The Effects of Hormones Vitamins and Minerals and Salts Thread

Discussion in 'Advanced Marijuana Cultivation' started by billy4479, Jan 28, 2013.


    billy4479 Moderator

    Hi Everybody Its been a while since I posted on here hope every body has been well .... So here it is a thread that probley has been a long time coming ..On here id like every body to post what they have seen learned and studied about Hormones Vitamins minerals and salts ...Plant nutrition Is my favorite field of study in the horticultural society many grower pick up a premixed bottle of 9-16 elements works great the use IBA based rooting hormones ,,they use cytokines and auxins .. found in deffernt products But only a few growers feel the need to ask how and why these work ... And what else works for Example the element platinum showed a 20 % increase in hydoponic tomatoes source hydroponic food production Arthur howard mresh phd very good book by the way highly recommend... Now most of us would grow broke in a cycle using a element like platinum ..But the reason for mentioning such a element is to inspire not to discourage the mind on what is possible in the world of plant nutrition ....Now lets be clear on what this thread is and is not ...This is not a thread claiming I know everthing there is to know and im your new grower god bow down before me no that is not what this thread is ..ThIs a thread saying I am humble wanting and willing to learn ..I have some to share and more to learn I hope you come here humble to willing to learn and at least entertain new idea To share what you know and to learn something new with each vist ..Is curiosity not a human trait is that not why you click on this thread to begin with ?

    BLOCKER Active Member

    Im a beginner wanting to learn about plant nutrition apparently cannabis specific nutes are over priced.. when its just different balances of NPK and micros.. where do i learn what a cannabis plant needs and where to find a cheap alternative to cannabis specific nutes?, thanks
    billy4479 likes this.

    Slipon Well-Known Member

    maybe look in to SS then ? (SS =super soil`s) there are many soil recipes out there (google)
    I have had good success with mixing a "organic" soil mix both for my Veg pots and flowering pots so I did`t had to add any nutritions beside some black strap molasses doing flowering

    stuff like Kelp Meal, Mearl/dolomite lime,worm casting, different guano`s humus, Epsom salt, vitimins, aso aso. I dont mind useing blood/bone Meal either some don't like it and use other stuff ..

    do some researches, much of the stuff you will need can be bough pretty cheap at plants schools aso
    I also know Ppl. who have there own compost/worm garden to make there own soil mix (if you have the space ?)

    most importent for weed IMO is that the PH is with in the right ranges (6-7) and have a good micro life and is airy and light (stuff like Perlite and lega nuts can be used here, I also like a bit of coco fibers)
    billy4479 likes this.

