Agriculture Reference
In-Depth Information
branches, days to flowering and days to physiological
maturity (Lalinia et al., 2012).
Therefore, there is an important requirement to
develop varieties of legumes that are tolerant to various
abiotic stresses in order to increase their yield. Various
biotechnological techniques provide researchers and
plant breeders with an opportunity to develop such
varieties, which are not only stress resistant but are also
high yielding and have high nutritional value.
Basic steps involved in MAS
Identi cation of molecular markers linked to genes
(To select plants containing target genes)
Marker validation
(Testing the reliability of markers to predict the phenotype)
16.2 approaches to incorporate stress
tolerance mechanisms
Marker-assisted backcrossing (MABC)
(To transfer the quantitative trait loci (QTL)/gene from the
donor genotype into the targeted genotype)
16.2.1 Gene pyramiding and marker-
assisted selection
The main aim of plant breeding is to select desirable traits
in a particular plant species and accumulate more advan-
tageous combinations of genes in new varieties.
Compared to other crops and cereals, legumes are more
prone to biotic stresses such as diseases and insect pests,
while among abiotic stresses, the common types affecting
them include drought, heat, cold/frost and salinity, which
have impact on the overall growth and yield of the plant.
Therefore, a new approach should be adopted to develop
high-yielding varieties that are resistant to various biotic
and abiotic stresses, by combining conventional and non-
conventional techniques. With the availability of newer
tools and techniques in plant breeding programmes, such
as marker-assisted selection (MAS) and marker-assisted
gene pyramiding, it has now become easier to develop
genotypes having desirable genes.
Figure 16.1 General steps of marker-assisted selection (MAS).
resistance against various diseases. Marker-assisted pyr-
amiding has been used widely for pyramiding disease
resistance genes in plants; for example, major genes for
resistance against blight (Huang et al., 1997; Singh et al.,
2001), blast (Hittalmani et al., 2000) and gall midge
(Kumaravadivel et al., 2006) have been pyramided in
rice. In wheat, the resistance genes for powdery mildew,
namely Pm2 + Pm4a, Pm2 + Pm21, Pm4a + Pm21 , have
been effectively pyramided in a single selected line (Liu
et al., 2000). Rust resistance genes, i.e. Lr41 , Lr42 and
Lr43 , have also been accumulated using the MAS gene
pyramiding approach (Cox et al., 1994).
16.2.1.2 General scheme of marker-assisted
gene pyramiding
The main objective of gene pyramiding is to obtain
an ideal genotype containing all desirable traits. Servin
et al. (2004) proposed that gene pyramiding breeding
consists of two basic steps, the pedigree step and the gene
fixation step (Figure  16.2). It involves accumulating
target genes present in diverse populations with desired
allele frequencies into one crossed population and then
the selected parents are intercrossed to fix all the target
genes into an ideal genotype (Servin et al., 2004).
16.2.1.1 Marker-assisted gene pyramiding
Gene pyramiding is the process of combining several
genes together into a single genotype: a genotype con-
taining all the target genes forms the end product
of  gene pyramiding. It is mainly used to improve
qualitative traits such as disease and insect resistance,
and is often performed using MAS. MAS, also called
marker-assisted breeding (MAB), is a technique that
can help to make traditional breeding methods more
efficient. Unlike genetic engineering, which transfers
isolated gene sequences into a genome, MAB does not
involve gene isolation and transfer of gene sequences
into the genome. Rather, it involves the use of genetic
markers associated with particular traits (Figure  16.1)
(Collard & Mackill, 2008). DNA markers and isozymes
are commonly used to mark those gene(s) that confer
16.2.1.3 Genetic markers
The genetic differences between the species are repre-
sented by the genetic markers. They act as 'signs' or
'flags' and are located in close proximity to the genes.
They neither represent the target genes nor do they
affect the phenotype of the trait of interest since they
are only linked to genes controlling the trait. Like genes,
 
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