GRAVITROPISM Frank [47] was perhaps the first to establish that curvatures due to gravitational
stimulus were directly connected with growth and described this phenomenon by the term geotropism.
The precise gravity-perceiving mechanism is still unknown. Since the root cap (terminal 500 m) appears
to be the seat of perception, it was thought for a long time that amyloplasts (organelles filled with starch
grains) were, in fact, the gravity-perceiving mechanism. However, the use of a mutant lacking amylo-
plasts in the cap, yet exhibiting nearly normal gravitropic response, has ruled out this mechanism [48].
Whatever may be the mechanism of perception, the ultimate response is asymmetrical growth, resulting
in a positive (root) or negative (shoot) curvature.
Strong evidence suggests that auxin controls the gravitropic response and the Cholodny-Went the-
ory has been invoked to explain the growth asymmetry that results in the shoot bending away from, and
the root toward, the gravity vector. It is well established that when shoot segments are placed horizon-
tally, more auxin is recovered from the lower than from the upper half. Using bioassay and^14 C-labeled
auxin, differences between the upper and lower halves of the maize coleoptiles were demonstrated, but
the activity on the upper halves did not differ from that of the vertical halves [38]. However, Naqvi and
Gordon [49], working on^14 C-labeled auxin transport kinetics, demonstrated that horizontal reorientation
of maize coleoptiles reduced the transport intensity (capacity) of the upper half without affecting the ve-
locity. Thus a lateral auxin concentration gradient enhanced movement from the upper toward the lower
half. This lateral movement has also been demonstrated in the maize mutant amylomaiz [50]. McClure
and Guilfoyle [51], using a molecular biology approach, have shown a clear correlation between auxin-
controlled gene expression and the gravitropic response of soybean hypocotyls. Molecular genetic stud-
ies on the phenotype of auxin-resistant mutants have further substantiated that auxin played an important
role in root gravitropism [52].
- Apical Dominance
The integrity of the complicated form of higher plants depends to a great extent on regulations that inte-
grate the various component parts. This gives a characteristic form or shape that is repeatable in time and
space. The stems assume characteristic geometry due largely to the extent of biochemical influence ex-
erted by the apex on the development of lateral (axillary) bud meristems. This phenomenon of apical dom-
inance (growth correlation or compensatory growth, i.e., preventing or slowing of lateral bud growth by
the apex) is of major importance in integration of the plant body, and parallel examples are found in
mosses and ferns [53]. Awareness of this role of the apex has undoubtedly influenced pruning practices
in horticulture and crop production. Interested readers are referred to excellent reviews on the subject
[54,55].
In a pioneering work using Vicia faba, Thimann and Skoog [56] demonstrated that auxin diffused in
agar blocks from the excised shoot apices, Rhizopusfilterate, or human urine can partially inhibit lateral
bud growth. Later, Leopold [57] showed that when the shoot apex of ‘Wintex’ barley (Hordeum vulgare)
was destroyed, tillering was profuse unless the apex destroyed was replaced by an auxin. These observa-
tions indicated that auxin from the apex exerts an influence on Vicialateral bud growth as well as on bar-
ley tillering. Studies on inhibiting auxin emanating from an apical bud, using inhibitors such as TIBA or
morphactin, have shown that the lateral bud growth was effectively enhanced. These studies thus provide
evidence in support of a direct role, but others question it and assign an indirect role to auxin in control-
ling this phenomenon [54]. In intact tobacco, petunia, and Arabidopsis thalianaplants, use of transgene-
mediated auxin and/or ethylene deficiencies, along with mutants insensitive to auxin or ethylene, supports
the idea that apical dominance is the result of the auxin/cytokinin ratio rather than auxin-induced ethylene
production [58].
- Root Formation
The most apparent auxin control of cell division is the formation of roots. Early evidence indicating the
presence of active buds on cuttings to promote root development below led to the identification of auxin
as the root-forming hormone [13]. The ease with which roots can form on cuttings varies enormously;
shoot cuttings from some plants produce roots simply if their basal cut end is left in water, whereas other
species do so only rarely. Root formation shows polarity and always occurs at the morphological basal
end, even if the cuttings are inverted upside down. Because auxin moves basipolarly, it was logical to be-
lieve that root formation at the basal end is a consequence of the movement of auxin to the lower tissues.
Removal of rich sources of auxin (i.e., buds and young leaves) reduces the number of lateral roots formed.
This capacity is restored, however, if auxin is substituted for these organs. Tissue culture studies have pro-
PLANT GROWTH HORMONES 507