    billy4479 Moderator

    [h=2]The Big Five[/h]We’ll cover five major types of plant hormones: auxin, gibberellin, cytokinin, ethylene, and abscisic acid. These hormones can work together or independently to influence plant growth.
    [h=2]AUXIN[/h]You’ve seen auxin in action. Well you haven’t seen the actual auxin molecule itself with the naked eye, but you’ve seen what it can do to a plant grown near a window. Have you ever wondered how a plant bends towards sunlight? Well, it has to do with auxin in the stem. Darwin and his son were curious about it, too. (Published in: The Power and Movement in Plants) However, they didn’t know at the time what exactly was causing plants to bend toward the light. Auxin itself wasn’t discovered until the late 1920s, and it was the first of the 5 major types of plant hormones to be studied. Auxin has lots of jobs but most importantly it stimulates growth, and if a plant doesn’t naturally produce auxin itself, it will die. So you can see auxin is pretty important. The technical alias for auxin is indole-3-acetic acid or IAA (just incase you ever see it written is “IAA” – it means the same thing as “Auxin”).
    Auxin is involved in cell growth and cell expansion, so it is produced primarily in parts of the plant that are actively growing like the stem (specifically, the very tiptop of the stem). This is where it gets interesting. Auxin is transported (read: active process - requires energy) in one direction in a plant - downward from the top to the bottom, like a one-way road from the stem tip to the roots. It is the only plant hormone known to do this. Therefore the concentration of auxin is highest at the top of the plant and decreases as you get closer to the roots, this controls the overall shape of the plant and helps keep the primary stem of a plant the leader.
    Have you ever seen the top of a single stem of tree that is pruned sprout into more than 20 new stems? That is because auxin maintains apical dominance it prevents lots of lateral buds and branches from growing on the side of the stem. When you prune the primary stem of a plant, the source of the auxin is removed, then no single stem is dominant anymore – apical dominance is removed.
    Back to our bendy plant in the windowsill, remember how auxin is involved in making cells longer? Well auxin will move to the shaded side of the plant stem and cause those cells to grow longer, while the cells on the sunny side of the plant stay the same size. That will cause the plant to bend to one side – toward the sun!
    [h=2]GIBBERELLIN[/h]Gibberellin causes some similar effects in plants as auxin, but it is a very different hormone. Gibberellins were discovered originally in Japan. A fungus called Gibberella fujikuroi infected rice plants and caused them to grow too tall and fall over. The infectious fungus produced a chemical that stimulated the growth in rice plants. The chemical was isolated and named Gibberellin after the fungus. It was later found that plants naturally produce variations of these chemicals!
    Gibberellins play an important role in several developmental stages in plants, but their claim to fame is making stems longer. Gibberellins promote stem elongation between nodes on the stem. A node is a place on a stem where a leaf attaches, so gibberellins elongate the internodes. It is easiest to see the absence of gibberellin in dwarf plants and rosette plants – there is very little space between nodes on a stem and the leaves are clustered toward the base of the plant.
    What’s the big deal about knowing how to control stem elongation in plants? Well, when would it be beneficial to know how to make a plant stem shorter or longer? Biologists can prevent plants in a greenhouse from making gibberellins to keep them a manageable size. That’s handy. Or what if you’re a farmer and your business is something that comes from the stem of a plant? Longer stems would mean more profit for you, right? Gibberellins sprayed on sugar cane in Hawaii elongate the stem between the nodes. Longer stems mean more stored sugar. More sugar to sell means more coin! Knowing about plant hormones just makes cents!
    [h=2]CYTOKININ[/h]Who knew that fish could play a role in the discovery of a plant hormone? Aged herring sperm DNA can promote cell division. The molecule that is responsible for this was named kinetin. Soon after, a substance that had the same biological effect as kinetin was found in plants, it stimulated plant cells to divide when in culture with auxin. The substance was named cytokinin and it is involved in cell division and in the making of new plant organs, like a root or a shoot. Cytokinins are produced in the root apical meristems (very tip of the roots) and travel upward hitching a ride with water and traveling up the stem through the xylem. The movement of cytokinins is passive – it does not require energy!
    Cytokinins are like the fountain of youth in plants. They delay senescence or the natural aging process that leads to death in plants. In the cell cycle, cytokinins promote the movement from the G2 phase to the M phase. In other words, they encourage cells to divide!
    Cytokinins are involved in repair, too. If a plant becomes wounded, it can fix itself with the help of cytokinins and auxin. Remember how some hormones work together to affect plants? Well if the concentration of auxin and cytokinin are equal, then normal cell division will take place. If the concentration of auxin is greater than cytokinin then roots will form. If the concentration of auxin is less than cytokinin then shoots will form.
    [h=2]ETYLENE[/h]Have you ever noticed that if you put a really ripe, brown banana right next to a bunch of green bananas, the unripe bananas will ripen and turn yellow much faster? How does that happen? Well, the brown banana is communicating with the green bananas using a hormone called ethylene. Ethylene is a plant hormone that affects ripening and rotting in plants. It is a particularly interesting plant hormone because it exists as a gas. No other plant hormone is gaseous! Ethylene can be produced in almost any part of a plant, and can diffuse through the plant’s tissue, outside the plant, and travel through the air to affect a totally different plant. How cool is that!
    Here’s how it was discovered. Tomato farmers noticed something weird happening with their crops. Back in the day many farmers used kerosene heaters in their greenhouses to warm the air so that they could grow tomatoes during the winter. With the advent of electricity, some farmers switched to new, fancy electric heaters, but they soon found that their tomatoes were not ready to be picked at the same time the way they were when the greenhouses were warmed with kerosene heaters. The burning of the kerosene in the heaters produced a molecule similar to ethylene that synchronized the ripening of the tomatoes!
    The formation of ethylene requires oxygen, and the agricultural industry has used this tidbit of information to their advantage. If you control the partial pressure of oxygen and carbon dioxide in a truck carrying produce (specifically low O2 high CO2) you can prevent ethylene synthesis and thus slow the ripening process. This is helpful when fruits and vegetables are grown in one region of the world and then shipped many miles away to be sold. Growers don’t want their produce to go bad before you even have a chance to buy it!
    [h=2]ABSCISIC ACID[/h]When our bodies need water we feel thirsty. The “thirst signal” signifies that we’re dehydrated and we need a drink of water. When a plant needs water, for example during a drought, it doesn’t have too many options. A rain dance is pretty much out of the question. Plants produce a chemical messenger, called abscisic acid, to alert the rest of the plant that it is water stressed. Abscisic acid is made in droughted leaves, droughted roots, and developing seeds and it can travel both up and down in a plant stem in the xylem or phloem sounding the alarm.
    Think back to transport in plants, how does water typically move through a plant? (Reminder: soil -> roots -> stem -> leaves -> air) Water molecules exit a plant through tiny pores in the leaves called stomata. Each stoma (singular) has two kidney bean shaped bodyguards on either side of the pore, whose job it is to open and close the stoma. When the guard cells are full of water, or turgid, the stoma is open. When water leaves the guard cells, they become flaccid, and the stoma is closed.
    Now imagine you’re a thirsty plant. It hasn’t rained in weeks and there is no moisture in the soil around your roots. You’re running dangerously low on water. What can you do to prevent yourself from losing any more precious H2O? Close the stomata! How do plants do it? Abscisic acid travels to the guard cells, sending a message that water is scarce. The guard cells spring to attention, and a rush of charged particles exit the guard cells, which subsequently triggers water inside the guard cell to leave, too. The guard cells shrivel and the stomata close! No more water is able to exit the plant through the stomata.
    That’s a brief overview on the five major types of plant hormones: auxin, gibberellin, cytokinin, ethylene, and abscisic acid. Remember that hormones are potent little chemical messengers, but they would lose their effectiveness if they hung around and built up in the tissues of the plant. So they are broken down and replaced over time.
    There is so much more to learn about plant hormones! A great textbook for those who want all the wonderful nitty-gritty details is Plant Physiology by Taiz and Zeiger.

