Agriculture Reference
In-Depth Information
3.4.2 Fungicide response genes
DNA sequences of genes offer the same potential and problems for research on
fungicide resistance as they do for avirulence and have already been used in
population research. There is the substantial additional complication, however, that
in most cases, several genes are involved in control of the response to a fungicide.
All site-specific fungicides have a target enzyme and in some cases, mutation of the
target site has been shown to be involved in resistance, including mutation of
β
-tubulin in resistance to benzimidazole fungicides (Koenraadt
et al.
, 1992),
cytochrome b in resistance to QoIs (Sierotzki
et al.,
2000a, 2000b; Zheng
et al.,
2000) and the cytochrome P450 14
-demethylase (P45014DM) in resistance to
triazoles (Délye
et al.
1997, 1998; Wyand and Brown, 2005). In other cases,
however, resistance is conferred by mutations in non-target proteins, such as those
involved in active transport of the fungicide across the plasma membrane (Hayashi
et al.
, 2001; Schoonbeek
et al.
, 2001; Vermeulen
et al.
, 2001).
α
(a) A single gene encoding the target protein
QoI resistance is associated with mutation of amino acid residue 143 in the wild-type
cytochrome b sequence from glycine to alanine (G143A). Although this is not the only
mutation of cytochrome involved in resistance to QoI fungicides (Avila-Adame and
Koller, 2003; Kim
et al.
, 2003; Pasche
et al.
, 2005), it is much the most common and
gives rise to complete resistance. Fraaije
et al.
(2002) used PCR primers to devise
a real-time PCR system to estimate the frequencies of G143 and A143 alleles in
B. graminis
f.sp.
tritici
and showed that the frequency of A143 increases on exposure
of powdery mildew to QoI fungicides applied to wheat crops. Fraiije
et al.
(2005) used
a similar system to investigate frequencies of the A143 allele in field populations of
M. graminicola
and the dispersal of QoI-resistant ascospores.
Another resistance caused by a mutation of a single nucleotide is that of single
fungi to benzimidazoles. Wheeler
et al.
(1995) devised a PCR test based on
sequence variation in the
β
-tubulin gene to diagnose resistance in
R. secalis
.
(b) More complex genetic systems
Resistances to most fungicides are genetically more complex, and for these, it will
therefore be correspondingly more challenging to devise diagnostic tests for
fungicide resistance based on DNA sequences. Délye found that mutation from
tyrosine to phenylalanine at amino acid residue 136 (Y136F) of P45014DM
correlated with resistance to triadimenol, a triazole fungicide, in
Erysiphe necator
(formerly
Uncinula necator
), the grapevine powdery mildew fungus (Délye
et al.
,
1997) and in
B. graminis
f.sp.
hordei
(Délye
et al.
, 1998).
Research by Wyand and Brown (2005) on
B. graminis
supported this conclusion
but also showed that the situation is considerably more complex, such that a diagnostic
test for the Y136F mutation is not sufficient to characterise resistance. In
B. graminis
f.sp.
hordei
, three of the four known phenotypes of triazole resistance were associated
