Plant Hormones
Types of hormones
 Like animals, plants use hormones to
produce functional and structural changes.
 Types of hormones include
 Auxins
 Gibberellins
 Cytokinins
 Ethylene
Auxin
 Auxins are a type of hormone that produces
cell elongation and allows the plant to grow
in length and to develop.
 The natural Auxin is a compound called
indoleacetic acid or IAA
Auxin
Auxin associated with phototropism - early experiments
demonstrate tip as receptor.
Auxin continued
 Auxin is made at the apical meristem of a
shoot and moves down at a rate of 10 mm
per hour to the region of elongation and
stimulates cell growth.
Question????
 At this speed, how do you think it is
transported through the cell?
Auxin moves down the plant
Answer
 Too slow for phloem transportation
 It is carried by the parenchyma tissue
from one cell to the next.

Auxins do two things to help with cell
elongation:
1. Soften the cell walls so the plant can
expand
2. Increase the rate of cell division
Demonstration of transported chemical
Loosening of cell wall
Lateral growth
 Auxins produced in the meristem inhibits growth
of side branches (lateral stems)
Cytokinins
Cytokinins
 You will remember from Mor 4 that cytology
is the study of cells.
 The hormone, Cytokinins stimulate cell
division (mitosis and cytokinesis)
 Cytokinins are produced by actively growing
tissue and can move throughout the plant in
the xylem, phloem and parenchyma cells.
Function of cytokinins

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Promotes cell division.
Lateral bud development.
Delay of senescence (AGING).
Stomata opening.
Rapid transport in xylem stream.
Cytokinins are used with Auxins to
help plants to grow and develop
Gibberellin
Effects of Gibberellins
 General cell elongation.
 Breaking of dormancy.
 Promotion of flowering.
Gibberellins
 Gibberellins are growth hormones that allow the
plant to grow in length.
 Gibberellins were discover in Asia when some
farmers noticed that rice would grow too fast and
tall which was caused by a fungus called
Gibberella.
 Gibberellins are usually produced in roots and
young leaves and stimulate a growth in leaves and
stems but not in roots.
 Gibberellins are also found to increase seed
germination and flower development
Discovered in association with Foolish
disease of rice (Gibberella fujikuroi)
uninfected
infected
Gibberellins and plant growth
Ethylene
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C = C
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Functions of ethylene
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Gaseous in form.
Rapid diffusion.
Affects adjacent individuals.
Senescence (aging) and abscission (the
natural process by which leaves or other
parts are shed from a plant).
 Interference with auxin transport.
 Initiation of stem elongation and bud
development.
Ethylene gas
 Years ago, farmers use to heat fruit in a
kerosene oven to make them ripen faster.
Scientist learnt that it was not the heat but the
ethylene gas byproduct that made it ripen
 We now know that plants develop their own
ethylene as a hormone that makes fruit ripen
Ethylene
 Scientist also discovered that ethylene is the
opposite to Auxin and inhibits cell growth
Thanks Class
Have a great day!!!