Cambridge International A Level Biology Revision Guide

Chapter 15: Coordination
Auxins and elongation growth
Plants make several chemicals known as auxins, of
which the principal one is IAA (indole 3-acetic acid,
Figure 15.36). Here, we refer to this simply as ‘auxin’ in
the singular. Auxin is synthesised in the growing tips
(meristems) of shoots and roots, where the cells are
dividing. It is transported back down the shoot, or up the
root, by active transport from cell to cell, and also to a
lesser extent in phloem sap.
N
H
CH 2 COOH
Figure 15.36 The molecular structure of indole 3-acetic acid,
IAA.
Growth in plants occurs at meristems, such as those at
shoot tips and root tips (Chapter 5). Growth occurs in three
stages: cell division by mitosis, cell elongation by absorption
of water, and cell differentiation. Auxin is involved in
controlling growth by elongation (Figure 15.37).
cellulose microfibrils
of the plant cell wall
auxin
cell surface
membrane
auxin
receptor
stimulation
H +
K +
H O
H + 2
aquaporin
K +
H 2 O
Figure 15.37 The binding of auxin to its receptor is thought to
activate a membrane protein, which stimulates the pumping
of protons out of the cell into the cell wall where they lower
the pH and break bonds. Potassium ion channels are also
stimulated to open leading to an increase in potassium ion
concentration in the cytoplasm. This decreases the water
potential so water enters through aquaporins.
H +
Auxin stimulates cells to pump hydrogen ions (protons)
into the cell wall. This acidifies the cell walls which leads
to a loosening of the bonds between cellulose microfibrils
and the matrix that surrounds them. The cells absorb water
by osmosis and the pressure potential causes the wall to
stretch so that these cells become longer, or elongate.
The details of this process are shown in Figure 15.37.
Molecules of auxin bind to a receptor protein on the
cell surface membrane. The binding of auxin stimulates
ATPase proton pumps to move hydrogen ions across the
cell surface membrane from the cytoplasm into the cell
wall. In the cell walls are proteins known as expansins
that are activated by the decrease in pH. The expansins
loosen the linkages between cellulose microfibrils. It is
not known exactly how they do this, but it is thought that
expansins disrupt the non-covalent interactions between
the cellulose microfibrils and surrounding substances,
such as hemicelluloses, in the cell wall. This disruption
occurs briefly so that microfibrils can move past each
other allowing the cell to expand without losing much of
the overall strength of the wall.
Gibberellins
Gibberellins are plant growth regulators that are
synthesised in most parts of plants. They are present in
especially high concentrations in young leaves and in
seeds, and are also found in stems, where they have an
important role in determining their growth.
Gibberellins and stem elongation
The height of some plants is partly controlled by their
genes. For example, tallness in pea plants is affected by a
gene with two alleles; if the dominant allele, Le, is present,
the plants can grow tall, but plants homozygous for the
recessive allele, le, always remain short. The dominant
allele of this gene regulates the synthesis of the last enzyme
in a pathway that produces an active form of gibberellin,
GA 1
. Active gibberellin stimulates cell division and cell
elongation in the stem, so causing the plant to grow tall.
A substitution mutation in this gene gives rise to a change
from alanine to threonine in the primary structure of the
enzyme near its active site, producing a non-functional
enzyme. This mutation has given rise to the recessive
allele, le. Homozygous plants, lele, are genetically dwarf as
they do not have the active form of gibberellin. Applying
active gibberellin to plants which would normally remain
short, such as cabbages, can stimulate them to grow tall.
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