Agriculture Reference
In-Depth Information
INTRODUCTION
In this chapter, we provide a short description
of transformation methods and recount the results
of published transformation experiments aimed at
understanding and/or improving a number of
wheat agronomic and end-use properties. We also
discuss some of the current limitations of wheat
transformation technology and prospects for its
impact on wheat improvement. We confi ne our
coverage to wheat transformation studies pub-
lished in English, with occasional reference to
results from transformation studies in rice (
Oryza
sativa
L.) or model species.
There are several excellent published reviews
of wheat transformation, some of which cover in-
depth topics only mentioned in passing in this
chapter. Patnaik and Khurana (2001) present a
detailed history of transformation methods. Vasil
(2007) brings both historical and futuristic per-
spectives to the subject, emphasizing how this
technology will help solve problems facing agri-
culture worldwide. Other good overviews of this
subject are by Sahrawat et al. (2003) and Jones
(2005).
Agrobacterium
-based transformation is
treated in detail by Jones et al. (2005) and Shrawat
and Lörz (2006).
Wheat transformation is the introduction of
DNA into wheat cells via means other than
sexual hybridization. For the purposes of this
chapter, transformation is a process that results
in integration and inheritance of new DNA. By
these criteria, the fi rst published report of
successful wheat transformation was by Vasil
et al. (1992). DNA can also be introduced
and expressed transiently in a nonintegrative
form, but this does not result in inheritance of
new genetic material.
Wheat transformation allows experimentalists
to introduce DNA from any source. The genes
for new or improved traits no longer need come
from sexually compatible relatives; even novel
synthetic genes made
in vitro
can be utilized.
However, engineering completely novel traits
into plants is challenging and in practice, consid-
erations of mRNA and protein stability and how
introduced proteins may interact with existing
pathways can limit the choice and donor species
of candidate genes. In a genetic transformation
experiment, only a single or a few genes are intro-
duced, allowing, at least in theory, changes to be
made without modifying the background geno-
type. This differs from traditional breeding by
which two entire genomes are combined in an
initial cross. To recover the original genotype
with a new trait after such a cross, several rounds
of backcrossing need to be undertaken with selec-
tion for the new DNA of interest in each round.
Even in extensively backcrossed material, genes
closely linked to the gene of interest from the
source parent will persist in the progeny, so-called
linkage drag. In contrast, genetic transformation
allows the experimenter to introduce only known
genes. An interesting special case exists when the
introduced DNA has been isolated solely from a
sexually compatible species (so-called intra- or
cis-genics) (Rommens 2007). Although the
methods used are different, nothing other than
gene location, and possibly expression levels, may
distinguish the plants produced in this way from
the products of conventional crossing or mutation
breeding.
WHEAT TRANSFORMATION: METHODS
AND RESULTS
Wheat transformation protocols consist of several
steps: culturing of explants that contain cells
capable of multiplying and organizing into
somatic embryos; introduction of DNA into
those totipotent cells; identifi cation of cell clus-
ters or embryos that have integrated the incom-
ing DNA into chromosomes; and regeneration of
fertile plants.
Targets for wheat transformation
In wheat, there are few cells that are known to be
capable of division, organization into somatic
embryos (differentiation), and regeneration of
shoots and roots into fertile plants. To serve as
transformation targets, such cells must be located
