HORMONES & SIGNALLING MOLECULES

 INDEX for HORMONES & SIGNALLING MOLECULES
Indoles & Indole Acetic Acids
Ascorbic Acid
Abscissic Acid
Karrikins
Gibberellins
Jasmonates
Phytochromes
Cytokinins
Salicylates
Strigolactones
Brassinolide
Sulfur Compounds
Volatiles / Gases

Phytohormones are plant hormones which govern the way the plant grows in some aspect or other. There are many differing kinds of phytohormones, with various modus-operandi. Auxins are a certain class of phytohormones, based upon Indole Acetic Acid and analogues. there are many other classes.

Signalling Molecules are usually volatile compounds which are able to travel through the air to other plants, where they may variously stimulate the growth of nearby similar plants, or suppress the growth of competing plants, or alert nearby plants of the same species to the fact that the plant emitting the volatile substance(s) is doing so because it is under attack from some pathogen and is signalling nearby similar plants to prepare defences against the pathogen(s). There may also be other circumstances where signalling molecules are emitted.

Some signalling molecules go through the soil or the root system to nearby plants or through the hyphae of fungi to other plants, with similar motives.


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.






ASCORBIC ACID


ASCORBIC and DEHYDROASCORBIC ACIDS
A 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


ABSCISIC ACID
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 synthesised within plants from Zeaxanthin via trans-ViolaXanthene to produce first Xanthoxin (not to be confused with Xanthotoxin) which is eventually transformed into Abscisic Acid.

PHASEIC 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.






KARRIKINS - plant growth regulators




KARRIKINS
Karrikins are lactones. Hot forest fires (and any burning plant material as long as it is hot enough) will generate substances called Karrikins, which are wafted around in the smoke. Karrikins (in particular KAR1) are plant growth regulators. They encourage and promote the germination of seedlings, which would be especially useful to the continued growth of plants razed to the ground by a forest fire, as long it didn't get too hot below the soil to damage the seed. These could be the seeds of trees burnt, or other seeds belonging to other plants that were also scorched. The triggering of seed germination by Karrikins requires the synthesis of Gibberellic Acid (another plant hormone - described below) and the presence of light. This is an adaptation by plants; they have 'learned' that there is a lot of sunshine that can percolate to ground level after a forest fire, and that this would now be an opportune time to dominate the scorched and razed land. They use the Karrikins which were within the smoke but may have been picked up by any rain-drops which watered any scattered seeds helping them to germinate. Karrikins are key germination triggers for many plant species prone to forest or grassland fires in hot climates, such as in the Metronome. Karrikins are able to trigger germination of certain plants such as Thale Cress (Arabidopsis thaliana) far more effectively than can the related previously known phyto-hormones the Strigolactones.

There are four known Karrikins (which are butenolides - [a five membered ring and lactone] which is fused to a 4H-Pyran (aka Oxine) ring [a six-membered ring]). They are known as KAR1, KAR2, KAR3 and KAR4. KAR2 is the simplest, lacking any additional methyl side-groups and chemically is ; KAR3 and KAR4 possess two methyl side-groups.






GIBBERELLINS


GIBBERELLIC ACID
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 synthesised from ent-Kaurene which possesses a 6-membered ring in place of the 5-membered ring.






JASMONATES


JASMONIC ACID
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.

METHYL JASMONATE
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.

ETHYLENE
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.






PHYTOCHROMES


PHYTOCHROME B - a signalling molecule

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 and  Phytochromes






CYTOKININS


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
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.

KINETIN
Kinetin, a 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
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
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 the 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
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

METHYL SALICYLATE
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
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


TURGORIN
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.

TURGORIN LMF1
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 revently 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), 2-AminoThioPhenol, PhenoThiazine, MethaneSulfonic Acid, Pyruvic Acid, Sulfinic Acid, EthaneSulfinic Acid, PropylSulfenic Acid, 2-AminoThioPhenol, 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 (Cabbage).


DJENKOLIC ACID
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 Ammonia (NH3).



The pyruvate is present as a salt of Pyruvic Acid.

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
(hormones and/or signalling molecules)

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 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 the 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-sensitise 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 hyponastic response.

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. .

HORMONES & SIGNALLING MOLECULES

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