- Bioassay
Only a bioassay can detect the physiological activity of a substance at hormonal concentrations, and thus
development of a quantitative bioassay provided the beginning for all hormonal work [13]. A number of
bioassays have been developed to measure the activity of auxins. Well known among them are the (1)
Avenacoleoptile curvature test, (2) Avenacoleoptile section test, (3) split pea (Pisum sativum) stem cur-
vature test, and (4) cress root inhibition test. Modern instruments of separation and quantitation, such as
high-performance liquid chromatography (HPLC) and gas chromatography coupled with mass spec-
trometry (GC/MS), are commonly used. Another extremely sensitive detection method is immunoassay
(a type of bioassay), and commercial kits are available for determining picogram quantities of plant hor-
mones [36].
- Tropisms
In nature, the orientation of shoot and root is of crucial importance to seedlings developing from seeds
oriented at all angles in the soil. For survival, shoots therefore need to be oriented toward the light so that
photosynthesis can begin before the stored food reserves are depleted, and roots must be oriented toward
the gravitational vector, to obtain water and ions and to secure anchorage and mechanical support. Shoots
are thus considered to be positively phototropic, and roots, negatively phototropic. On the other hand,
shoots are negatively gravitropic and roots positively gravitropic. These two responses are of great eco-
logical importance and also have relevance to plant and crop productivity.
Tropisms (from the Greek word trope, “turn”) have been divided into three phases: perception, trans-
duction, and response. To explain the transduction and response phases, caused by photo or gravity stim-
ulation, the Cholodny-Went theory states: “Growth curvatures, whether induced by internal or by exter-
nal factors, are due to an unequal distribution of auxin between the two sides of the curving organ. In the
tropisms induced by light or gravity the unequal auxin distribution is brought about by a transverse po-
larization of the cells, which results in lateral transport of the auxin.”
Although the validity of the Cholodny-Went theory has been questioned without an alternative ex-
planation [37], others have come to its defense [38,39]. Li et al. [40], using auxin-responsive messenger
RNAs (mRNAs) called small auxin up RNAs (SAURs) as a molecular probe, have supported the idea of
asymmetric distribution of auxin at the sites of action during tropistic response. However, Jaffe et al. [41],
using the pea mutant ‘Ageotropum’, found that roots appear to be neither negatively phototropic nor pos-
itively gravitropic but grow in the direction of increasing soil moisture.
PHOTOTROPISM When the exposure of light falling on plant organs becomes differential, a curva-
ture develops, so that reorientation takes place in such a way that the organ is evenly illuminated. This re-
sponse of the plant or its organ(s), where the plane of curvature is determined by the spatial relationship
of the organ and the light stimulus, is known as phototropism.
The Cholodny-Went theory suggests that unilateral illumination causes auxin to move laterally to the
darkened side, causing the organ to curve toward light. Bioassay and^14 C-labeled auxin studies using eti-
olated maize coleoptile tips have shown that there was no difference in auxin yield between evenly illu-
minated segments (first or second positive range) and their dark control segments. But unilateral illumi-
nation caused asymmetry in auxin yield between the two halves of the tip segments [42,43]. These
observations demonstrated the consequence rather than the cause of the lateral asymmetry in auxin trans-
port. In such a cause-and-effect relationship, it is important to differentiate between the two. Using
[^14 C]IAA and maize coleoptiles, Naqvi and Gordon [44], studying the [^14 C]IAA transport kinetics of eti-
olated maize coleoptiles, provided the first evidence that bilateral illumination, in the first positive range,
caused retardation of basipetal auxin transport intensity (capacity) without affecting the velocity. The lat-
eral asymmetry observed was thus a consequence of the resultant concentration gradient. Based on their
observations and other evidence, Naqvi and Engvild [31] proposed that “photolysis of a carotenoid (vio-
laxanthin) produces compounds (similar to or identical with ABA) that inhibit the basipetal auxin trans-
port. Unilateral stimulation produces an asymmetry of inhibition and, hence, the curvature.” It is now
known that in maize coleoptiles, ABA is the most dominant hormone after IAA [45]. Therefore, any
change in its concentration would influence auxin transport. Further support for the lateral transport the-
ory came from the observation that in bean (Phaseolus vulgaris), auxin transport inhibitor DPX-1840 in-
hibited basipetal as well as lateral transport, resulting in growth retardation and the loss of phototropic re-
sponse [46]. These results suggest that basipetal and lateral transports are essential for photo-stimulated
differential growth.
506 NAQVI