What
causes a plant to “grow-up,” roots to grow down, or plants to grow towards a
light source? The answer is plant hormones
or plant growth regulators as they are sometimes called. In this particular case the hormone involved
is the naturally occurring plant substance called indole-3-acetic acid or IAA
for short.
IAA’s primary function in life is to stimulate
cell growth especially near growing points.
In stems the more IAA present the more cell elongation occurs. If you place a living, growing stem on its
side it will naturally bend to return to growing in an upright position. Here’s
how it works in our horizontal stem example.
In response to basic gravity IAA accumulates in the bottom portion of
the stem. This increased concentration
causes the cells at the bottom of the stem to elongate (grow) at a faster rate
then those cells at the top causing the stem to naturally curve upwards. In roots IAA has the opposite effect. The more IAA presents the slower the growth
rate causing a natural downward growth.
Phototropism, or the growth of plants towards a light source, is also
under the control of IAA. IAA is
degraded by sunlight and thus accumulates in the shady side of stems causing
cells on the shady side to elongate at a faster rate then those on the sunny
side curving the stem to naturally grow towards the light. Amazing!
