Agriculture Reference
In-Depth Information
or sodium azide, or through larger chromosomal changes caused by -ray irradiation. For
disease resistance work, the former is preferable as the smaller changes generated are less
likely to disrupt other genetic processes and traits. Because multiple changes occur with
either method, further crossing and selection of mutant lines is usually required in order
to produce a variety suitable for commercial release.
A highly successful example is the development of durable resistance to powdery
mildew (
Blumeria graminis
f. sp.
hordei
) controlled by resistance at the
mlo
locus in
barley. The fi rst mutant at the locus was induced by X-rays in 1942 and since then, a series
of new independent mutation events have been developed. Some of these mutants have
been widely deployed in high yielding cultivars across Europe. While plenty of naturally
generated variation has existed for this trait, there has been a long history of major resis-
tance genes losing their effectiveness through changes in the pathogen population. The
mlo
mutants have provided a source of resistance in adapted backgrounds that appears
to be effective against all races, that is it does not conform to the gene-for-gene system
identifi ed by Flor (1971). Initially, necrotic leaf spotting that accompanied the resistance
and which reduced grain yield, hindered the use of
mlo
mutants. Subsequent breeding
work eliminated those problems and from the 1980s onwards many varieties carrying
mlo
resistance have been grown throughout Europe without any virulence to the resistance
being detected (Helms Jørgensen, 1992).
It is of interest that the protein coded by the
mlo
gene is quite different from all other
resistance genes so far identifi ed, as the wild type allele codes for a cell membrane recep-
tor and it is the non-functional allele of this which provides resistance (Büschges
et al.,
1997). It is also noteworthy that the mutant
mlo
locus increases susceptibility to the
rice blast fungus
Magnaporthe grisea
(Jarosch
et al.,
1999) and the spot blotch fungus
Bipolaris sorokiniana
(Kumar
et al.,
2001).
Although
mlo
mutants have been widely used, natural variation at the
mlo
locus has also
been found from landrace collections made in Ethiopia that provided similar resistance.
Where mutation will have a more profound effect is where it provides resistance to an
important pathogen where naturally occurring resistances have not been found and espe-
cially where alternative control strategies are unavailable or economically costly. This is
the case for many soil-borne diseases, where variation is not present or where screening
is very time consuming, costly and/or unreliable. The recent report (Hershey, 2007) of a
mutant line with resistance to barepatch disease caused by
Rhizoctonia solani
(AG8) in
the wheat variety Scarlet in the USA is therefore of great interest, although it is yet to
be seen whether this resistance will operate in the fi eld. The resistance gene will be even
more valuable if it is found to operate against
Rhizoctonia
species in other crops.
6.4
Breeding methodology and selection strategies
for inbreeding crops
There are three general methods used for breeding for disease resistance: (a) pedigree
selection based on simple crosses between two parents, (b) backcrossing and (c) mass
selection methods (see Figure 6.1). Using different permutations of these methods in dif-
ferent generations of the breeding process provides a diversity of specifi c options that can
be tailored to the trait, the specifi c genes involved, the scale and budget for the program
and the breeder's personally favoured strategies.
