Brief history of the discovery of plant hormones:
Auxin, polar auxin transport and tropisms.

In the 1700's, Stem girdling (removal of ring of bark and phloem - see figure) was was observed by horticulturists to result on swelling on the stem just above where the phloem was removed. In some cases, roots would grow from the swollen regions. In addition, girdling also had some effects on shoot growth. These observations led to the notion that there were materials in plant sap that moved from the top of the plant downwards that were affected root growth and other things that moved up from the roots that were required for shoot growth.

Another observation supporting the notion that some growth-regulating substance moved from the shoot downward is the formation of roots on stem cuttings. In addition, if leaves are removed root formation is greatly reduced indicating that the leaves are the source of the growth substance. We now know that the substance is the naturally occurring auxin, indole-3-acetic acid (IAA) and that addition of auxin will result in root formation. Application of auxin to cutting to facilitate root formation is a standard practice in the horticultural industry.

As mentioned in class, studies on tropic responses were critical in the discovery of the plant hormone Indole-3-acetic acid (IAA). We have already seen how Darwin's experiments on phototropism showed that there was some substance that moved down from coleoptile tips to control cell elongation. In the presence of unilateral light, the substance becomes redistributed so that more flows down the shaded side and stimulated the cells to elongate faster than the cells on the lighted side, which actually elongate slower since the redistribution of the growth substance causes less to move down the lighted side. As we discussed in class, Darwin also came to similar conclusions with regard to gravitropism based on experiments where the root cap was surgically removed or altered. In the case of roots, the root cap was shown to be the site of graviperception and the location of the changed redistribution of the growth substance.

The avena coleoptile curvature bioassay was essential for the studies on phototropism and gravitropism and eventually for the discovery that the indole-3-acetic acid (IAA) was the endogenous auxin in plants.

Moreover, the bioassay results also indicated that IAA transport was a key factor in controlling plant growth and that for the most part, IAA is made in the tip region and moves from the tip to the base in a polar fashion. Polar auxin transport was studied using the curvature bioassay as depicted below.

That auxin transport was specifically controlled was shown in experiments using radioactive IAA (see below). Agar blocks containing labeled IAA (donor blocks) were placed on one end of an excised stem or coleoptile segment and an agar block containing no auxin or unlabelled auxin was placed on the other end to receive any radioactive IAA that was transported. IAA was found to only move in a basipetial fashion (that is from the top end to the bottom end).

In both phototropism and gravitropism, lateral redistribution of auxin has been shown to play a role in establishing the differential cell elongation that causes stem or root curvature. In the root, auxin that moves down from the shoot in cells in the central vascular tissues. When the auxin reaches the root cap, some is redistributed and transported in the epidermal and outer cortical cells. Upon gravistimulation, the flow of auxin is laterally altered so that more move back to the elongation zone on the lower surface than on the upper surface. Because roots are more sensitive to auxin than shoots, cell elongation is inhibited on the lower side and the root curves down.

One explanation for why shoots bend up and roots bend down in response to gravity, even though gravity causes auxin to be laterally redistributed to the lower side of both, has to do with different sensitivities of these plant parts. Specifically, elongation assays with applied auxin have shown that roots are more sensitive than shoots to auxin. Most plant hormones show does-responses that increase to a maximum. However, above the maximum, the hormone can be inhibitory. Thus, if in a vertical plant (non-gravistimulated) the endogenous hormone is at the level depicted by the red dashed line in the figure above, root growth is already in the downward slope of the supraoptimal response. However, in the shoot, the endogenous level is below the optimum for growth. After rotating the plant to a horizontal position, the level of auxin increases on the lower surface of both. In the root, this increases the level further into the inhibitory range but in the shot, growth will be promoted as the level of auxin move closer to the response optimum. It is not clear why roots and shoots have different dose-response relationships. Perhaps the level of auxin receptors differs.