Our seeds are sprouted and curving toward the light (preferably blue). The curvature of the coleoptile is thought to be due to a difference between the auxin levels on the two sides of the coleoptile. Went found that unilateral light caused more auxin to be produced in agar blocks under the shaded side of coleoptile tips (compared with blocks under the lighted side).
This distribution of auxin toward the shaded side provides a concentration gradient across the coleoptile. This results in differential stimulation of growth. Growth is more rapid on the shaded side of the coleoptile than on the lighted side.
The fact that the lighted side is inhibited compared to dark controls indicates a redistribution of auxin such that the final concentration on the lighted side is less than in dark controls. Here expression of an auxin sensitive DR5:GUS reporter construct indicates the lateral distribution of auxin with light coming from the right. The differential growth on the two sides of the stem have resulted in its curvature. This closeup shows the detail of the auxin-sensitive promoter indicating the presence of auxin at high concentration on the shaded side of the stem.
When an auxin transport inhibitor (NPA) eliminates both the auxin lateral distribution and the curvature response.
But how does light accomplish the needed distribution of auxin across the coleoptile? It has taken many decades of work to figure that out.
It turns out that the light signal apparently causes increased synthesis on both lighted and shaded sides. However, synthesis is about three-fold greater on the shaded side. On the other hand, the synthesis was not due to NEW synthesis. Rather the auxin was made by conversion from "bound" forms...auxin bound to amino acid R-groups and/or sugars.
It is important that we also realize that amount of hormone is not the only determining factor in plant growth responses. In the late 1970s Trewavas pointed out that sensitivity of the responding cells was a critical factor.
Sensitivity is different in different tissues. Roots are clearly more sensitive to auxin concentration than are coleoptiles. It takes more auxin to stimulate coleoptiles than it does to stimulate roots. It is also true that sensitivity changes through development...and this is largely unexplored but important. For example the cells that respond to the light-stimulated auxin are located a certain distance down a coleoptile. Younger cells above this zone and older cells below this zone do not respond even though the auxin differential is essentially the same as for cells within the responding zone!
The distribution of auxins that causes differential growth in phototropism of coleoptiles also occurs in coleoptiles and roots held horizontally (perpendicular to the gravity vector).
A range of older experiments have investigated the role of gravity and auxin in gravitropism in roots.
In the gravity perception cells, starch grains fall against the inner and outer tangential walls of the columella cells in the root cap. Here are micrographs of a root tip and a close-up of a columella cell.
Gravity has caused the starch grain statoliths to fall inside this statocyte, lodging them against other cell components. This apparently is the perception mechanism of gravitropism.
This position of the starch statoliths in the statocytes causes a redistribution of auxin such that IAA accumulates in the tissue of the lower side of the horizontal root.
If you wonder how increased auxin on the lower side would cause the root tip to curve downward, then you are thinking along a good track. But if you remember the comment above about differential sensitivity in tissues, you can explain this response. Here is a graph to assist you in your thinking:
Think about the red arrow as indicating the concentration of the auxin on the shaded side of the coleoptile or root. Think of it as the concentration on the side of the coleoptile or root facing the gravity source. Perhaps now you understand why coleoptiles are positively phototropic, and negatively gravitropic, and you understand why roots are negatively phototropic and positively gravitropic.
Auxin effects also include stimulation of root formation, growth of stems and leaves, growth of flower parts, expansion of fruits, apical dominance, and the delay of leaf abscission. The stimulation of growth responses in some of these examples is accomplished by a specific sequence of events.
The effect of auxin can be demonstrated to occur within 10 minutes of application.
Moreover, the acid-induced rapid growth responses have been observed in both dicots and monocots.
These rapid growth kinetics are associated with acid-induced wall loosening and turgor driven growth. The pertinent information supporting that idea is shown below.
The hypothesis to explain auxin-signalled acid-induced growth is shown below.
One final point to make is that acid-induced growth is short-lived. While tremendous growth is possible under that mechanism, the cell walls become thin and eventually the cell breaks through the wall with its turgor and lyses. For a growth response to be sustained, additional wall material must be added to the wall. There is little surprise that sucrose is synergistic in many hormone-mediated growth responses. We saw this in a previous graph:
It is also true that longer-kinetics of sustained growth involve cooperation between hormone signals. This relationship between auxin and gibberellic acid is shown here.