The following points highlight the two main mechanisms of Gravitropism. The mechanisms are: 1. Sensory Mechanism 2. Reaction Mechanism.
Gravitropism: Mechanism # 1.
Sensory Mechanism:
Plants can perceive the direction of the gravitational stimulus in relation to their orientation, suggesting the existence of some sort of gravity-sensing or geo-perceptive mechanism. Primarily, geo-perception occurs in the root cap or shoot-tip region as their removal reduces geotropic sensitivity without affecting elongation.
Haberlandt and Nemec (1990), suggested the presence of a number of gravity-sensitive inclusions named statocyte in each organ, containing high density gravity-sensitive inclusions named statoliths.
In most cases, the statoliths are sedimentable starch grains that are abundant in certain cells of the root cap. It has been suggested that the sedimentation of the statoliths at the bottom of the statocytes by the gravity vector force leads to the unilateral stimulation at the cellular level. Thus the change in intracellular orientation helps the plant organs to perceive the stimulus very rapidly (Fig. 15.3).
The minimum duration of the stimulus for the detectable gravitropic response is the presentation time (1 sec to 5 min) and the time taken from the application of the stimulus to the appearance of curvature is the reaction time that can vary from 10 min to several hours.
Change in orientation is rapidly detected by the plant organ, suggesting the rapid operation of the geo-perceptive mechanism. This is explained by the starch-statolith hypothesis. If the unilateral stimulation is dependent on the sedimentation of the statoliths, then the sedimentation must be very rapid.
Of all the cellular organelles and sub-cellular particles only the starch grains sediment within the presentation time. The starch grains remain in a group (1 to 8) within a membrane to form an amyloplast.
There are usually 4 to 12 amyloplast per statocyte which is confined to the columella (the central core) of the root cap and in the cells of bundle sheath or endodermis all along the shoot.
There are, however, many plants such as Sphagnum, moss and certain fungi which do not possess any sedimentable starch grains in their cells still they exhibit gravitropism, suggesting the different nature of statoliths. In the geotropically sensitive rhizoids of Chara fragilis the statoliths are granules of barium sulphate. These observations, however, do not support the starch-statolith hypothesis.
Starch grains can be removed from the cells of coleoptiles and roots by keeping them in a solution of high concentration of kinetin and GA3 at 300 C for 35 hours. Lepidium sativum roots fail to perceive the geotropic stimulus when treated like this and exhibit straight growth in any direction.
The gravitropic response of the roots is recovered by the reformation of the amyioplasts by placing the same in water and light for 20-24 hours. Essentially similar results have been obtained in the roots of red and white clover and in the coleoptiles of Triticum durum.
If the root cap is removed, the geotropic responsiveness is lost, and with the regeneration of a new cap, geo-response is restored. Careful microscopic studies suggest that immediately after the detachment of the cap, starch grains begin to develop from pro-plastids in the cells of the quiescent centre.
These starch grains help the root to renew its graviresponse. In non-photosynthetic albino mutants of Zea mays, the leaf base statocytes do not contain starch grains and the gravitropic responsiveness is not found.
If the leaf sheath cells are supplied with sucrose, there is the formation of amyloplast with the concomitant appearance of gravitropic responsiveness.From all the available evidence, it can be unequivocally concluded that the amyloplast sedimentation is an integral part of the gravity-sensing mechanism in roots and shoots of higher plants.
Gravitropism: Mechanism # 2.
Reaction Mechanism:
Cholodny (1926) and Went (1926), proposed the classical hypothesis that like phototropism, the differential auxin concentration is the cause of geotropic curvature.
In vertical shoots, there is symmetrical auxin distribution from the apex to the growing zone. If positioned horizontally, auxin concentration becomes higher at the lower half owing to lateral redistribution. Auxin promotes shoot growth at concentrations in which it generally inhibits root growth.
It seems that higher concentration in the lower side of the shoot causes more growth, producing negative curvature and less growth in root, producing positive curvature. How far this hypothesis is valid may be discussed in different plant organs.
(a) Geotropism in Roots:
The most well-studied organ for the gravitropic response is the root. It has to depend on other parts of the plant for the supply of organic nutrients and hormones or their precursors.
These materials are transported down the root throughout the stele. Auxins, particularly IAA, gibberellins, cytokinins and ABA have been identified in the root tissues. Transport of IAA towards the root tip takes place only through the stele.
When the root cap of maize is removed, the gravitropic response is lost until a new cap is regenerated. If one half of the cap is removed, a large curvature is produced towards the side of the half-cap in any kind of orientation.
As the root cap is the controlling point and as it is a little but distant from the elongation zone of curvature development it is obvious that there is some sort of communication between the two regions.
It is clear that the cap is the source of a growth inhibitor on the lower side of the root, that inhibits or reduces growth there, causing the formation of a positive geotropic curvature. A mica or metal foil barrier inserted longitudinally inside the cap greatly reduce the bending ability of the root, which is again suggesting that the cap is the source of an inhibitor moving down laterally inside the cap (Fig. 15.4).
The Cholodny-Went hypothesis suggests that the inhibitor compound involved in the geotropic responses of roots is auxin. But there are many conflicting reports. Bridges et al., found that IAA was confined only to the stele in maize roots.
