Polarity in Plants
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Formale Metadaten
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| Lizenz | Keine Open-Access-Lizenz: Es gilt deutsches Urheberrecht. Der Film darf zum eigenen Gebrauch kostenfrei genutzt, aber nicht im Internet bereitgestellt oder an Außenstehende weitergegeben werden. | |
| Identifikatoren | 10.3203/IWF/C-2027eng (DOI) | |
| IWF-Signatur | C 2027 | |
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| Produktionsjahr | 1997 |
Technische Metadaten
| IWF-Filmdaten | Film, 16 mm, LT ; F, 19 min |
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Transkript: Englisch(automatisch erzeugt)
00:20
Polarity is a fundamental property of all living organisms.
00:24
It often expresses itself morphologically, as in this beech tree. The beech seedling, which at first develops underground, is already showing polarity. Its radical anchors it in the soil, whilst its shoot grows upwards.
00:42
It elongates and raises the cotyledons into the light. The growth process shown here takes two weeks. Morphological polarity corresponds to a functional polarity. Leaves are active in photosynthesis, as the cotyledons are already.
01:03
Roots absorb water and mineral salts from the soil. Not only the seedling as a whole, but also its individual organs show polar organization. The radical, shown here for example in a chenopodium seedling, shows polarity very distinctly.
01:25
The cells at the root apex form a cap, which protects the region of cell division or meristematic zone. They secrete mucus over the surface, and so make it easier for the root to force its way into the soil.
01:42
Then, at a certain distance behind the apex, root hairs grow out. They serve to absorb water and mineral salts. After the development of the seedling root, the shoot axis, or hypocotyl, elongates, raising the cotyledons into the light.
02:02
The polarized organization of roots can be seen particularly well in grasses, as shown here in a barley seedling.
02:26
The sloughed off root cap cells stand out clearly in dark field illumination. The meristematic zone, the zone of elongation immediately behind it, and the root hair region can be recognized.
02:40
The root zones demonstrate polarity in this succession. Many of the root surface cells have grown out to form root hairs. The cytoplasm in the root hair undergoes streaming, shown here at five times the normal speed.
03:01
The most important function of protoplasmic streaming is to transport the barely visible Golgi vesicles alongside the large vacuole towards the tip. There, the vesicles are incorporated into the tip of the elongating root hair.
03:21
Root hairs also grow apically, that is, in a polarized manner, made visible by time-lapse cinematography at 20 times normal speed. The polarity of plant organs manifests itself not only morphologically and in the manner of growth, but also physiologically, as demonstrated by an experiment on barley roots.
03:44
When seedlings are placed on an agar medium at pH 5.3, which contains the indicator bromocresol purple, the region around the root apex changes color in a few minutes.
04:02
The medium turns yellow towards the front of the root hair region and towards the rear of the zone of elongation. Surrounding the anterior of the elongation zone and in the region of the apical meristematic zone, it turns red. The yellow color indicates a decrease and the red color an increase in pH.
04:24
The cells in the yellow region therefore release protons into the medium, whilst the cells in the red region take up protons. This is explained in the diagram. The release of protons is coupled with the uptake of cations and of elongation growth.
04:45
The uptake of protons is coupled with the uptake of anions from the soil and organic nutrients from the apoplast, as well as with the growth of protoplasm. First, let us consider proton efflux from the rear of the zone of elongation and the zone of root hair growth.
05:09
As in all plant cells, the membrane potential is negative. This is caused by a surplus of organic and also inorganic anions, for example, chloride ions in the cytoplasm.
05:24
For the sake of clarity, these ions are ignored in the account which follows. The efflux of protons results from the activation of proton pumps. During this process, ATP is used up.
05:44
On the outer side of the membrane, a positive potential is therefore built up. That is, there is an excess of positively charged particles. At the same time, proton expulsion increases the negative charge on the inner side of the membrane. The cell becomes hyperpolarized.
06:03
In the opposite direction, cations, especially the essential iron potassium, are taken up by the cell. This takes place through ion channels which are opened up due to hyperpolarization. This typical secondary active transport is therefore driven by the proton gradient across the cell membrane.
06:32
Potassium uptake leads to a raising of the osmotic potential of the cell, a prerequisite for elongation growth.
06:49
Now, about the conditions in the front of the elongation zone and in the apical meristem. Here, proton influx proceeds with the uptake into the growing cell of anions,
07:02
such as chloride, nitrate and hydrogen phosphate, as well as organic nutrients. The uptake of anions is brought about by carrier proteins. These are integral components of the membrane with high substrate specificity
07:23
which catalyze the transport of materials through the membrane. Through the loss of protons, a negative potential is built up on the outside of the route.
07:46
The simultaneous uptake of protons and anions is termed proton-anion simport, and it ensures the absorption of minerals from the soil.
08:01
An electrical field is built up between the regions of the elongation and root-hair zones and the root tip. The flow of protons follows this electrical field. The other ions also move in the electrical field, but do so more slowly and are not considered here.
08:23
The root tip thus shows a distinct polarity with respect to tissues, some of which show efflux and some influx of protons. Morphological and physiological polarity is shown not only by roots but also by leaves.
08:42
This is shown particularly well in the leaves of the Canadian waterweed Elodea canadensis. The plant lives submerged in fresh water. In the light, oxygen bubbles often collect on the underside of the leaves. They are formed by the cleavage of water during photosynthesis and float up to the surface.
