Agriculture Reference
In-Depth Information
alleles (i.e.
a
a
B
B
) the plant is resistant to the disease,
as the pest opened (or overcame) the
A
resistance gene,
but could not open the
B
resistance gene. Similarly,
when the pest genotype is homozygous for the reces-
sive virulent alleles
b
b
but has no
a
virulence alleles
(i.e.
A
A
b
b
) the plant is also resistant to the disease,
in this case because the pest opened (or overcame) the
B
resistance gene, but could not open the
A
resistance
gene. In the last example, the pest genotype is homozy-
gous recessive for both the
a
a
and
b
b
virulence alleles
(i.e.
a
a
b
b
) and can open (or overcome) both the
A
and
B
resistance genes in the host, and hence the host
plant is susceptible to infection by the disease.
However, the situation is far from being this simple.
Resistance to pests or diseases can be the result of either
qualitative (single gene) or quantitative (multiple gene)
inheritance. Resistance that is controlled by a single
gene will result in distinct classes of resistance (usually
resistant or susceptible) and are referred to as
specific
or
vertical resistance
. Resistance that is controlled by
many genes will show a continually variable degree of
resistance and is referred to as
non-specific, field, gen-
eral
or
horizontal resistance
. Throughout this text the
terms used will be vertical or horizontal resistance.
Vertical resistance is associated with the ability of sin-
gle genes to control specific races of a disease or pest. The
individual alleles of a major gene can be readily identi-
fied and transferred from one genotype to another. In
many cases the source of the single gene resistance is
derived from a wild or related species and backcross-
ing is the most common method to introduce the allele
into a commercial background. Segregation of single
genes can be predicted with a good degree of reliability
and the selection of resistant genotypes can be relatively
simply achieved by infection tests with specific pathogen
races.
The primary disadvantage of vertical disease resis-
tance is that new races of the pathogen are quite likely
to arise that will be able to completely overcome the
resistance. These new races may, in fact, have existed at
a low level within the population of the pathogen before
the resistance was even incorporated into the new cul-
tivar or that cultivar was grown in agriculture. Thus,
of course, the 'resistance' can be overcome relatively
quickly. In addition, introduction of vertical resistance
will increase the selective advantage of any mutant that
arises in the pathogen population that can overcome the
resistance. And as only a single mutation is required,
must be made to ensure that other yield reducing fac-
tors (e.g. other air-borne fungi or other diseases) are
kept under control in the trial.
Types of plant resistance
It has been claimed that for each gene in the host that
controls resistance there is a gene in the pathogen that
determines it, the pathogen will be
avirulent
(unable to
overcome the resistance and hence unable to infect or
injure the host) or
virulent
(able to infect or injure the
host). This gene-for-gene hypothesis has been likened
to a set of locks and keys. A simple example of this
lock and key situation is shown in Table 3.1. In the
first example, the host plant genotype has no resistance
genes, so any pest genotype will be able to infect the host
plant irrespective of the presence or absence of virulence
alleles. In the second example, the host plant has one (or
two) dominant genes for resistance to pest
A
strain (i.e.
A_bb
), and as the pest genotype has no virulence genes,
the plant is resistant to the disease. In the third example,
the host plant again has one (or two) dominant resis-
tance genes against the
A
strain of pest but now the pest
genotype is homozygous recessive (i.e. two copies) of
the virulence gene (
a
a
) and therefore unlocks (or can
overcome) the resistance gene (
A
) in the host plant, and
the plant is susceptible to the disease. In the last three
examples the host plant has one or more copies of the
dominant resistance genes to both pest
A
and
B
strains,
(i.e.
A_B_
). When the pest genotype is homozygous for
the recessive virulent alleles
a
a
but has no
b
virulence
Table 3.1
Possible phenotypic plant response (i.e. resistant
to disease or susceptible to disease) in various combinations
of dominant alleles conferring single gene plant resistance
(capital letters A or B represent resistance genes), and
recessive alleles conferring susceptibility (a or b) and the
'matching' alleles in the pest where a
or b
confer virulence
and A
and B
give avirulence.
Plant genotype
Pest genotype
Plant response
aabb
Any virulence gene
Susceptible
A_bb
No virulence gene
Resistant
a
a
B
B
A_bb
Susceptible
a
a
B
B
A_B_
Resistant
A
a
b
b
A_B_
Resistant
a
a
b
b
A_B_
Susceptible
