plant hormones and growth in plants Flashcards
chemical coordination
Plants are multicellular organisms living in a complex and ever-changing environment. The key limitations on plants are that they are rooted - they are not mobile, and they do not have a rapidly responding nervous system. They are, however, coordinated organisms that show clear responses to their environment, communication between cells, and even communication between different plants.
The timescales of most plant responses are slower than animal responses, but they still respond as a result of complex chemical interactions. Plants have evolved a system of hormones - chemicals that are produced in one region of the plant and transported both through the transport tissues and from cell to cell and have an effect in another part of the plant. Important plant hormones include auxins, gibberellins, abscisic acid (ABA), and ethene. These chemicals have a wide range of functions within the plant.
Plants produce chemicals which signal to other species - for example, to protect themselves from attack by insect pests - and may communicate with other plants. They also produce chemical defences against herbivores. In plant responses, chemicals are essential.
The growth of plants, from the germination of a seed to the long-term growth of a tree, is controlled by plant hormones. You will look at the different chemicals and their roles in isolation, but in fact the growth and form of a plant are the result of the interaction of many different hormonal and environmental factors.
Scientists are still unsure about the details of many plant responses.
There are a number of reasons for this. Plant hormones work at very low concentrations, so isolating them and measuring changes in concentrations is not easy. The multiple interactions between the different chemical control systems also make it very difficult for researchers to isolate the role of a single chemical in a specific response.
Outlined are a number of key aspects of plant growth with the role of hormones highlighted.
some roles of plant hormones
auxins- control cell elongation, prevent leaf fall (abscission), maintain apical dominance, involved in tropisms, stimulate the release of ethane, involved in fruit ripening
gibberellin-cause stem elongation, trigger the mobilisation of food stores in a seed at germination, stimulate pollen tube growth in fertilisation
ethene-causes fruit ripening, promotes abscission in deciduous trees
ABA (abscisic acid)-maintains dormancy of seeds and buds, stimulates cold protective responses, for example, antifreeze production, stimulates stomatal closing
plant hormones and seed germination
For a plant to start growing, the seed must germinate.
• When the seed absorbs water, the embryo is activated and begins to produce gibberellins. They in turn stimulate the production of enzymes that break down the food stores found in the seed. The food store is in the cotyledons in dicot seeds and the endosperm in monocot seeds. The embryo plant uses these food stores to produce ATP for building materials so it can grow and break out through the seed coat. Evidence suggests that gibberellins switch on genes which code for amylases and proteases - the digestive enzymes required for germination. There is also evidence suggesting that another plant hormone, ABA, acts as an antagonist to gibberellins (interferes with the action of gibberellin), and that it is the relative levels of both hormones which determine when a seed will germinate.
experimental evidence
Experimental evidence supporting the role of gibberellins in the germination of seeds includes:
-Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins. These seeds do not germinate. If gibberellins are applied to the seeds externally, they then germinate normally.
• If gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate as they cannot make the gibberellins needed for them to break dormancy. If the inhibition is removed, or gibberellins are applied, the seeds germinate.
auxins
Auxins such as indoleacetic acid (IAA) are growth stimulants produced in plants. Small quantities can have powerful effects. They are made in cells at the tip of the roots and shoots, and in the meristems. Auxins can move down the stem and up the root both in the transport tissue and from cell to cell. The effect of the auxin depends on its concentration and any interactions it has with other hormones. Auxins have a number of major effects on plant growth.
They stimulate the growth of the main, apical shoot. Evidence suggests that auxins affect the plasticity of the cell wall - the presence of auxins means the cell wall stretches more easily.
Auxin molecules bind to specific receptor sites in the plant cell membrane, causing a fall in the pH to about 5. This is the optimum pH for the enzymes needed to keep the walls very flexible and plastic. As the cells mature, auxin is destroyed. As the hormone levels fall, the pH rises so the enzymes maintaining plasticity become inactive. As a result, the wall becomes rigid and more fixed in shape and size and the cells can no longer expand and grow.
• High concentrations of auxins suppress the growth of lateral shoots. This results in apical dominance. Growth in the main shoot is stimulated by the auxin produced at the tip so it grows quickly.
The lateral shoots are inhibited by the hormone that moves back down the stem, so they do not grow very well. Further down the stem, the auxin concentration is lower and so the lateral shoots grow more strongly. There is a lot of experimental evidence for the role of auxins in apical dominance. For example, if the apical shoot is removed, the auxin-producing cells are removed and so there is no auxin. As a result, the lateral shoots, freed from the dominance of the apical shoot, grow faster. If auxin is applied artificially to the cut apical shoot, apical dominance is reasserted and lateral shoot growth is suppressed.
Low concentrations of auxins promote root growth. Up to a given concentration, the more auxin that reaches the roots, the more they grow. Auxin is produced by the root tips and auxin also reaches the roots in low concentrations from the growing shoots. If the apical shoot is removed, then the amount of auxin reaching the roots is greatly reduced and root growth slows and stops. Replacing the auxin artificially at the cut apical shoot restores the growth of the roots. High auxin concentrations inhibit root growth.
gibberellins
As you have learnt, gibberellins are involved in the germination of seeds. They are also important in the elongation of plant stems during growth. Gibberellins affect the length of the internodes - the regions between the leaves on a stem. Gibberellins were discovered because they are produced by a fungus from the genus Gibberella that affects rice. The infected seedlings grew extremely tall and thin. Scientists investigated the rice and isolated chemicals - gibberellins - which produce the same spindly growth in the plants. It was then discovered that plants themselves produce the same compounds.
Plants that have short stems produce few or no gibberellins. There are well over a hundred different naturally produced gibberellins. Scientists have bred many dwarf varieties of plants where the gibberellin synthesis pathway is interrupted. Without gibberellins the plant stems are much shorter. This reduces waste and also makes the plants less vulnerable to damage by weather and harvesting.
synergism and antagonism
Most plant hormones do not work on their own but by interacting with other substances. In doing so, very fine control over the responses of the plant can be achieved. If different hormones work together, complementing each other and giving a greater response than they would on their own, the interaction is known as synergism. If the substances have opposite effects, for example one promoting growth and one inhibiting it, the balance between them will determine the response of the plant. This is known as antagonism. Our knowledge of plant hormones and the mechanisms by which they have an effect is still far from complete - this is an active and important area of research.