    Cannabiology Member

    I think one that you missed, which may be of particular interest to people on this site is Florigen - the hormone responsible for inducing flowering in photo period plant species (such as, oh, lets say, cannabis?).
    Transcription of the Florigen gene doesn't occur until after 12 hours of perpetual darkness, hence the 12/12 cycle, 12 hour minimum to get the plants to flower. I would assume it to be possible to apply such a hormone to a plant, while keeping it on a 18/6 cycle, or even 24 hour, and in theory, it would still produce flowers. In such a case, I could see the benefits of flowering a plant under 18/6, because if it were still able to flower while receiving the extra light, its possible that this will indeed create larger buds. Of course, this hormone has yet to be synthetically produced, so we have to seek alternatives...
    One thing that comes to mind, is that hormones are signalling compounds and are naturally very mobile within the vascular system of any organism. So I would think, if you were to somehow able to get half of a plant on a 12/12 cycle, and induce flowering, that the florigen hormone would be then influencing the entire plant, therefor causing the entire plant to flower, which only exposing smaller portions of it to a 12/12 light cycle.
    billy4479 and mushead like this.

    shagalicious Active Member

    i believe florigen production begins after 2 hours of total dakness and if interrupted cannot begin anew for 2 more
    genetics would dictate the rate of production and the plant's threshold for veg-flower transition

    awesome thread btw
    billy4479 likes this.

    billy4479 Moderator

    That sounds pretty amazing I think I remember reading about a graft where on plant was in one room on a 12/12 light cycle and was grafted to anther plant in a room with 18/6 on the light cycle.. Both plants flowered because flowering was induced by a hormone shared by both plants ...Its been years since I read that study and had almost forgot it thanks for sharing your knowledge friend ..

    billy4479 Moderator

    Plant hormones help coordinate growth, development, and responses to environmental stimuli.

