Agriculture Reference
In-Depth Information
These technologies have identified specific molecular
markers that may be used in breeding programmes via
marker-assisted selection (MAS) to improve stress toler-
ance (Dita
et al.,
2006). In legumes, the practical
application of MAS for the genetic enhancement of
resistance or tolerance to stress has been limited.
Schenider
et al.
(1997) reported that MAS may be useful
to select drought-tolerant common bean.
variation, including callus cultivation and somatic
embryogenesis, has the ability to generate genetic vari-
ation (Larkin & Scowcroft, 1981; Dita
et al.,
2006). The
ability to produce agronomically useful somaclones via
organogenesis and somatic embryogenesis has been
reported in pea (Griga
et al.,
1995) and pigeon pea
(Chintapalli
et al.,
1997).
Indeed, combining mutagenesis techniques with
MAS through TILLING (see Section 1.7.9) will make
mutagenesis more suitable for legume enhancement.
The main problem with these techniques is the high
quantity of individuals required to find the desired trait.
Nevertheless, by using
in vitro
selection systems this dis-
advantage can be reduced (Dita
et al.,
2006).
1.7.3 Gene pyramiding assisted by MaS
Pyramiding different resistance or tolerance traits into a
genotype helps plant breeders to achieve resistance to
abiotic stress. In legumes there are numerous examples
of introgression and pyramiding of favorable alleles and
QTLs. However, MAS has been used to help in gene
pyramiding to overcome stresses in only a few cases
(Dita
et al.,
2006). Nevertheless, Schneider
et al.
(1997)
indicated that MAS may be useful to select drought-
tolerant common bean.
There are some exceptions where MAS has facilitated
breeding efforts in several legume crops to combat
important biotic stressors (Dita
et al.,
2006). For example,
MAS was successfully used for the breeding of soybean
resistant to cyst nematode (Diers, 2004), of pinto bean
resistant to common bacterial blight (Mutlu
et al.,
2005)
and of narrow-leaved lupin (
Lupinus angustifolius
L.)
resistant to phomopsis stem blight (Yang
et al.,
2002)
and anthracnose (Yang
et al.,
2004). Moreover, when
resistance is conferred by single genes and/or easily
overcome by new pathogen races, the gene pyramiding
strategy facilitated by MAS can be an efficient method
(Dita
et al.,
2006).
Legume cultivars having appropriate combinations of
resistance and/or tolerance to biotic and abiotic stresses,
achieved through gene pyramiding, could provide durable
resistance, and MAS can be a valuable tool to guide and
identify the pyramiding of these genes (Dita
et al.,
2006).
The generation of markers based on genes with
altered expression patterns in response to stresses could
result in more effective and targeted MAS. Some of
these genes may be good candidates for future MAS
studies in legumes (Dita
et al.,
2006).
1.7.5
In vitro
selection
In vitro
selection is one of the important classical
breeding methods (Svabova & Lebeda 2005) and has
been used for both biotic and abiotic stresses. In legumes,
in vitro
selection was applied to alfalfa (
Medicago sativa
)
for selection of resistance to
Colletotrichum trifolii
(Cucuzza & Kao, 1986),
Fusarium oxysporum
(Cvikrova
et
al.,
1992) and
Verticillium albo-atrum
(Koike & Nanbu,
1997). However, no resistant lines were reported in
these studies (Dita
et al.,
2006).
Putative stress-resistant lines derived from both con-
ventional breeding and transgenic approaches could be
screened using
in vitro
selection. This is suitable for some
abiotic stresses, where appropriate screening methods are
unavailable or have low efficiency. Somaclonal variation
and
in vitro
mutagenesis followed by
in vitro
selection
offer an alternative way for breeding (Dita
et al.,
2006).
1.7.6 transcriptomics
Repression of genes or transcriptional activation is an
important tool in the control of stress responses in plants
(Chen
et al.,
2002; Dita
et al.,
2006). Thus, identification
of differentially expressed genes is particularly impor-
tant to understand stress responses in plants. To achieve
this objective, tools such as microarrays (Schena
et al.,
1995), serial analysis of gene expression (SAGE)
(Velculescu
et al.,
1995), suppression subtractive hybrid-
ization library (Diatchenko
et al.,
1996), and quantitative
measurement of transcription factor (TF) expression
have been developed in addition to older techniques
such as Northern blotting (Dita
et al.,
2006).
In legumes, transcriptomic techniques are useful
ways of breeding to combat environmental stresses. Jain
1.7.4 Somaclonal variation and
in vitro
mutagenesis
Tissue culture provides a big range of genetic variation
in plants, which can be incorporated in plant breeding
programmes (Jain, 2001; Dita
et al.,
2006). Somaclonal
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