Plant Coordination (I) -- Phytohormomes P.11
Co-ordination in Plants (I) --- Plant Growth Substances
Plants’ gro_____ and dev______is controlled by prom______and inh______ which are generally called plant growth substances (phytohormones, plant hormones). In addition, certain responses of plants are controlled by the action of some plant growth substances.
Growth in plants can be divided into three stages : cell d______, cell en______and cell dif______. The action and distribution of plant growth substances also reflect these stages of growth and development.
Types of growth substances / General actionCytokinins / cell division
Auxins and Gibberellins / cell enlargement and differentiation
Abscisic acid / dormancy
Ethene / ageing (senescence)
I) TROPISMS
Tropisms are gr movements carried out by part of a plant towards or away from a di______ stimulus. If the growth is towards the stimulus then the response is p______ and if it is away from the stimulus the response is n______. Stems, for example, generally bend towards the light therefore they are positively phototropic, whereas roots tend to be negatively phototropic.
In addition to light and gravity, plants may respond to touch (as in climbing plants), or water h_____tropism, (as in roots), or chemicals-ch____tropism, (pollen tubes grow down the style).
A) Phototropism
Early work on phototropism
The following list is a summary of results from experiments performed by Charles Darwin in 1880 and biologists working on phototropism in the early part of this century.
Most work was done on the young seedlings of oat.
The organ used was the coleoptile-- a protective sheath surrounding the plumule (germinating shoot) of some monocot plants. eg. grass
The next list summaries the conclusions that were drawn from these results.For each conclusion say which result or results supports it.
Summary of results from the
early classical work on phototropism / Link the conclusion and related result(s) / The list summaries the conclusions which were drawn from these results.
1. If the tip of the coleoptile was removed, the coleoptile did not grow. Pieces of coleoptile or stem immersed in an auxin solution elongated. / w / w / A. The tip produces something which is essential for growth. 1
2 Coleoptiles grew towards illuminated side; the bending took place just behind the tip. / w / w / B. The effect of light is perceived by the tip. 3
3 If the tip was removed or covered by a light-proof material, no growth curvature was shown. / w / w / C. The stimulus (auxin) is produced at the tip. 4 (i) (ii)
4 The decapitated coleoptile would, however, grow towards the light if
i) the tip was replaced on the stump or
/ w / w / D. The stimulus (auxin) produced by the tip passes from there to a lower part of the coleoptile which bends. 2, 3
ii) the tip was replaced on the stump with a thin layer of gelatin between the two cut surfaces. / w / E. The stimulator substance is water soluble. 4(ii)
5 The decapitated coleoptile did not grow towards the light if cocoa butter (a fat), or mica or platinum foil was placed between the two cut surfaces instead of gelatin. / w / w / F. The stimulator substance is not soluble in fat, neither is it an electric stimulus. 5
6 If the decapitated coleoptile was kept in the dark and the cut-off tip replaced at one side of the stump, the stump bent away from the side on which the tip was replaced. / w / w / G. Unilateral illumination causes a greater concentration of the growth stimulating substance on the darker side of the coleoptile. This causes greater growth on that side, with consequent bending towards the light. 6
The growth hormone was first extracted by the Dutch botanist F. W. Went in 1928. He removed coleoptile tips and placed them on agar jelly for several hours. He then found that the agar could cause growth to be resumed in a decapitated coleoptile i.e. the growth hormone had diffused out of the tip into the jelly.
By allowing the hormone to diffuse from the coleoptile tip into an agar block and then from that block to others, he prepared a range of hormone concentrations and was able to show that the degree of cur of the coleoptile was proportional to the con of the hormone in the block..