    • In general, plant hormones control plant growth and development by affecting the division, elongation, and differentiation of cells.
      • Some hormones also mediate shorter-term physiological responses of plants to environmental stimuli.
      • Each hormone has multiple effects, depending on its site of action, its concentration, and the developmental stage of the plant.
    • Some of the major classes of plant hormones include auxin, cytokinins, gibberellins, brassinosteroids, abscisic acid, and ethylene.
      • Many molecules that function in plant defenses against pathogens are probably plant hormones as well.
      • Plant hormones tend to be relatively small molecules that are transported from cell to cell across cell walls, a pathway that blocks the movement of large molecules.
    • Plant hormones are produced at very low concentrations.
      • Signal transduction pathways amplify the hormonal signal many-fold and connect it to a cell’s specific responses.
      • These include altering the expression of genes, affecting the activity of existing enzymes, or changing the properties of membranes.
    • Response to a hormone usually depends not so much on its absolute concentration as on its relative concentration compared to other hormones.
      • It is hormonal balance, rather than hormones acting in isolation, that control growth and development of the plants.
    • The term auxin is used for any chemical substance that promotes the elongation of coleoptiles, although auxins actually have multiple functions in both monocots and dicots.
      • The natural auxin occurring in plants is indoleacetic acid, or IAA.
    • In growing shoots, auxin is transported unidirectionally, from the shoot apex down to the base.
      • The speed at which auxin is transported down the stem from the shoot apex is about 10 mm/hr, a rate that is too fast for diffusion, but slower than translocation in the phloem.
      • Auxin seems to be transported directly through parenchyma tissue, from one cell to the next.
      • This unidirectional transport of auxin is called polar transport, and has nothing to do with gravity.
        • Auxin travels upward if a stem or coleoptile is placed upside down.
      • The polarity of auxin transport is due to the polar distribution of auxin transport protein in the cells.
      • Concentrated at the basal end of the cells, auxin transporters move the hormone out of the cell and into the apical end of the neighboring cell.
    • Although auxin affects several aspects of plant development, one of its chief functions is to stimulate the elongation of cells in young shoots.
      • The apical meristem of a shoot is a major site of auxin synthesis.
      • As auxin moves from the apex down to the region of cell elongation, the hormone stimulates cell growth, binding to a receptor in the plasma membrane.
      • Auxin stimulates cell growth only over a certain concentration range, from about 10?8 to 10?4 M.
      • At higher concentrations, auxins may inhibit cell elongation, probably by inducing production of ethylene, a hormone that generally acts as an inhibitor of elongation.
    • According to the acid growth hypothesis, in a shoot’s region of elongation, auxin stimulates plasma membrane proton pumps, increasing the voltage across the membrane and lowering the pH in the cell wall.
      • Lowering the pH activates expansin enzymes that break the cross-links between cellulose microfibrils and other cell wall constituents, loosening the wall.
      • Increasing the membrane potential enhances ion uptake into the cell, which causes the osmotic uptake of water.
      • Uptake of water increases turgor and elongates the loose-walled cell.
    • Auxin also alters gene expression rapidly, causing cells in the region of elongation to produce new proteins within minutes.
      • Some of these proteins are short-lived transcription factors that repress or activate the expression of other genes.
      • Auxin stimulates a sustained growth response of making the additional cytoplasm and wall material required by elongation.
    • Auxins are used commercially in the vegetative propagation of plants by cuttings.
      • Treating a detached leaf or stem with rooting powder containing auxin often causes adventitious roots to form near the cut surface.
      • Auxin is also involved in the branching of roots.
        • One Arabidopsis mutant that exhibits extreme proliferation of lateral roots has an auxin concentration 17-fold higher than normal.
    • Synthetic auxins, such as 2,4-dinitrophenol (2,4-D), are widely used as selective herbicides.
      • Monocots, such as maize or turfgrass, can rapidly inactivate these synthetic auxins.
      • However, dicots cannot and die from a hormonal overdose.
        • Spraying cereal fields or turf with 2,4-D eliminates dicot (broadleaf) weeds such as dandelions.
    • Auxin also affects secondary growth by inducing cell division in the vascular cambium and by influencing the growth of secondary xylem.
    • Developing seeds synthesize auxin, which promotes the growth of fruit.
      • Synthetic auxins sprayed on tomato vines induce development of seedless tomatoes because the synthetic auxins substitute for the auxin normally synthesized by the developing seeds.
    • Cytokinins stimulate cytokinesis, or cell division.
      • They were originally discovered in the 1940s by Johannes van Overbeek, who found that he could stimulate the growth of plant embryos by adding coconut milk to his culture medium.
