Presentation 18 – Plant Hormones (for Li, K, Sr)

• Chemical messengers produced by one plant organ affect cells

in another location within the plant

• Very small concentrations can cause substantial changes in the

plant

• Most often used to regulate the growth and maturity of the

plant

• Picture on the left shows a potato grown in the dark. Because of

the absence of light, an imbalance of growth hormones causes

many sprouts to grow out of the potato in many different

directions. This causes the potato to rapidly “eat up” the starch

remaining in the tuber.

• Picture on the right is a dramatic representation of a plant shoot

growing towards the light (candle). You may have seen this in

plants that grow in shaded areas on potted plants in window

sills.

• One important function of plant hormones is to coordinate

growth responses called tropisms

• Plant organs respond to stimuli by growing towards them

(positive tropism) or way from them (negative tropism)

• Phototropism is response toward light: shoot tips of plants are +

phototropic while the root tips are – phototropic

• Thigmotropism is response to mechanical pressure or “touch,”

some plant organs like tendrils (modified stems) are +

thigmotropic, wrapping around other plants or trellises

• Gravitropism/geotropism is response to the force of gravity.

Shoot tips are – gravitropic, root tips are + gravitropic. In the

picture on the lower right, no matter how you orient the seed

when planting it, the roots will always grow downward.

• Phototropism has been much studied since the 1800’s.

• How do plants curve towards the light? This involves a

difference in growth of the cells on both sides of a plant shoot.

(A coleoptile is a special name for the covering of a grass shoot).

The cells on the side of the plant that faces away from the

source of light should elongate more that the cells on the side

of the plant that faces the light source. This causes the plant to

appear to “bend” towards a given source.

• Experiments by Charles and Francis Darwin as well as FW Went

showed that this happens because of the distribution of a plant

hormone called auxin.

FW Went’s Experiment

• The Darwins discovered that the substance that caused the

bending was produced by the plant’s shoot tip even if the

bending itself occurred on the shoot, a short distance away

from the tip.

• Went discovered that the substance that caused the bending

was a chemical called “auxin,” (from the Gr. word “to

grow/increase”). Auxin was produced by the shoot tip of plants

in response to light. When a shoot tip was cut and placed on a

porous material, like a block of agar, auxin would diffuse into

the agar block.

• Went wanted to demonstrate that it was the difference in the

concentration of auxin that caused the difference in growth.

He took the agar block with the auxin and placed it on various

positions on top of shoots that had their tips cut off.

• In the drawing above, the first two shoots were the – controls.

They showed no growth differences. The third had the auxin-

agar block placed evenly on top. It too showed no growth

difference. This is because the auxin was distributed evenly on

both sides of the plant shoot. The fourth and fifth shoots had

the auxin-agar block placed a little bit off-center, the fourth

had it placed on the right, the last had it placed on the left

side. This caused the shoots to curve as auxin caused the cells

on the side it was on to elongate more.

• There are five major classes of plant hormones (and many more minor classes). This table summarizes their origins,

locations, and major functions. All of these hormones work together to ensure proper growth and development and

proper response to stimuli. Familiarize yourself with this table. Look up the meanings of the encircled words.

growth/elongation

“fountain of youth”

hormone

elongation and

proper development

stress hormone

maturation hormone causing

fruit ripening and leaf aging