the 5-membered ring.
Jasmonic Acid, and its metabolites, is a plant hormone and is derived from
Linolenic Acid. It plays roles in regulating plant development and growth, including growth inhibition, senescence, tendril coiling (but obviously not in Lodgepole Pines), seed germination, flower development, flower form, flowering time, flower opening, the number of open flowers, and leaf fall. It also has a hand in tuber formation of potatoes, yams and onions.
It also plays a role in the wounding response and systemic acquired resistance. It acts as a defence chemical against insects, interfering with their digestive processes.
Jasmonic Acid can be converted into the ester Methyl Jasmonate within the plant, which plays similar roles in plant defence as Jasmonic Acid. Plants produce both chemicals in response to stress or damage. Methyl Jasmonate also signals to remoter parts of the plant (via propagation through the air since it is both aromatic and volatile) forearming them against similar damage or attack, so that they are prepared. It is thus also a signalling molecule. But Methyl Jasmonate is a gas which is not very active in plants, but as a gas is able to waft over to nearby plants whereupon it diffuses into the pores of the leaves of nearby un-damaged plants, where, acted upon by water, it gets converted into the water-soluble Jasmonic Acid. The Jasmonic Acid then attaches itself to specific receptors in cells triggering the leafs' defence mechanism.
Methyl Jasmonate can also induce ethylene formation. Ethylene, H2C=CH2, is a gas and plant hormone that enhances the ripening of nearby fruits which is used extensively in green-house agriculture.
PHYTOCHROME B - a
Thale Cress is the most studied plant in history because it is relatively easy to perform experiments with it. Thale Cress has been found to channel light from the sun/sky down through the plant stems and along the roots, as though they were optical fibres. When that light gets to the roots it triggers photoreceptors detectors in the roots (just like it triggers photoreceptors in the stems) which stimulates gravitotropic rot growth, directing roots downwards. Only certain wavelengths of light are propagated along stems and roots and it is these wavelengths to which
Phytochrome B responds, in turn activating Elongated Hypocotyl 5 (Hy5 in shorthand), a transcription factor mediating the gravitotropic response.
Alas, both Phytochrome B and Hy5 are complex biological molecules which cannot be drawn in this website.
Further Reference: Phytochrome Signalling Mechanisms
Cytokinins are a class of phytohormones which promote cell growth by division (cytokinesis). There are several natural Cytokinins divided into two groups, the Adenine-type (such as Kinetin (originally discovered in
Millet), Zeatin (first found in
Zea Mays) and
6-BenzylAminoPurine (a synthetic cytokinin) and those based on
phenylurea such as
N-N'-DiPhenylUrea (found in coconut milk) and the synthetic
Thidiazuron which is used extensively in tissue culture and rooting hormones.
Zeatin is one of the Cytokinins aka plant hormones. It was initially found in species of Zea, and is present in Zea Maize. It is derived from the
purine base Adenine, which has similarities to other Purines such as Guanine and the Xanthines Theophylline, Caffeine and Theobromine.
Zeatin (not to be confused with Zeathanthin) belongs to the family of plant-growth hormones called Cytokinins (which modulate cell division and shoot formation) and is also found in
Coconut milk. It promotes the growth of lateral buds and can be applied artificially by spraying on to the meristems of plants where it induces cell division leading to bushier plants. Cytokinins are highly synergistic with Auxins, augmenting each other. The ratios of these two groups of plant hormones control most of the main growth periods over a plants lifetime with the cytokinins countering some of the effects of the auxins. The ratio of the two govern where growth occurs, increased cytokinin induces more shoot growth whilst more auxin induces root formation.
cytokinin first found in
Millet, but in 1996 was found to naturally exist in the DNA of cells from almost every organism tested so far. It is thus ubiquitous and is thought to be produced from Furfural (aka Furfuraldehyde) which is derived as an oxidation product from the DeOxyRibose sugar in DNA, and then furfurals further reaction with the Adenine bases in DNA.
