Genomics-Assisted Breeding for Fusarium Head Blight Resistance in Wheat - Translational Genomics for Crop Breeding: Biotic Stress - page 48

Genomics-assisted wheat breeding has been
reviewed recently by Gupta et al. (2010), who
also list a few cases where genomics-assisted
breeding for FHB resistance improvement was
performed. Gupta et al. (2010) stated that
marker-assisted selection (MAS) is being prac-
ticed for improvement of a variety of traits in
wheat around the world. Marker-trait associ-
ations have been discovered for a number of
genetically relatively easy but difficult-to-score
traits. Hence MAS has been found useful for
the improvement of such traits. However, for
improvement of highly complex polygenic traits,
novel approaches using high throughput and
dense genotyping and new selection strategies
such as AB-QTL mapping, mapping-as-you-
go, marker-assisted recurrent selection, asso-
ciation mapping, and genome-wide selection
(GS) should be further developed (Gupta et al.
2010). This statement appears very appropriate
for FHB resistance breeding. Assessing genetic
differences in FHB resistance is not trivial and
requires well-replicated disease resistance tests
usually done in specific nurseries. Several reports
showed that MAS was efficient for the fixation
of a limited number of well-characterized QTL,
as outlined in the following paragraphs.
In addition to detectable QTL, FHB resis-
tance is also modulated by an unknown num-
ber of medium- to small-effect QTL that usually
remain undiscovered in conventional QTL map-
ping. Selection for such minor resistance factors
was only possible through sophisticated pheno-
typic selection until recently. In order to better
exploit the potential of genomics-assisted breed-
ing for polygenic traits, genome-wide selection
(GS), first proposed by Meuwissen et al. (2001),
offers great promise also for crop plant improve-
ment (Heffner et al. 2009, 2010). So far no
peer-reviewed research paper using GS for FHB
resistance has been published, but initial results
appear highly encouraging (Rutkoski et al. 2012;
Hofstetter et al. 2011). Up to now, only a subset
of the reported FHB resistance QTL has been
independently validated and an even lower num-
ber of QTL have been applied in marker-assisted
breeding (Buerstmayr et al. 2009).

Genomics-Assisted Breeding for
FHB Resistance

Although a number of transgenic approaches for
improving FHB resistance have been performed
(see Buerstmayr et al. 2012 for review), this
article does not cover transgenics. This chapter
focuses entirely on genomics-assisted breeding
for resistance improvement by utilizing the
plant's native resistance genes. In order to
apply marker-assisted breeding, it is necessary
to know the position and effect of resistance
QTL. Whereas mapping of QTL is still
resource-intensive, because it requires extensive
investments in genotyping and phenotyping, the
application of markers indicative of resistance
QTL in cultivar improvement is comparably
quick and easy.
Table 4.1 illustrates a survey of published
QTL validation and marker-assisted germplasm
improvement studies for Fusarium head blight
resistance in wheat. The earlier studies in the
field of genomics-assisted breeding were not
yet explicit MAS projects, but relied on QTL
validation populations that were used to assess
the resistance-improving effects of various QTL
alleles in specific populations. In more recent
studies, MAS has been performed by moving
the desired QTL alleles into regionally adapted
lines and evaluating the selection response asso-
ciated with the specific QTL. Until now, no FHB
resistance QTL has been cloned, therefore linked
markers are the only option for MAS. Currently,
an almost perfect marker is available only for
Fhb1 : the marker Umn10 (Liu et al. 2008). Very
recently, initial results on genomic selection for
improving FHB resistance have been published
(Rutkoski et al. 2012).

MAS for the Major FHB Resistance
Gene Fhb1

The first, and to date best, validated FHB resis-
tance QTL is derived from the Chinese spring
wheat cultivar Sumai-3, and is mapped to chro-
mosome arm 3BS. This QTL was indepen-
dently discovered by Waldron et al. (1999), Bai