From the beginning of germination display roots a tendency to grow downwards, while shoots grow
upwards (anisotropic growth). A germinating seed can be turned
upside down several times and the root will still start to bend downwards. This behaviour is
another example for a tropism, a movement triggered by a
stimulus. The question is: what is the controlling stimulus?
"It is not the soil’s humidity that causes the direction of the root, since the root will grow downwards and the shoot will grow upwards even if the plant is placed in an earth-filled tube where the upper part is moist and the lower is dry. If the plant is put into a water-filled tube the lower part of which is exposed to light while the upper is darkened, the direction of growth will still not change: a proof that it is independent of light." (A. de CANDOLLE, 1834/38)
In 1806 showed the British physiologist A. KNIGHT in a decisive experiment that the direction of root growth is controlled by gravity. He fixed seedlings on a vertically standing wheel that was turning around its axis due to a small millwheel fuelled by a fast-flowing stream. The rotation caused a centrifugal force. The effect caused by this force was also observed when the wheel was in a horizontal position: at a fast rotation grew all roots outwards (in the direction of the centrifugal force), at a low rotation did they grow in an angle of about 45° (the result of both centrifugal force and gravity).
KNIGHT thought at first that the root tips where pulled downwards by their own weight, but this assumption was soon refuted, since the downward movement occurs also when the weight of the root tips was compensated for by an opposite weight (JOHNSON, 1828).
A. B. FRANCK introduced the term geotropism in 1868, set it of against phototropism, and distinguished three types:
positive geotropism (like in the example just given)
negative geotropism, and
transversal geotropism.
Positive and negative geotropism together are also called ortho-geotropism. It causes the vertical (orthotropic) orientation of the plant’s axis. Shoot growth is mostly negatively geotropic since shoots grow upwards even in complete darkness. Phototropism can therefore be understood as a secondary process, usually of the same direction as the negative geotropism. Transversal geotropism is a direction of growth that is vertical to the shoot axis. The direction of many lateral shoots, side roots, leaves, etc. is described by this term. All directions diverting from that of the shoot axis are called plagiotropic. Orthotropic organs (shoot axis, main root) are usually of radial symmetry, plagiotropic organs (leaves, side roots, etc.) are mostly of dorsiventral organisation.
J. v. SACHS extended the experiments of KNIGHT and supplemented them by the invention of
the clinostat (1879) that helped him to stop the bending (compensation of gravity by the
centrifugal force). By marking the single parts of the root with water colour marks (today
exchanged against other types of labelling) did he show that in the root,
too, it is not the
tip itself that elongates but the section above (a growth zone). Plant anatomical studies
proof that the elongation of one side is caused by an elongation of the respective cells.
We have not yet answered the questions how the stimulus is perceived and how it is transmitted.
In 1892 postulated F. NOLL (1858 – 1908, professor at Bonn, later at Heidelberg) the existence of microscopically small, mobile components in the cells of the root tip exerting a pressure on the plasma of the bottom side of the cell. The stimulus produced by the local pressure would then have to be converted into a stretching of the cell and into an increased growth by the cell’s plasma. The plant anatomists B. NEMEC at Prague and the Austrian G. HABERLANDT who regarded the central cells of the calyptra as the location for the mobile particles following gravity were able to confirm this suggestion (statolith theory). They could identify the particles as amyloplasts.
NOLL’s suggestion remained insecure in one point, since the relocation of the statoliths does not cause a stretching of the starch-containing cells’ walls, but an elongation of the cells in the growth zone. The question for stimulus forwarding is inevitable. A definite answer has not been given yet, though it is known that auxine participates. Its distribution pattern is not identical with that observed in phototropism, though. The following observations indicate its participation:
The amyloplasts’ mobility decreases from tip to base just like the influence of gravity upon the auxin transport. Strong centrifugal forces cause relocations of the amyloplasts and simultaneous relocations of auxin in the basal parts, too.
Mutants with smaller amyloplasts display a smaller rate of relocation, less influence of gravity upon the auxin transport, and less geotropic bending. The latter does not mean that the mutant has a generally reduced ability to bend since the phototropic reactions of mutant and wild type do not differ.
When talking about phototropism did we mention the behaviour of the Phycomyces sporangium. A comparable example is known for geotropism, too. It is the reaction of the single-celled rhizoids of the stonewort Chara. They are transparent, indifferent towards light, and the movements of their particles can be easily observed with a microscope. The existence of strongly refractive cellular inclusions ("Glanzkörper") in the tip of the rhizoid the relocation of which correlates with a geotropic bending is especially striking (J. BUDER,1961). Based on these observations tried A. SIEVERS and his collaborators (Botanical Institute of the Universität Bonn) to make out a direct correlation between the relocation of the "Glanzkörper" and the geotropic bending by microscopical, electron microscopical, and biochemical analyses. Experiments with cytochalasin B that stops the activity of microfilaments showed that the relocation of the statoliths is an actively controlled process involving microfilaments. Electron microscopical images showed that Golgi-vesicles participate in the cellular growth, and that they are moved to those areas where cellular elongation takes place as soon as bending starts. In summary, one can draw the following conclusions:
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Statoliths hinder cellular growth in the areas where they are close to the plasma (exactly the opposite to what NOLL postulated). In places where the statoliths sediment at first do they cause slight denting in. |
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Golgi-vesicles gather at the side opposite to the statoliths where they participate in the beginning synthesis of the cell wall that leads finally to bending. By putting weight on the root tips can the growth rate of the single sections be exactly monitored. |
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If statoliths are relocated to a vertically growing rhizoid, is the regular longitudinal growth inhibited. |
Even though the observations made with Chara are fairly impressive, do they not explain the causal chain in multicellular roots where stimulus perception and gravitropic reaction take place in different cells even separated by other, non-reactive cells. The findings do stress another point, though: they show that the black box between stimulus perception and reaction can be assembled from different system components.