STRIGOLACTONES - plant growth hormones
Strigolactones are double-lactones
5-DeoxyStrigol is the pre-cursor to the Strigolactones produced within Chameleon. Strigolactones are Plant Hormones which stimulate the growth of symbiotic mycorrhizal fungi. They also inhibit the formation of branches on established plants whilst at the same time promoting the germination of nearby seedlings. The seedlings then germinate and have more room around them allowing more sunlight to dance upon their fledgling leaves. These Strigolactones are produced within Chameleon itself and are derived from Carotenoids.
Sorgomol is another Strigolactone produced not only in Chameleon but also in
Sorghum from which it derives its name. It is a potent germination stimulant for seeds of the root-parasitic weeds of
Great Millet (Sorghum bicolor) and is probably instrumental in the propensity for Chameleon to spread rampantly. One Strigolactone (not as [far as your author knows] produced by Chameleon) is called
Sorgolactone, which gets its name from the Sorghum plants it helps germinate. Another is (+)-
Orobanchol (which is also not reportedly produced by Chameleon), presumably so-called from plants of the genus Orobanche (
Broomrapes) it helps germinate.
Strigone has been isolated from root extracts of Chameleon and is also a potent seed germination stimulant, depending upon the four possible stereoisometric arrangements of the compound, some of which will not be produced naturally.
Karrikins (shown above) are structurally similar to Strigolactones, but much simpler. They are produced naturally in the smoke from forest fires when plant matter burns at high temperature and are much more effective plant hormones, helping seeds to germinate after a fire, than are the Strigolactones themselves.
TWO SALICYLATES - hormone and signalling molecules
ASPIRIN / ACETYLSALICYLIC ACID
Aspirin, or Acetyl Salicylic Acid, is also found in plants, being a plant hormone (phytohormone) which not only helps the plant grow but also is involved in a pathway signalling the presence of plant pathogens and mediating the plant defence against the pathogens.
Once activated by a pathogen, it is also involved in inducing resistance to the pathogen in parts of the plant not yet infected. The signalling process also invokes the conversion of salicylic acid into the volatile ester, methyl salicylate, whereupon it can then drift through the air to other nearby plants to prime them against the presence of a nearby pathogen or pest, warning of their proximity by remote control. Methyl Salicylate is also called Oil of Wintergreen, and is indeed produced by the
Wintergreen plants, such as Round-Leaved Wintergreen, some species of
Gaultheria, most members of the
Pyrolaceae Family, some species of plants of the Genus
Betula and all species of plants of the
Spiraea family, including Dropwort and Meadowsweet
To humans Methyl Salicylate, aka Oil of Wintergreen, being an ester, smells sweet, hence the name Meadowsweet. Methyl Salicylate is, however, not only toxic but also an insect pheromone. By this means the plant is also able to attract beneficial insects that will help kill the invading herbivorous insect pests. It is commercially extracted not from any Wintergreen plant, but from twigs of the Sweet Birch tree (Betula Lenta. Oil of wintergreen is used a fragrance in certain products not necessarily including perfumes, and in deep-heat liniments and in trace amounts as a flavour in some chewing gums, candies and mouth-washes as an alternative to spearmint and peppermint for it is also an anti-septic. Like most essential oils, it is poisonous in greater amounts.
BRASSINOLIDE - growth enhancer
Brassinolide looks similar to a steroidal compound, but actually on closer examination has a 7-membered
lactone ring in place of a 6-membered carbon ring. This was first isolated from the pollen from Oil-seed Rape and it turns out to be a
plant hormone or Auxin capable of enhancing the growth of Oil-seed Rape (by this means, and possibly by others, once established, Oil-seed Rape is able to block out all competitors).
TURGORINS - a hormone and signalling molecules
Turgorins are auxins, plant hormones (phytohormones), which are involved in the thigmonastic response of plants. Turgorin itself is one such Turgorin, being the Sulfate of the Glucoside of Gallic Acid. In Mimosa pudica the Glucoside of Gallic Acid is sulfated by the enzyme Sulfotransferase (ST) which transfers a sulfate moiety from 3'-PhosphoAdenoside-5'-PhosphoSulfate (PAPS) to the Glucoside of Gallic Acid.