09:05
Electron microscopy of a leaf section shows the distinctly polarized construction of the Elodea leaf. At the leaf edge, there are only two cell layers, the upper epidermis with relatively large cells
09:21
and the lower epidermis with smaller cells. Both layers of cells contain chloroplasts. With bromocresol purple, differential proton transport can be demonstrated between the upper and lower sides of the leaf, shown here by time-lapse cinematography.
09:43
The cells on the lower side export protons into the medium, which there becomes yellow in color. Cells on the upper side take up protons, so the medium here turns purple. Electrophysiological measurements have shown that a proton stream of several microamperes per square centimeter
10:04
flows from the lower to the upper side of the leaf. Polarized proton flow will be explained in a diagram of a transverse section of a leaf. First, let us look at the smaller cells on the lower side of the leaf.
10:25
Here, proton pumps, which are localized in the plasma membrane, drive protons into the surrounding medium. Whilst the inner side of the membrane becomes hyperpolarized,
10:41
a positive potential builds up on the outside. The protons expelled into the medium combine with hydrogen carbonate to form carbonic acid.
11:03
This breaks down to water and CO2. Carbon dioxide diffuses through the plasma membrane and into the chloroplasts, where it is used in photosynthesis. Many submerged aquatic plants provide themselves with carbon in this way.
11:29
Elodia also uses hydrogen carbonate directly as a source of carbon. Now the events on the upper surface of the leaf.
11:49
The uptake of anions is connected with the influx of protons. Here again, there are carriers which facilitate the uptake of anions.
12:02
As in the root, this is also associated with proton anion symport. On the outer side of the membrane, as shown previously in roots, a negative potential is built up by migration of protons.
12:24
The mineral salts' nutrition of submerged aquatic plants, such as elodia, therefore takes place through the leaves. The driving force for this is the electrical potential of the membrane and proton flux.
12:41
Between the upper and lower sides of the leaf, an electrical gradient is set up. The flux of protons follows this gradient between lower and upper sides, as shown here for a part of the leaf in three dimensions.
13:13
So much for polarity in higher plants. Simple organisms also show polar organization,
13:21
as in this tubular alga vorcheria. Its large multinucleate filaments grow at the tip. Apical growth prevents the formation of a further growth zone in the immediate vicinity of the apex. The first branching of the filament occurs at a distance of some hundreds of micrometers.
13:43
The development of a side branch is induced by exposure to blue light provided here through a fine light guide. First, chloroplasts accumulate near the illuminated region. After two to three hours, under the influence of local illumination,
14:04
the cell begins to grow out so that the filament branches. Such a response to blue light is shown using time-lapse cinematography. This experiment points to the dominance of the growing apex
14:22
and in addition shows that polarity can be induced by external factors such as, for example, light. Induction of polarity has been particularly well studied in brown algae, as in Pelvisia fastigiata.
14:42
This alga grows in the upper part of the intertidal zone. It produces a large number of practically unpolarized egg cells which are released into the sea. Because of their high specific gravity, the eggs sink to the bottom.
15:03
Only after fertilization by simultaneously released spermatozooids do the eggs develop polarity. This is induced by signals from the environment. Light plays an especially important role in plants and algae.
15:20
In unilateral white light, the zygote forms a rhizoidal initial on the side facing away from the light, as shown here by time-lapse cinematography. Later, the rhizoid anchors the alga to the rocks. On the side facing the light, the thallus initial develops.
15:41
The thallus-rhizoidal axis is at first weak, but during the course of the next 10 to 15 hours it becomes stabilized and is irreversible thereafter. After the outgrowth of the rhizoidal initial, the first unequal cell division occurs, which clearly divides the embryo into a thallus cell and a rhizoidal cell.
16:04
For this polarization, only the blue part of the spectrum is effective. It is because of this that in unilateral blue light, the germination of the zygotes is likewise polarized and unidirectional. Here again, the rhizoidal initials form on the side away from the light,
16:23
while thallus initials develop on the side facing the light. Further, cell divisions follow. In contrast, in unilateral red light, zygotes germinate in all directions.
16:43
They possess no special receptors for the induction of polarity in this spectral region. The incident red light becomes concentrated to the right by the lens-like action of the cells. Nevertheless, the zygotes grow out to form rhizoids at any point.
17:10
In linear polarized white light, the zygotes germinate parallel to the electrical light vector. This observation is interpreted as indicating a dichroic arrangement,
17:21
that is, of a preferred orientation of the blue light receptors. Unusually, a zygote has even formed two rhizoidal initials. This confirms that polarity is induced afresh by light and not that a previously determined axis has merely been rotated.
17:41
As an early indication of polarization, the zygote secretes an increased amount of mucilage on the pole facing away from the light. This is advantageous in attaching the zygote to its natural substratum. The mucilage is stained here with toluidine blue.
18:05
After five to ten hours, a rhizoid grows out from the pole previously richer in mucilage.
18:20
Polarity, which can be induced by other physical and by chemical signals in addition to light, is passed on at each cell division to the daughter cells. Thus, an entire polarized organism can develop from a single polarized zygote. This polarity is abandoned only during the formation of egg cells
18:43
so that the next generation is able to adapt to the new environment.