Agriculture Reference
In-Depth Information
taining diagnostic nucleotide changes, including
an
Aci
I restriction enzyme site which permitted
the development of a cleavage amplifi cation poly-
morphic sequence (CAPS) based marker. The
robust
VRN
-
A1
CAPS marker can be used in
molecular breeding applications and to determine
the allelic state of germplasm and breeding
material.
(Cuthbert et al., 2006b). Similarly, the FHB-
resistance QTL identifi ed on chromosome 6B has
been mapped and named
Fhb2
(Cuthbert et al.,
2006a). The move from large chromosomal seg-
ments to sectors closely linked to the gene(s)
controlling the trait of interest will facilitate
marker-assisted selection for quantitative traits
such as FHB resistance.
Precocious sprouting of mature wheat grain
can occur when wet, humid conditions exist prior
to harvest. Mature wheat grain can germinate
while plants are still in the fi eld. The kernels are
considered to be sprout-damaged, when there are
signs of growth such as swollen, exposed germs
and the appearance of root and shoot tips. Prehar-
vest sprouting (PHS) can have a serious impact
on grain yield and on end-use quality (Derera
1989; Kruger 1989), including noodle quality
(Nagao 1995). Preharvest sprouting resistance is
a complex trait affected by genotype, environ-
ment, plant diseases, and spike morphology.
Thus, molecular markers should be effective tools
to select for PHS resistance, because markers are
unaffected by the growing environment.
Because of the importance of this trait to wheat
production, several QTLs for PHS resistance
have been identifi ed in wheat (Anderson et al.,
1993; Bailey et al., 1999; Kato et al., 2001; Osa et
al., 2003; Mares et al., 2005; Mori et al., 2005).
These QTLs for PHS resistance have been
mapped to at least 10 of the 21 wheat chromo-
somes including 1A, 3A, 3B, 3D, 4A, 4B, 5A, 5D,
6B, and 7D. The location(s) of QTLs varied con-
siderably depending on the specifi c mapping
population, but PHS-resistance QTLs on group
3 and 4 chromosomes have been reported in mul-
tiple studies (Anderson et al., 1993; Kato et al.,
2001; Groos et al., 2002; Kulwal et al., 2005;
Mares et al., 2005; Mori et al., 2005). The impor-
tance of QTLs on the group 3 chromosomes
appears to be related to the pleiotropic effects of
the seed coat color alleles and the
TaVp1
genes.
In maize, the
Vp1
gene reportedly codes for a
dormancy-related transcription factor (Osa et al.,
2003). Introgression of major QTLs for
PHS resistance can increase grain dormancy
(Kottearachchi et al., 2006). Marker-assisted
selection for PHS resistance is desirable since it
COMPLEX TRAITS
Many agriculturally important traits, such as
grain yield, are quantitatively inherited and phe-
notypes are distributed over a range. The distri-
bution tends to indicate that the trait is controlled
by multiple genes, each giving a small effect, and/
or the trait is infl uenced by environmental condi-
tions such that trait expression is controlled by a
combination of genetic and environmental com-
ponents. The mapping of quantitative trait loci is
an important step toward the identifi cation of the
single genes that control a signifi cant proportion
of the phenotypic variation of quantitatively
inherited traits (Lander and Botstein 1989). The
continued development of high-density molecu-
lar maps (Somers et al., 2004) will permit the
fi ne-mapping that is necessary to move from
genomic regions to individual genes responsible
for quantitatively inherited traits.
This progression is evidenced in the isolation
of QTLs associated with Fusarium head blight,
a destructive disease which reduces grain yield
and negatively impacts end-use quality of wheat.
Pathogenesis can include toxin production which
has serious health implications even when present
in low levels, for both livestock and humans
(Gilbert and Tekauz 2000). Various QTLs for
FHB resistance have been identifi ed on several
wheat chromosomes, including 2D, 3A, 3B, 4B,
5A, 5B, 6B, 7A, and 7B (McCartney et al., 2007),
and these impact different components of host-
plant resistance. Arguably, the most important
QTL associated with FHB resistance is from the
Chinese wheat cultivar Sumai-3 and is located on
chromosome 3BS (Liu and Anderson 2003; Zhou
et al., 2003). Recently, the locus on 3BS was
mapped as a Mendelian trait and named
Fhb1