Another Turgorin is Turgorin LMF1 (Turgorin Leaf Movement Factor 1) which is the double Glycoside of Gentisic Acid (rather than of Gallic Acid as for Turgorin itself). One of the glycosides (the pentose) is related to
DeoxyRibose. Both Turgorins are involved in the leaf movements in
Sensitive Plant (Mimosa pudica). These same phytohormones are involved both in the tactile and the diurnal closing of its leaves.
The Turgorins are very likely also involved in the mechanism for opening and closing the stomata whereby plants are able to transpire. Thus they are probably involved not only in the regulation of temperature of the plant but also for the transportation of themselves throughout the plant via the sap.
VOLATILE SULFUR COMPOUNDS
(possibly some signalling molecules)
It has only recently been discovered that when
Sensitive Plant Mimosa pudica is sensitive to touch when it emits a cocktail of volatile and odorous compounds, many containing sulfur, both organic and inorganic compounds, some of which smell foul. They include
Sulfur Dioxide (SO2),
S-propyl Propane-1-Thiosulfinate, and
ThioFormaldehyde - the latter being a fleeting and highly unstable compound which has never before been found to be emitted by a plant.
The foul-odour compounds are PropaneSulfenic Acid,
2-AminoThioPhenol and S-propyl Propane-1-Thiosulfinate. These compounds are not, as was once thought, produced by microbes, but are produced within the roots of the plant and emitted when the roots are agitated by touched either by human skin, or by moving soil, but not by touching the roots with other materials such as by glass or metal objects. Some of these sulfur compounds are to be found in onions and garlics.
THere are microscopic sac-like protuberances or hairs just 0.5mm long on the roots which contain a higher proportion of potassium K+ and Chlorine Cl- ions than surrounding tissue, and when the hairs are stimulated they release the highly potent odour substances. The levels of these ions both drop and the sacs deflate having expelled their odorous compounds. (The sacs are reminiscent of glandular trichomes as found on
Stinging Nettles) These ions are involved in the initiation of the emission of the odorous substances. Only a tiny amount of the odorous compounds is necessary to fill a room with a disgusting smell. It is not known whether the smell is targeted at predators and scavengers or to fend off the roots of competing plants. They do seem to be signalling molecules, but to what target or end is unknown. It may be that the smell (to humans and other mammals) is irrelevant.
Glandular root hairs which secrete small organic molecules have also been found on Sorghum plants and on Apple trees.
Sensitive Plant is not the only plant in the Mimosaceae family to emit smells when disturbed; 40 species from nine genera within the Mimosoidae sub-family produce
Carbon Disulfide, CS2 and 19 of those 40 produce
Carbonyl Sulfide, C=O=S. However,
Sensitive Plant is not one of the 40 Mimosaceae species to emit Carbon Disulfide. Carbon Disulfide is a flammable gas with an unpleasant smell which, in the presence of any water vapour contained in the air (due to humidity), decomposes into
Carbon Dioxide CO2 and another toxic flammable gas with obnoxious odour,
Hydrogen Sulfide H2S. Carbonyl Sulfide is released by deep ocean vents, volcanoes and by organisms in the ocean. Some is oxidized in the atmosphere to Sulfuric Acid H2SO4. Because it is continually created and destroyed, Carbonyl Sulfide exists in the atmosphere in a secular equilibrium - indeed it is the most abundant sulfur compound present in the air (at about 0.5ppb ± 10%) since. Carbonyl Sulfide is also given off by some cheese and by cooked vegetables from the Brassicaceae family (
It is thought, but not proven, that both
Carbon Disulfide and
Carbonyl Sulfide are produced when the
non-proteinogenic amino acid (NPAA)
Djenkolic Acid (present in the plant) is cleaved with the aid of an enzyme, yielding
Cysteine, ThioFormaldehyde and a Pyridinium ion, hydrolysis of which produces first
2-Amino-Acrylate (aka α-Amino-Acrylate), then Pyruvate and
The pyruvate is present as a salt of
Djenkolic Acid is yet another NPAA (non-proteinogenic amino acid) which also contains two atoms of sulfur. It is a near-dimer, with two symmetrical parts sharing only a central carbon atom, being
Cysteine radicals. It is toxic causing nephrotoxicity and is found in the non-native Jengkol beans, a legume (from the
Fabanaceae family) found in South-East Asia. It is toxic because of its insolubility which precipitates sharp needle crystals of Djenkolic Acid in the renal tract. It is thus a mechanical toxin rather than one which uses its chemistry to interact with the body. As such, it has a similar modus operandi to the raphides (sharp needle crystals) of Calcium Oxalate found in species of Oxalis (Wood-Sorrel) and in Rhubarb. If the beans are boiled before consumption this tragedy is avoided. Djenkolic Acid is also found in the seeds of other Legumes, such as
Leucaena esculenta and
Pithocolobium ondulatum, both non-native to the UK.