      • A decade later, Folke Skoog and Carlos O. Miller induced cultured tobacco cells to divide by adding degraded samples of DNA.
      • The active ingredients in both were modified forms of adenine, one of the components of nucleic acids.
      • These growth regulators were named cytokinins because they stimulate cytokinesis.
    • The most common naturally occurring cytokinin is zeatin, named from the maize (Zea mays) in which it was found.
    • Much remains to be learned about cytokinin synthesis and signal transduction.
    • Cytokinins are produced in actively growing tissues, particularly in roots, embryos, and fruits.
      • Cytokinins produced in the root reach their target tissues by moving up the plant in the xylem sap.
    • Cytokinins interact with auxins to stimulate cell division and differentiation.
      • In the absence of cytokinins, a piece of parenchyma tissue grows large, but the cells do not divide.
      • In the presence of cytokinins and auxins, the cells divide, while cytokinins alone have no effect.
        • If the ratio of cytokinins and auxins is at a specific level, then the mass of growing cells, called a callus, remains undifferentiated.
        • If cytokinin levels are raised, shoot buds form from the callus.
        • If auxin levels are raised, roots form.
    • Cytokinins, auxins, and other factors interact in the control of apical dominance, the ability of the terminal bud to suppress the development of axillary buds.
      • Until recently, the leading hypothesis for the role of hormones in apical dominance—the direct inhibition hypothesis—proposed that auxin and cytokinin act antagonistically in regulating axillary bud growth.
      • Auxin levels would inhibit axillary bud growth, while cytokinins would stimulate growth.
    • Many observations are consistent with the direct inhibition hypothesis.
      • If the terminal bud, the primary source of auxin, is removed, the inhibition of axillary buds is removed and the plant becomes bushier.
        • This can be inhibited by adding auxins to the cut surface.
    • The direct inhibition hypothesis predicts that removing the primary source of auxin should lead to a decrease in auxin levels in the axillary buds.
    • However, experimental removal of the terminal shoot (decapitation) has not demonstrated this.
      • In fact, auxin levels actually increase in the axillary buds of decapitated plants.
      • Further research is necessary to uncover all pieces of this puzzle.
    • Cytokinins retard the aging of some plant organs.
      • They inhibit protein breakdown by stimulating RNA and protein synthesis and by mobilizing nutrients from surrounding tissues.
      • Leaves removed from a plant and dipped in a cytokinin solution stay green much longer than otherwise.
      • Cytokinins also slow deterioration of leaves on intact plants.
      • Florists use cytokinin sprays to keep cut flowers fresh.
    • A century ago, farmers in Asia noticed that some rice seedlings grew so tall and spindly that they toppled over before they could mature and flower.
      • In 1926, E. Kurosawa discovered that a fungus in the genus Gibberella causes this “foolish seedling disease.”
      • The fungus induced hyperelongation of rice stems by secreting a chemical, given the name gibberellin.
    • In the 1950s, researchers discovered that plants also make gibberellins. Researchers have identified more than 100 different natural gibberellins.
      • Typically each plant produces a much smaller number.
      • Foolish rice seedlings, it seems, suffer from an overdose of growth regulators normally found in lower concentrations.
    • Roots and leaves are major sites of gibberellin production.
      • Gibberellins stimulate growth in both leaves and stems but have little effect on root growth.
      • In stems, gibberellins stimulate cell elongation and cell division.
      • One hypothesis proposes that gibberellins stimulate cell wall–loosening enzymes that facilitate the penetration of expansin proteins into the cell well.
      • Thus, in a growing stem, auxin, by acidifying the cell wall and activating expansins, and gibberellins, by facilitating the penetration of expansins, act in concert to promote elongation.
    • The effects of gibberellins in enhancing stem elongation are evident when certain dwarf varieties of plants are treated with gibberellins.
      • After treatment with gibberellins, dwarf pea plants grow to normal height.
      • However, if gibberellins are applied to normal plants, there is often no response, perhaps because these plants are already producing the optimal dose of the hormone.
    • The most dramatic example of gibberellin-induced stem elongation is bolting, the rapid formation of the floral stalk.
      • In their vegetative state, some plants develop in a rosette form with a body low to the ground with short internodes.
      • As the plant switches to reproductive growth, a surge of gibberellins induces internodes to elongate rapidly, which elevates the floral buds that develop at the tips of the stems.
    • In many plants, both auxin and gibberellins must be present for fruit to set.
      • Spraying of gibberellin during fruit development is used to make the individual grapes grow larger and to make the internodes of the grape bunch elongate.
        • This enhances air circulation between the grapes and makes it harder for yeast and other microorganisms to infect the fruits.
    • The embryo of a seed is a rich source of gibberellins.