This became the basis for a bio-assay technique to investigate the relative amounts of hormone in various plant tissues
Q. Identify the following components in phototropic response : stimulus, receptor, transmission, effector, response.
B) Geotropism
Early work on Geotropism
w
The following list is a summary of results from the early classical work on geotropism in roots. / Link the related conclusion from the result(s) / Summary of the conclusions which were drawn from these results.1. a / 1 When the root was placed horizontally, it grew towards gravity. Bending took place just behind the tip. / w / w / A. The effect of gravity is perceived by the tip. 2(i),(ii)
2. b / 4 If the tip was removed, the root
i) no longer responded to gravity,
/ w / w / B. The auxin is produced at the tip. 3(i)
ii) continued to elongate. / w / w / C. The stimulus perceived by the tip passes from there to the area behind the tip which bends. 1
3. / If the tip was replaced on a decapitated root, the root
i) responded to gravity, and
/ w / w / D. The auxin of both root and shoot is the same 4,5
ii) ii) had a lower growth rate than it had without the tip. / w / w / E. The auxin produced at the root tip retards root growth. 3(ii)
4. / A coleoptile tip on a decapitated root also retarded growth. / w / w / F. The auxin produced at the root tip stimulates coleoptile growth 5.
5. / Root tips placed at one side on decapitated coleoptiles caused a positive curvature. / w / w / G. The root is more sensitive to auxin than is the stem. 4,5
6. / Agar blocks containing IAA (an auxin) when placed on decapitated roots caused
i) a retardation of growth if placed on straight and / w / w / H. Increasing the auxin level, or even bringing it up to its normal value for roots, retards root growth. 2(ii), 3(ii), 4, 6(i)&(ii), 8
ii) a negative curvature if placed at one side (i.e. a curve towards the side on which the block was placed), as also did the one-sided replacement of root tips. / w
7. c / 7 Very low concentrations of IAA applied to decapitated roots led to an acceleration of growth. / w / w / I. Very low concentrations of IAA accelerate root growth.7, 8
8. d / 8 There was more auxin in the lower half of horizontally placed roots than in the upper. / w
C) Synthesis and Transport of auxins
At first the amounts of hormone extracted were too small for it to be analysed and the term a_____ was applied to it.Later it was found that a wide range of auxins occur in plant tissues. Probably the most important and the first to be identified was indoleacetic acid (IAA).
a) Transport of Auxin
Q. What was the hypothesis under investigation ?Q. What conclusion can be drawn from the experiments ?
· Auxins are made continuously in the shoot ap and yo leaves. A little auxin is also made in roots.
· Movement away from the tip is p (unidirectional).
· It moves from cell to cell and is eventually inactivated and degraded by enzymes.
· Long distance transport can also occur via the vascular bundle (mainly phloem) from shoots to roots.
b) Auxins in stem and root
Indolelacetic acid is an auxin which can be manufactured artificially. Its effects, when applied at different concentrations to a plant, can be measured accurately.
1 Both root and stem have a range of auxin concentration which promotes growth and a range which inhibits growth. Which organ is the more sensitive to auxin ?2 Describe in words the effect of auxin on the growth of
a) the stem,
b) the root.
3 Externally applied auxin is absorbed more readily by dicotyledons than by monocotyledons. Suggest a use to which this property can be put. / The figure shows the growth responses of roots and stems to varying concentrations of externally applied auxin.
c) Auxin and Geotropism
d) A possible mechanism for Geotropism:
Tests have shown that auxin tends to accumulate on the l_____ side of the root and the shoot but the effect it produces in each is different.In the shoot the auxin causes increased growth and the shoot produces an upward curvature.
Roots, however, are much more s______ to auxin and the amount which collects on the lower side of the root actually i______growth, while that on the upper side may promote it, so that the root bends downwards towards gravity.
e) The gravity-sensing mechanism
The perception of gravity by the plant has led to a search for mobile structures which could fall under g to the lower sides of the cells.The root c__ and endo______ of many plants have been found to contain mo st gr______which could act as statoliths in this way and the time they take to fall across the cell coincides well with the presentation time required for a response to gravity.
They also disappear along with geotropic sensitivity if the plant is kept at low temperatures and reappear again together with geotropic sensitivity on return to normal temperatures. /
In some unknown way the displacement of these st_____ grains affects the distribution of growth substances such as auxins and abscisic acid.