LOW MOLECULAR WEIGHT VOLATILES/GASES
Nitric Oxide, NO and Hydrogen Peroxide H2O2 or HO-OH are two other gases (besides Ethylene, HC=CH, discussed above) that are used extensively by plants as signalling molecules. Both
(hormones and/or signalling molecules)
Hydrogen peroxide and
Nitric Oxide are generated by a wide range of biological and non-biological stress processes. However, the means by which nitric oxide are generated by biological means has not yet been elucidated. The cell response to both hydrogen peroxide and to nitric oxide are both complex and little understood, but they seem to be manifold. The responses of plants to both signalling molecules is also manifold, but it is known that hydrogen peroxide is involved in cell-death.
The subject is so complicated and so little is known about the actions or of the modus-operandi of either substance that your Author directs those interested to a 2001 research paper in pdf format by Neill, Desikan, Clarke, Hurst and Hancock from the Journal of Experimental Botany: H2O2 & NO as signalling molecules in plants (pdf).
Small molecules tend to have a great many effects and interactions with other molecules, so it is little wonder that both NO and H2O2 have such diverse and little-understood effects when their interactions are manifold. Unlike many much larger molecules, which sometimes are specific to only certain plants, these small molecules can effect changes and responses in a great many different plants and organisms. They are ubiquitous if not universal messengers and initiators.
NO, NITRIC OXIDE
Nitric Oxide is functional in many biological systems including mammals and humans where, amongst other effects, it modulates the flow of blood. In plants it is now known to affect seed germination. It may also be involved in Nitrogen Fixation in the
root nodules of plants belonging to (mainly) the Pea Family. NO is pivotal in the control of stomatal openings in plants (which are used by the plant to control the release of oxygen or of water vapour and to control the ingress of carbon dioxide which the plant requires in order to grow). Nitric Oxide is also produced when plants are under attack from some pathogen suggesting a role in defence. It is hugely important in the control of reactive oxygen species (ROS - which includes hydrogen peroxide) generated when the plant during metabolism, particularly in regard to Glutathione a major anti-oxidant in plants. There are many other at present little understood roles of NO in plants.
H2O2, HYDROGEN PEROXIDE
Hydrogen Peroxide is one of those reactive oxygen species (ROS) produced in plants during metabolism. It is also produced at the site of a wound in plants in order to hyper-sensitize damaged cells to undergo cell-death (apoptosis) and also to stiffen cell-walls. It is active in plants exposed to UV-B light protecting them from damage by activating several cell pathways, via gene expression. There are similarly many other at present little understood roles of H2O2 in plants.
C2H4, H2C=CH2, ETHYLENE
Ethylene is a gas and almost universal hormone for the ripening of all sorts of fruit. Ethylene is produced within the plants cells and especially when they are rapidly growing and multiplying (dividing), even more so in the dark. Both new growth and newly germinated seedlings produce elevated levels of ethylene. The gas escapes from the plant where it can influence other parts of the same plant such as leaf stalks, which start to bend more upright when the lower surfaces grow faster than the upper surfaces (like a bimetallic strip this forces it to bend): a
As the parts grow taller and reach more light, the extra light causes
phytochrome to assume a differing form which then dampens down the production of ethylene gas allowing the leaf to grow again.
For seedlings that are still underground (which receive no light) ethylene production is greatly ramped up thickening up the stem to give it more rigidity and then able to force its way through to the surface, sometimes through tarmac or other obstacle! but if it still cannot get through the stem ceases growing upwards and grows around the obstacle until it can reach the surface.