Scott and Wilkins (1968), are of the opinion that the auxin transport is polarized, that is from the base to the root apex, but the direction appears to be wrong for auxin to be involved in the geotropic response of roots. Kundu and Audus (1974), reported the occurrence of a growth inhibitor in Zea root caps having the physicochemical properties very similar to ABA.
Similar results have been obtained by many other workers. Rivier and Pilet (1974), on the other hand, have detected IAA in the maize root caps and Pilet (1977) reported more IAA than ABA in maize roots.
Suzuki et al. (1979), reported the presence of IAA, ABA and an unidentified inhibitor in the lower halves of Zea roots. They, however, are of the opinion that an unidentified compound might be responsible for root geotropism.
That ABA is the important inhibitor responsible for the production of geotropic curvature is supported by the following observations:
(a) Asymmetric IAA application does not induce the formation of geotropic curvature.
(b) Scott and Wilkins (1969), observed a positive geotropic response in Zea mays roots in light. This observation was supported by Wilkins and Wain (1974), who observed that the dark-grown maize root caps lack ABA.
(c) External application of ABA to the horizontally placed Zea roots in dark made them gravity sensitive.
Despite the above evidence, it is, however, still questionable whether ABA can undergo downward lateral transport in horizontally placed roots or not.
Hartung (1976, 1981), was unable to demonstrate either an asymmetric distribution or lateral transport of labeled ABA applied to the root caps of Phaseolus coccineus. Vicia faba and Zea mays. Suzuki et al. (1979), detected an asymmetric distribution of an unidentified inhibitor instead of ABA in horizontal Zea roots.
They concluded that this unidentified growth inhibitor, which is not ABA, is responsible for geotropic curvature. To answer the question as to how ABA is able to induce gravitropic response in horizontally oriented roots, they have postulated that ABA is necessary either as a precursor or as a controlling factor in the synthesis or release of the inhibitor.
(b) Geotropism in Coleoptiles and Shoots:
If the coleoptiles and dicotyledonous shoots are oriented horizontally, an auxin concentration gradient from tip to base is established. The higher concentration on the lower side induces more growth to produce the negative curvature. It is suggested that auxin undergoes downward lateral transport and thus becomes distributed asymmetrically in horizontal coleoptiles and shoots.
The presence of IAA in Zea coleoptile apices has been demonstrated by mass spectrometry (Greenwood et al., 1972). Using radioactive IAA, Goldsmith and Wilkins (1964), first unequivocally demonstrated its downward lateral transport in the horizontally-placed shoots. Shaw et al (1973), using tritiated IAA, observed its downward lateral transport in horizontally placed Zea mays coleoptiles.
Brauner and Appel (1960), observed that the insertion of a barrier into the horizontal coleoptile apex to prevent lateral transport markedly decreased the gravitropic bending ability of the coleoptile. Thus, the Cholodny-Went hypothesis seems to be valid at least in part of the overall response mechanism in the horizontally-placed coleoptiles.
Cane and Wilkins (1969), are of the opinion that differential longitudinal transport of IAA in basipetal direction in horizontal coleoptiles of Zea mays has an additive effect on the initial lateral transport to establish asymmetric distribution of IAA. Again, whether growth regulators other than IAA are involved or not, has not yet been clearly established.
Hall et al (1980), further criticized the Cholodny-Went hypothesis in coleoptiles as to whether the magnitude of asymmetry of IAA distribution is adequate for the changes in growth rate or not.
There has been no clear assessment of this criticism. But at present, this much can be emphasized that lateral transport of IAA toward the lower side of a horizontal coleoptile is obvious. Whether this process alone can initiate the changes in growth rate is not clear.
In Helianthus annuus and Cucumis sativus hypocotyls, negative gravitropic curvature appears to be due to a cessation of growth of the upper side and substantial increase of growth of the lower side caused by lateral transport of IAA.
It is still questionable whether IAA alone is responsible for the graviresponse of the shoot or whether any other hormone is involved. Phillips (1972), detected a substantial asymmetry of distribution of GA in horizontally placed Helianthus shoot. Still it is difficult to assess the significance of this asymmetry.
In addition to the growth regulators, asymmetric distribution of K+ and Ca2+ has also been detected in gravitropically stimulated hypocotyls of Helianthus annuus (Bode, 1960; Goswami and Audus, 1976). Higher K+ and Ca2+ concentration have been found in the ash in the lower half of the hypocotyls in comparison with the upper half.
The significance of this asymmetrical distribution of ions in horizontally-placed hypocotyls is still not known. This may be a consequence of IAA asymmetry.
In horizontally-oriented hypocotyls of soybean, gravitropism leads to a rapid asymmetry in the accumulation of a group of auxin-stimulated mRNAs called SAURs (small auxin up-regulated RNAs).
In vertical seedlings these mRNAs remain symmetrically distributed throughout the hypocotyl region. If oriented horizontally SAURs begin to accumulate on the lower half within 20 minutes and geotropic bending starts after 45 minutes.
The existence of a lateral gradient in the SAUR gene expression is indirect evidence for the existence of a lateral gradient of auxin within short period of time of gravitropic response. Because of the quick response, expression of SAUR genes is a convenient probe for the lateral transport of auxin during photo- and gravitropism.
The GH3 gene family is also up-regulated within 5 minutes of auxin treatment. This is used as a molecular marker for the presence of auxin. By the use of the GUS reporter gene fused to GH3 promoter it is possible to detect the lateral gradient in auxin concentration occurring during both photo- and gravitropism.