Cambridge International A Level Biology Revision Guide

Chapter 17: Selection and evolution
Breeding for resistance to various fungal diseases,
such as head blight, caused by Fusarium, is important,
because of the loss of yield resulting from such infections.
Successful introduction of an allele giving resistance
takes many generations, especially when it comes from a
wheat grown in a different part of the world. To help with
such selective breeding, the Wheat Genetic Improvement
Network was set up in the UK in 2003 to bring together
research workers and commercial plant breeders. Its aim is
to support the development of new varieties by screening
seed collections for plants with traits such as disease
resistance, or climate resilience (Figure 17.19), or efficient
use of nitrogen fertilisers. Any plant with a suitable
trait is grown in large numbers and passed to the
commercial breeders.
Wheat plants now have much shorter stems than they
did only 50 years ago. This makes them easier to harvest
and means they have higher yields (because they put
more energy into making seeds rather than growing tall)
(Figure 17.20). The shorter stems also make the plants
less susceptible to being knocked flat by heavy rains, and
means they produce less straw, which has little value and
costs money to dispose of.
Figure 17.20 Harvesting wheat.
Most of the dwarf varieties of wheat carry mutant
alleles of two reduced height (Rht) genes. These genes code
for DELLA proteins which reduce the effect of gibberellins
on growth (page 355). The mutant alleles cause dwarfism
by producing more of, or more active forms of, these
transcription inhibitors. A mutant allele of a different
gene, called ‘Tom Thumb’, has its dwarfing effect because
the plant cells do not have receptors for gibberellins and so
cannot respond to the hormone.
Rice, Oryza sativa, is also the subject of much selective
breeding. The International Rice Research Institute, based
in the Philippines, holds the rice gene bank and together
with the Global Rice Science Partnership coordinates
research aimed at improving the ability of rice farmers to
feed growing populations.
The yield of rice can be reduced by bacterial diseases
such as bacterial blight, and by a range of fungal diseases
including various ‘spots’ and ‘smuts’. The most significant
fungal disease is rice blast, caused by the fungus
Magnaporthe. Researchers are hoping to use selective
breeding to produce varieties of rice that show some
resistance to all these diseases.
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Figure 17.19 Wheat breeders are attempting to produce
new varieties of wheat that will be able to grow in the higher
temperatures that global warming is expected to bring.