      • After hydration of the seed, the release of gibberellins from the embryo signals the seed to break dormancy and germinate.
      • Some seeds that require special environmental conditions to germinate, such as exposure to light or cold temperatures, will break dormancy if they are treated with gibberellins.
      • Gibberellins support the growth of cereal seedlings by stimulating the synthesis of digestive enzymes that mobilize stored nutrients.
    • First isolated from Brassica pollen in 1979, brassinosteroids are steroids chemically similar to cholesterol and the sex hormones of animals.
      • Brassinosteroids induce cell elongation and division in stem segments and seedlings at concentrations as low as 10?12 M.
      • They also retard leaf abscission and promote xylem differentiation.
      • Their effects are so qualitatively similar to those of auxin that it took several years for plant physiologists to accept brassinosteroids as nonauxin hormones.
    • Joann Chory and her colleagues provided evidence from molecular biology that brassinosteroids were plant hormones.
      • An Arabidopsis mutant that has morphological features similar to light-grown plants even when grown in the dark lacks brassinosteroids.
      • This mutation affects a gene that normally codes for an enzyme similar to one involved in steroid synthesis in mammalian cells.
    • Abscisic acid (ABA) was discovered independently in the 1960s by one research group studying bud dormancy and another investigating leaf abscission (the dropping of autumn leaves).
      • Ironically, ABA is no longer thought to play a primary role in either bud dormancy or leaf abscission, but it is an important plant hormone with a variety of functions.
      • ABA generally slows down growth.
      • Often ABA antagonizes the actions of the growth hormones—auxins, cytokinins, and gibberellins.
      • It is the ratio of ABA to one or more growth hormones that determines the final physiological outcome.
    • One major affect of ABA on plants is seed dormancy.
      • The levels of ABA may increase 100-fold during seed maturation, leading to inhibition of germination and the production of special proteins that help seeds withstand the extreme dehydration that accompanies maturation.
      • Seed dormancy has great survival value because it ensures that the seed will germinate only when there are optimal conditions of light, temperature, and moisture.
    • Many types of dormant seeds will germinate when ABA is removed or inactivated.
      • For example, the seeds of some desert plants break dormancy only when heavy rains wash ABA out of the seed.
      • Other seeds require light or prolonged exposure to cold to trigger the inactivation of ABA.
      • A maize mutant that has seeds that germinate while still on the cob lacks a functional transcription factor required for ABA to induce expression of certain genes.
    • ABA is the primary internal signal that enables plants to withstand drought.
      • When a plant begins to wilt, ABA accumulates in leaves and causes stomata to close rapidly, reducing transpiration and preventing further water loss.
      • ABA causes an increase in the opening of outwardly directed potassium channels in the plasma membrane of guard cells, leading to a massive loss of potassium.
      • The accompanying osmotic loss of water leads to a reduction in guard cell turgor, and the stomata close.
      • In some cases, water shortages in the root system can lead to the transport of ABA from roots to leaves, functioning as an “early warning system.”
      • Mutants that are prone to wilting are often deficient in ABA production.
    • In 1901, Dimitry Neljubow demonstrated that the gas ethylene was the active factor that caused leaves to drop from trees that were near leaking gas mains.
      • Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection.
      • Ethylene production also occurs during fruit ripening and during programmed cell death.
      • Ethylene is also produced in response to high concentrations of externally applied auxins.
    • Ethylene instigates a seedling to perform a growth maneuver called the triple response that enables a seedling to circumvent an obstacle as it grows through soil.
    • Ethylene production is induced by mechanical stress on the stem tip.
    • In the triple response, stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally.
    • As the stem continues to grow horizontally, its tip touches upward intermittently.
      • If the probes continue to detect a solid object above, then another pulse of ethylene is generated, and the stem continues its horizontal progress.
      • If upward probes detect no solid object, then ethylene production decreases, and the stem resumes its normal upward growth.
    • It is ethylene, not the physical obstruction per se, that induces the stem to grow horizontally.
      • Normal seedlings growing free of all physical impediments will undergo the triple response if ethylene is applied.
    • Arabidopsis mutants with abnormal triple responses have been used to investigate the signal transduction pathways leading to this response.
      • Ethylene-insensitive (ein) mutants fail to undergo the triple response after exposure to ethylene.
        • Some lack a functional ethylene receptor.
    • Other mutants undergo the triple response in the absence of physical obstacles.
      • Some mutants (eto) produce ethylene at 20 times the normal rate.
