Plant Responses

1. Plants must respond to the environment in order to avoid stress, avoid being eaten and to survive long enough to reproduce. Plants must respond to both biotic factors (living components of the environment) and abiotic factors (non-living components of the environment such as high temperatures and drought). Plant Responses Nastic movements are plant movements that occur due to environmental stimuli. Unlike tropic movements, the direction of the response is not dependent upon the direction of the stimulus. Nastic movements are generally caused by changes in the turgidity of cells. In the venus flytrap plant, action potentials triggered by contact between insects and the hairs of their leaves cause cells at the hinge of the leaf to take up water and swell, causing the leaf to close. • Tropisms are directional growth responses in which the direction of the response is determined by the direction of the external stimulus. • Phototropism – shoots are positively phototrophic – they grow towards light. • Geotropism – roots are positively geotropic – they grow towards the pull of gravity. • Chemotropism – on a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place. • Thigmotropism – shoots of climbing plants such as ivy, wind around other plants or solid structures and gain support.

2. Auxin: Phototropism • Phototropism is the directional growth response towards or away from light. Most plant shoots grow towards light, whereas roots grow away from it. The distribution of auxin in a plant explains how response occurs: • Proteins called phototropins sit in the membranes of shoot cells. When they are hit by blue light, they become phosphorylated. If the light is coming from one side only, the phototropin on one side becomes phosphorylated while that on the shady side does not. • The phosphorylation of phototropin brings about a sideways movement of auxin. More auxin accumulates in the shady side of the shoot. • The presence of auxin stimulates cell elongation , so where there is more auxin, there is more growth. How do Auxins work? Auxins stimulate growth by cell elongation . Auxin increases the stretchiness of the cell wall by promoting the active transport of hydrogen ions into the cell wall. The resulting low pH provides the optimum conditions for wall loosening enzymes, expansins, to work. They break bonds within the cellulose so the walls so they become less rigid and can expand and become longer as the cell takes in water. This is a permanent effect.

3. Leaf Abscission Leaf abscission is when a tress drops its leaves. Many trees drop their leaves in autumn, which helps plants to survive through the winter by reducing water loss, reducing frost damage and avoiding fungal infections through damp leaves. Cytokinins stop the leaves of deciduous trees from senescing (ageing – turning brown and dying). They do this by making sure that the leaf acts as a sink for phloem transport, meaning that the leaf is guaranteed a good supply of nutrients. Ethene is an unusual hormone as it is a gas: it diffuses through the air spaces between cells. It tends to be produced by maturing or ageing fruits or leaves. Usually, auxins inhibit abscission: as long as a leaf is producing plenty of auxin, it will not fall off the tree. As autumn approaches and leaves age, the rate of auxin production declines. A drop in auxin levels makes the leaf more sensitive to ethene levels. More ethene is then produced and this in turn inhibits auxin production. There has therefore been a change in the balance of auxin and ethene – less auxin and more ethene. As a result, an abscission layer grows at the base of the leaf stalk (petiole). This is made of thin walled cells which are then weakened by the enzyme cellulase, which digests the walls, separating the petiole form the stem. The leaf has also grown a layer of protective tissue, the cells of which have suberin in their walls to prevent the entry of pathogens.

4. Apical Dominance Plants grow length ways by the division and elongation of cells. A region near the tip of the shoot called the apical meristem contains small, undifferentiated cells that are constantly dividing. Lateral buds are on either side of plant shoots. They also contain meristems, but they don’t become active until as long as the apical meristem is in position and functioning. Apical bud removed; lateral buds grow The presence and growth of the apex of the shoot inhibits sideways growth from the lateral buds. • The mechanism of apical dominance was thought to be related to auxin that is produced in the apical meristem: normal auxin concentrations of auxin in the lateral buds inhibit growth and low auxin concentrations promote growth. Evidence for this mechanism of apical dominance: • If we cut off the tip of a shoot and apply synthetic auxin, the shoot will continue to show apical dominance • If auxin transport inhibitor is applied below the apex of the shoot, the lateral buds will grow. • However, a scientist called Gocal disproved this direct causative link, and now, 2 other hormones are thought to be involved: • Abscisic acid inhibits bud growth: High concentrations of auxins in the shoot may keep abscisic levels high in the bud. When the tip is removed (the source of auxin), the abscisic acid levels fall and the bud starts to grow. • Cytokinins promote bud growth: high concentrations of auxin make the shoot apex a sink for cytokinins produced int he roots, so most of the cytokinins will go to the shoot apex. When the apex is removed, cytokinin spreads more evenly around the plant, promoting growth in the buds.

5. Gibberellins and Stem Elongation Gibberellin causes the rapid growth of stems in some kinds of plant. If gibberellin is applied to the stems of dwarf bean plants, the stems begin to grow rapidly. The stems get longer as the lengths of the internodes increase. • Experimental Evidence • So it’s been found that gibberellin can cause stem elongation, but how do we know it does so in nature? • Scientists firstly investigated gibberellin concentrations in small (homozygous recessive for ‘le’ allele) and tall pea plants (homozygous dominant for ‘Le’ allele), that were otherwise genetically identical. The plants with higher gibberellin concentrations were taller. • In order to synthesise gibberellins, plants must convert a substance known as GA20 to GA1. Scientists took a plant that had a mutation that meant GA20 couldn’t be produced. They grafted its shoot onto a homozygous recessive plant which could not convert GA20 to GA1. The new plant produced after the graft grew tall: the grafted shoot has provided the enzyme that allows GA20 to be converted to GA1, and all of the GA20 present in the plant can now be converted. • This confirmed that gibberellins directly causes stem elongation. Gibberellins have been shown to cause growth in the internodes by stimulating cell elongation (by loosening cell walls) and cell division (by stimulating production of a protein that controls the cell cycle)

6. Commercial Uses of Plant Hormones • Auxin • Sprayed onto developing fruits to prevent abscission – fruits stay on toe plant for longer • Sprayed onto unpollinated flowers to promote the growth of seedless fruits (parthenocarpy) • Artificial auxins used as selective herbicides . Synthetic auxins are transported in the phloem to all parts of the plant and they can act within the plant for longer as they are not a close fit for the enzymes that break them down. It is also thought that they may be able to enter cells through transporters in the plasma membrane, but cannot leave through the transporters that would usually allow natural auxins out by facilitated diffusion. The build up of the artificial auxin may be what kills the plant. Cytokinins can delay leaf senescence and so are sometimes used to prevent the yellowing of lettuce leaves after they have been picked. Cytokinins are also used in tissue culture to help mass produce plants. Ethenepromotes fruit ripening and promotes fruit drop in cotton, cherry and walnut. • Gibberellins • Delay senescence in citrus fruits, extending the time fruits can be left unpicked and making them available for longer in shops • Increasing yield of sucrose from sugar cane : gibberellin makes the stems of sugar cane elongate, and as sucrose is stored in the internodes of the stems, more sucrose is present. • Brewing: gibberellin stimulates germination in barley grains . Germinated barley grains produce amylase enzymes that break down starch to maltose, and the action of yeast on the maltose then produced alcohol.