Q. How this mechanism of gravity detection is similar to that in animals?
/
Reference Reading : Modern hypotheses on geotropism
Transmission of a growth inhibitor from the root cap to the zone of elongation has been shown, but this is not necessarily auxin. Several groups of workers have been unable to find auxin in the root caps of maize seedlings, a common experimental plant. Instead, abscisic acid, a well-known growth inhibitor, has been found. Ethene, another growth inhibitor, could also be involved.
Q. What can you conclude from the experiments shown on the right. Controls, using untreated agar, showed no curvature. When IAA was used instead of abscisic acid no significant curvature was obtained.
The current confusion surrounding tropism demonstrates the process by which knowledge of science progresses. One simple and elegant theory provided an explanation for geotropism. Then further investigations revealed facts that were not in agreement with the theory. Now, much of the theory must be discarded and new hypotheses put forward on the basis of new information and they, in turn, must be critically tested and assessed.
C) Other Effects of Auxins
a) Apical dominance
· Apical dominance is the phenomenon whereby the presence of a growing ap bud inhibits growth of lat buds.· It also includes the sup of lateral root growth by growth of the main root. Removal of a shoot apex results in lateral bud growth, that is bran .
This is made use of in pr when bushy rather than tall plants are required.
Auxins exert their influence in an unknown way, possibly by somehow 'attracting' nutrients to the apex.
Other plant growth sustances may also be involved that interact with auxins to produce the phenomenon.
Q. What plant growth substance is made in the shoot apex?
Q. Describe an experiment to show whether the growth substance you named is responsible for apical dominance.
b) Pollen tube growth, Fruit development and Parthernocarpy
· Germinating p grains are a rich source of auxins as well as stimulating the ov to produce more auxin.· This auxin is ‘retention’ of the o , which becomes the f_____ after fertilisation. Without it abscission of the flower normally occurs.
· After fertilisation, the ovary and the ripe s____ continue to produce auxins which stimulate fruit development.
A few natural cases where fruit development proceeds without fert , and therefore without seed development, for example banana, pineapple and some seed____ varieties of oranges and grapes.
Such development is called parthenocarpy. Unusually high a_____ levels occur in these ovaries. P______can sometimes be art______induced by adding auxins, as in tomato, squash and peppers. /
The strawberry fruit consists of a swollen red receptable, with a large number of small fruits (achenes) embedded in it. The figure shows ripe strawberry fruits which have been treated as described under each diagram.
Q. Comment on the relationship between seed, auxin and fruit growth in strawberry
Par______can also be induced by Gibberellins.
II) THE RANGE OF PLANT HORMONES
In recent years a wide range of other types of plant hormone have been discovered and the main groups together with their effects are shown in the table below.
Plant hormone / Effectsauxins e.g. IAA / · Phototropism, geotropism, apical dominance,
· root initiation, promotes cell elongation, parthenocarpy
· Synthetic auxins can be used to promote root growth in cuttings, kill broadleaved weeds (herbicide), prevent premature fruit fall etc.
gibberellins / · Promote cell division, cambial activity.
· Overcome genetic dwarfism. Promote cell & stem elongation.
· Promotes flowering & Parthenocarpy.
· Mobilise food stores in seeds.
· Induce new RNA and protein synthesis.
· Act independently or together with auxin
cytokinins / · Stimulate cell division and enlargement,
· root initiation and growth, shoot initiation and growth. Break dormancy.
· Appear to work with auxin to promote mitosis.
abscisic acid / · Accelerates abscission and senescence /aging in a variety of species.
· Inhibits germination in some species.
· Effects can be counteracted by auxins, gibberellins and cytokinins.
ethene (a gaseous hormone) / · Accelerates fruit ripening. Inhibits bud growth.
III) Other Processes Regulated by Plant Hormones
B) Gibberellins
a) Discovery of gibberellins
During the 1920s a team of Japanese scientists was investigating a particularly damaging worldwide disease of rice seedlings, caused by the fungus Gibberella (now called Fusariurn). Infected seedlings developed very long internode, and become thin and pale and eventually died or gave poor yields.