Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
356
Gibberellins and seed germination
Gibberellins are involved in the control of germination
of seeds, such as those of wheat and barley. Figure 15.38
shows the structure of a barley seed. When the seed is
shed from the parent plant, it is in a state of dormancy;
that is, it contains very little water and is metabolically
inactive. This is useful because it allows the seed to survive
in adverse conditions, such as through a cold winter, only
germinating when the temperature rises in spring.
The seed contains an embryo, which will grow to form
the new plant when the seed germinates. The embryo
is surrounded by endosperm, which is an energy store
containing the polysaccharide starch. On the outer edge of
the endosperm is a protein-rich aleurone layer. The whole
seed is covered by a tough, waterproof, protective layer
(Figure 15.38).
The absorption of water at the beginning of
germination stimulates the embryo to produce
gibberellins. These gibberellins diffuse to the aleurone
layer and stimulate the cells to synthesise amylase. The
amylase mobilises energy reserves by hydrolysing starch
molecules in the endosperm, converting them to soluble
maltose molecules. These maltose molecules are converted
to glucose and transported to the embryo, providing a
source of carbohydrate that can be respired to provide
energy as the embryo begins to grow.
Gibberellins cause these effects by regulating genes that
are involved in the synthesis of amylase. In barley seeds, it
has been shown that application of gibberellin causes an
increase in the transcription of mRNA coding for amylase.
It has this action by promoting the destruction of DELLA
proteins that inhibit factors that promote transcription
(Chapter 16 page 391). (DELLA stands for the first five
amino acids in the primary sequence of these proteins.)
QUESTION
15.12 a i Explain the advantages to plants of having
fast responses to stimuli.
ii The closure of a leaf of the Venus fly trap is an
example of the all-or-nothing law.
Explain why.
iii Suggest the advantage to Venus fly traps of
digesting insects.
b Outline how auxin stimulates elongation growth.
c i What are the phenotypes of plants with the
genotypes LeLe, Lele and lele?
ii When gibberellins are applied to dwarf pea
plants, the plants grow in height. Explain what
this tells us about dwarfness in pea plants.
embryo
water uptake initiates
germination
aleurone layer synthesises
amylase in response to
gibberellin
aleurone layer
gibberellin
amylase
starch
maltose
embryo synthesises
gibberellin in response
to water uptake
endosperm tissue
containing starch
reserves
Figure 15.38 Longitudinal section through a barley seed, showing how secretion of gibberellins by the embryo results in the
mobilisation of starch reserves during germination.