Transgenic Applications in Wheat Improvement - Wheat Science and Trade - page 393

and Lr21 genes were identifi ed by map-based
cloning (Cloutier et al., 2007; Feuillet et al., 2003;
Huang et al., 2003, respectively). Each sequence
encoded recognizable gene-for-gene resistance
proteins with typical coiled coil, leucine-rich
repeat, and nucleotide-binding-site domains.
Genetic transformation was used to prove that the
sequences did indeed control the resistance phe-
notypes. In the case of Lr1 , transformation of a
susceptible cultivar with the candidate gene con-
ferred resistance to a Puccinia triticina race carry-
ing avirulence gene Avr 1. The extent of resistance
was correlated with transgene dosage and expres-
sion levels (Cloutier et al., 2007). In the case of
Lr10 , overexpression of the candidate sequence
under control of the Ubi1 promoter conferred
enhanced resistance to the expected races of leaf
rust (Feuillet et al., 2003). In the case of Lr21 ,
transformation with the candidate sequence com-
plemented the absence of the equivalent allele in
'Fielder', a cultivar that had been susceptible to
the tester leaf rust isolate PRTUS6 (Huang et al.,
2003).
Transformation has been instrumental in
identifying some of the key components in the
complex network of wheat genes that control the
transition from vegetative to reproductive states.
In early experiments, Chong et al. (1998) reported
that an antisense construct to the wheat ver203
gene delayed heading in the fi rst generation of
wheat plants transformed by the pollen-tube
method. In later experiments an antisense version
of VER2 , a related gene expressed in young
leaves surrounding the shoot apex, resulted in a
44-day delay in heading after vernalization of
winter wheat (Yong et al., 2003). In wheat plants
carrying a sense-suppressed WAP1 transgene, a
wheat gene with similarities to the Arabidopsis
gene AP1 that induces fl owering, heading times
were delayed regardless of vernalization or pho-
toperiod (Murai et al., 2003). The authors con-
cluded that the WAP1 gene product accelerated
the autonomous phase transition from vegetative
to fl owering states (Murai et al., 2003). In the
fi rst application of RNAi strategy to confi rm the
identifi cation of a gene sequence isolated by map-
based cloning, Yan et al. (2004) showed that
decreased expression of the vernalization gene

VRN2 accelerated fl owering time in winter wheat
by more than four weeks, proving their hypoth-
esis that VRN2 encodes a repressor of fl owering.
Members of the same group also used RNAi to
show that the VRN1 gene encoded an inducer of
fl owering; reductions in VRN1 transcripts in
spring wheat transformed with an RNAi con-
struct delayed the transition of the shoot apex
from vegetative to reproduction growth, increas-
ing the time to heading by 2 to 3 weeks
(Loukoianov et al., 2005). Continuing this
approach, members of the same group used map-
based cloning to isolate a candidate gene encod-
ing a transcription factor that they hypothesized
was the product of a third vernalization gene,
VRN3. Transformation of winter wheat with a
sense construct of this gene resulted in its over-
expression and accelerated fl owering (Yan et al.,
2006).
Wheat transformation also was instrumental in
verifying the sequence and pleiotropic effects of
the Grain Protein Content ( GPC-B1 ) gene, which
modulates protein, zinc, and iron contents of
wheat grain. A quantitative trait locus from Triti-
cum turgidum ssp. diccocoides was found to increase
the levels of these three nutrients in domesticated
durum wheat carrying a segment of the 6B chro-
mosome from T. dicoccoides . The same segment
accelerated senescence of the fl owering plant. To
show that a single gene was responsible for all of
these phenotypes, Uauy et al. (2006) designed an
RNAi construct to decrease expression of all
genes homologous to GPC-B1 in a domesticated
hexaploid wheat cultivar. The resultant trans-
genic plants senesced more than 3 weeks later
than non-transgenic siblings and had more than
30% reductions in seed protein, zinc, and iron
contents. Having the complete sequence of the T.
dicoccoides GPC-B1 allele is allowing breeders to
use molecular markers to introgress it into domes-
ticated wheat.

Applications to understand or modify
seed properties
Wheat supplies about 20% of the food calories
consumed by the world's population. Thus it is
not surprising that understanding the genetic