Agriculture Reference
In-Depth Information
gene under control of a HMW-GS promoter.
They detected minor differences in the timing
of accumulation of transcripts for some seed
storage and other abundant proteins, compared
with seeds from non-transgenic control plants.
By the last time-point at 32 days after pollination,
the transcript compositions of transgenic and
control seeds were the same (Gregersen et al.,
2005).
continue to be a method for identifying the gene
sequences that determine or affect important
agronomic and utilization traits. The proven
success of RNA interference to generate allele-
and family-specifi c down-regulation of genes in
the wheat polyploid genome has no peer in tech-
nology at this time. Determining the sequences of
important genes is more than just an academic
exercise: knowing exactly which sequence con-
trols or infl uences a given trait provides wheat
breeders with a perfect molecular marker, not
only for detecting the presence or absence of a
gene but also for identifying relevant individual
alleles of that gene. Gene sequences are also
needed by geneticists seeking to detect useful
chemically induced mutations in known wheat
genes by targeting induced local lesions in
genomes (TILLING) (Slade et al., 2005). Such
alleles can potentially provide novel variation for
genes of interest.
Wheat transformation could benefi t from
technology improvements in effi ciency, in wid-
ening of the range of genotypes to which it
can be applied, in precision of insertion, in
better control of insertion structures, and in
better understanding of promoter activity. Basic
research progress in model plants with smaller
genomes, such as Arabidopsis and rice, is
likely to yield better understanding of position
effects, transgene silencing, and the complete
set of features that allow predictable expression
of transgenes in new genomic contexts. More
promoters are needed whose activity is restricted
to a limited number of plant tissues and/or
developmental stages. For most types of pest
resistances, it would be desirable to have a
promoter with high levels of activity in green
tissues or roots, but no activity in wheat
seeds. Another type of promoter that would be
valuable is one that can be activated by external
cues that an experimenter or grower could
control, for example, the ethanol-inducible
promoter described by Li et al. (2005a). Such
promoters need not come from wheat, but
would have to be active in wheat. In addition,
the inclusion of recognition sequences for site-
specifi c recombinases could allow marker gene
excision, site-specifi c integration, and stacking
Practical considerations
In summary, so far no consistent or large differ-
ences have been detected between transgenic and
non-transgenic wheat plants in transcripts, pro-
teins, or metabolites, except for those expected to
be directly affected by transgene expression.
Despite the limitations of transformation technol-
ogy discussed earlier, the majority of transgenic
wheat lines best studied in fi eld trials so far—
those with HMW-GS and
Ubi1
::
bar
or glypho-
sate resistance transgenes—have the same yields
and other agronomic characteristics as their non-
transgenic parents.
The practical consequences of the random
insertions and unpredictable structures of inte-
grated transgenes are that multiple transforma-
tion events for each construct may need to be
screened to fi nd those that behave predictably in
terms of inheritance, expression, and lack of non-
target effects. For functional genomics research,
8 to 10 independent transformants are usually
suffi cient to determine the identities of genes with
major effects. For applied research that intro-
duces new genes into wheat, 20 or more transgen-
ics may be needed to fi nd those with the desired
phenotypes. For transgenic plants destined for
commercialization, hundreds of different trans-
genic events may be needed to fi nd one line with
stable inheritance, an easily defi ned integration
structure, high and appropriate expression levels,
desirable agronomic characteristics, and, in some
cases, no selection genes.
FUTURE PERSPECTIVES
Indisputably, wheat transformation will have a
major impact on wheat breeding, because it will