      • Other mutants, called constitutive triple-response (ctr) mutants, undergo the triple response in air but do not respond to inhibitors of ethylene synthesis.
        • Ethylene signal transduction is permanently turned on even though there is no ethylene present.
    • The various ethylene signal-transduction mutants can be distinguished by their different responses to experimental treatments.
    • The affected gene in ctr mutants codes for a protein kinase.
      • Because this mutation activates the ethylene response, this suggests that the normal kinase product of the wild-type allele is a negative regulator of ethylene signal transduction.
      • One hypothesis proposes that binding of the hormone ethylene to a receptor leads to inactivation of the kinase and inactivation of this negative regulator allows synthesis of the proteins required for the triple response.
    • The cells, organs, and plants that are genetically programmed to die on a particular schedule do not simply shut down their cellular machinery and await death.
      • Rather, during programmed cell death, called apoptosis, there is active expression of new genes, which produce enzymes that break down many chemical components, including chlorophyll, DNA, RNA, proteins, and membrane lipids.
      • A burst of ethylene productions is associated with apoptosis whether it occurs during the shedding of leaves in autumn, the death of an annual plant after flowering, or as the final step in the differentiation of a xylem vessel element.
    • The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when roots cannot absorb water from the frozen ground.
      • Before leaves abscise, many essential elements are salvaged from the dying leaves and stored in stem parenchyma cells.
      • These nutrients are recycled back to developing leaves the following spring.
    • When an autumn leaf falls, the breaking point is an abscission layer near the base of the petiole.
      • The parenchyma cells here have very thin walls, and there are no fiber cells around the vascular tissue.
      • The abscission layer is further weakened when enzymes hydrolyze polysaccharides in the cell walls.
      • The weight of the leaf, with the help of the wind, causes a separation within the abscission layer.
    • A change in the balance of ethylene and auxin controls abscission.
      • An aged leaf produces less and less auxin, and this makes the cells of the abscission layer more sensitive to ethylene.
      • As the influence of ethylene prevails, the cells in the abscission layer produce enzymes that digest the cellulose and other components of cell walls.
    • The consumption of ripe fruits by animals helps disperse the seeds of flowering plants.
      • Immature fruits are tart, hard, and green but become edible at the time of seed maturation, triggered by a burst of ethylene production.
      • Enzymatic breakdown of cell wall components softens the fruit, and conversion of starches and acids to sugars makes the fruit sweet.
      • The production of new scents and colors helps advertise fruits’ ripeness to animals, which eat the fruits and disperse the seeds.
    • A chain reaction occurs during ripening: ethylene triggers ripening and ripening, in turn, triggers even more ethylene production—a rare example of positive feedback on physiology.
      • Because ethylene is a gas, the signal to ripen even spreads from fruit to fruit.
      • Fruits can be ripened quickly by storing the fruit in a plastic bag, accumulating ethylene gas, or by enhancing ethylene levels in commercial production.
      • Alternatively, to prevent premature ripening, apples are stored in bins flushed with carbon dioxide, which prevents ethylene from accumulating and inhibits the synthesis of new ethylene.
    • Genetic engineering of ethylene signal transduction pathways has potentially important commercial applications after harvest.
      • For example, molecular biologists have blocked the transcription of one of the genes required for ethylene synthesis in tomato plants.
      • These tomato fruits are picked while green and are induced to ripen on demand when ethylene gas is added.
    • Plant responses often involve interactions of many hormones and their signal transduction pathways.
      • The study of hormone interactions can be a complex problem.
      • For example, flooding of deepwater rice leads to a 50-fold increase in internal ethylene and a rapid increase in stem elongation.
        • Flooding also leads to an increase in sensitivity to GA that is mediated by a decrease in ABA levels.
        • Thus, stem elongation is the result of interaction among three hormones and their signal transduction chains.
      • Imagine that you are a molecular biologist assigned the task of genetically engineering a rice plant that will grow faster when submerged.
        • What is the best molecular target for genetic manipulation? Is it an enzyme that inactivates ABA, an ethylene receptor, or an enzyme that produces more GA?
      • Many plant biologists are promoting a systems-based approach.
        • Using genomic techniques, biologists can identify all the genes in a plant.
        • Two plants are already sequenced: Arabidopsis and the rice plant Oryza sativa.
        • Using microassay and proteomic techniques, scientists can determine which genes are inactivated or activated in response to an environmental change.
      • New hypotheses and approaches will emerge from analysis of these comprehensive data sets.

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