Download Auxin reading

Auxin
The first hormone that we will discuss is auxin. Its chemical abbreviation is IAA. This
is the hormone that Darwin discovered when he did his experiments with phototropism. He
did not discover the chemical structure of auxin, but he did describe its effect on the plant.
This hormone is produced in apical meristems and some other tissues. Auxin is an important
hormone because it has multiple target cells and is largely responsible for stimulating plant
growth, although it has some other actions as well. It stimulates plant growth by loosening
the cell wall. Plants can grow in one of two ways: they can increase their cell number or
they can increase their cell size. But a plant cell is constrained because there is a cell wall
surrounding the plasma membrane. If the cell is going to increase in size, this wall must
also increase in size. Remember the cell wall contains cellulose and a number of proteins; it
is a fairly rigid structure. Auxin loosens these macromolecules so that that cell wall can
expand as the cell grows. Some of these proteins in the cell wall are coded for by genes
named expansins. They were discovered by a faculty member in the Biology Department at
Penn State in University Park, Dr. Dan Cosgrove, and are also involved in this process of cell
wall expansion.
This is how phototropism and gravitropism work. Auxin works to stimulate cell
growth to change the direction of growth of a root or a stem. Figure 9.21 shows some of the
experiments that the Darwins did when they discovered the effect of auxin. Figure 9.21a,
Frame 1, shows the normal response in phototropism: the shoot bends toward the light. The
Darwins then covered the tip of the shoot to block the light, and as shown in Frame 2, the
phototropic response did not occur. If they covered the tip with a transparent cover, shown
in Frame 3, the response occurred. Finally, they covered the stem below the tip as shown in
Frame 4; again, the response occurred.
So as long as the tip is exposed to the light, phototropism occurs. They also cut off
the tip (not shown in Figure 9.21a), and there was no response to light. Their experiments
showed that there was something being produced in the tip that caused the response; this
something was auxin that was produced in the apical meristem. In later experiments done
by others, the tip was cut off and gelatin blocks inserted between the tip and the shoot.
When the tip and shoot are not in contact, the response does not occur. However, if the
gelatin block that was under the tip is placed on top of the shoot, the shoot grows (Figure
9.21b). This experiment demonstrated that the substance causing the response could
diffuse from the cells in the tip and be picked up by the cells in the stem. However, the
shoot grows straight, rather than bending as is seen in phototropism. So how does auxin
cause the phototropic response?
Figure 9.21a. Experiments by the Darwins Demonstrating that Something in the
Tip of the Shoot Caused Plant Growth
Figure 9.21b. Later Experiments that Showed the Substance Diffused and the Plant
Responded to Differences in Concentration of the Substance (Auxin)
The last picture in Figure 9.21b shows the results of an important experiment. It is
similar to the experiment described above, when the gelatin block that had been under the
tip was applied to the shoot and the shoot grew. However, in this experiment the block was
applied to one side of the shoot, not in the middle, so the auxin diffused down only one
side. This caused the shoot to bend and demonstrated how auxin works by changing its
concentration. The cells on the side that the auxin diffuses down elongate faster than the
cells on the other side. This causes the shoot to bend. When a plant bends toward the light,
the auxin concentration is higher on the side away from the light; these cells elongate more
and this causes the seedling to bend so that it's facing the light. This probably is the same
way that auxin works in roots. The distribution of the statoliths on the "bottom" of the root
cells after the plant is turned causes the calcium concentration to change, leading to more
auxin concentration at the cells on "top," those cells elongate more and push the root down.
This difference in auxin concentration on one side causes the bending either of the shoot in
response to light or the root in response to gravity.
Once you have completed the reading write a two paragraph summary about auxin and its effects on
plants. Include in your summary how auxin effects phototropism.