Agriculture Reference
In-Depth Information
function through the application of targeting
induced local lesions in genomes (TILLING)
(McCallum et al., 2000).
formed or that its microspores can be used to
develop doubled haploids. The other aspect is
that tissue culture techniques are known to create
somaclonal and gametoclonal variation—that is,
variation induced by the tissue culture protocol
when somaclonal variation is found in somatic-
derived cell cultures and regenerants and when
gametoclonal variation is found in gamete-derived
cell cultures and regenerants. Because all wheat
lines have some level of heterogeneity, experi-
ments must be carefully designed to separate
somaclonal or gametoclonal variation from culti-
var heterogeneity (Baenziger et al., 1989). Finally,
when somaclonal or gametoclonal variation is
found, they tend to be deleterious like most muta-
tions. However, somaclonal or gametoclonal
variation is not uniform in the regenerants, so
by increasing the number of regenerants,
those without deleterious effects can be
identifi ed.
Variation from
in vitro
tissue culture
Wheat tissue culture is used for many purposes:
(i) to rescue embryos from wide crosses made to
transfer genes from wild relatives (Sharma and
Ohm 1990; Raghavan 2003) or to create haploid
plants (Laurie and Reymondie 1991); (ii) to
use as source material for wheat transformation
(Vasil 2007); (iii) to create doubled haploid plants
(Shariatpanahi et al., 2006); (iv) to create soma-
clonal variation (Larkin et al., 1984). From a
breeding perspective, the embryo rescue proce-
dures are most important when they assist germ-
plasm introgression from wild relatives into
cultivated types or are used to create doubled
haploid lines. The importance of genes from wild
relatives for wheat breeding has been reviewed by
Friebe et al. (1996), and it would be diffi cult to
overestimate their importance. Many important
cultivars have genes from wild relatives. In both
cases, embryo rescue is needed because the endo-
sperm does not form, or forms poorly, and cannot
support the developing embryo, thus leading to
embryo death without rescue.
Similarly, immature embryo culture is gener-
ally used for and most important when it is used
as the recipient cells for wheat transformation.
Immature embryo culture can also be used as
donor cells for other tissue cultures such as sus-
pension cell cultures (Fellers et al., 1995), which
can then be used for transformation or for bio-
chemical selection. Doubled haploid breeding
and wheat transformation are discussed later in
separate sections, and the use of wheat synthetics
(one method of introgressing germplasm from
wild relatives) is discussed in Chapter 16. Hence
further discussion will be brief.
Wheat breeders should realize that almost
all tissue culture techniques, especially those
involving immature embryo culture and anther or
microspore culture, are genotype-dependent.
Basically some genotypes work better in culture
than others (Shariatpanahi et al., 2006), so that
one cannot assume any elite line will be trans-
Transgenic wheat and its impact on
wheat breeding
In the early 1990s, wheat became the last major
cereal grain species to be transformed (Vasil et al.,
1992, 1993; Weeks et al., 1993). The current state
of transgenic wheat research was recently reviewed
(Vasil 2007) and will be discussed further in
Chapter 18. As such, we will discuss transgenic
wheat from a breeder's point of view, and specifi -
cally how it will affect future wheat breeding and
its potential. It should also be understood that
transgenic wheat has been called genetically mod-
ifi ed (GM) wheat or genetically engineered wheat.
In a chapter that discusses the genetic improve-
ment of wheat, genetically modifi ed or genetically
engineered wheat could be used to describe
what all wheat breeders do through traditional
and evolving plant breeding methods; hence we
will use the term transgenic wheat in this
chapter.
As mentioned in our fi rst section, the principles
underlying wheat improvement include biology
of sexual recombination, Mendelian laws of
inheritance, and selection. Clearly, transforma-
tion can greatly change the amount and kind
