Biology Reference
In-Depth Information
resistant varieties, and an integrated approach
to both Striga and soil fertility. Of these
approaches, development of resistant crop culti-
vars has been recognized as the most effective
and feasible method. To date, Striga -resistant
sorghum cultivars - such as N13, SRN39,
Framida, 555, ICSVs, SRN39 derivatives (P401
to P409), Soumalemba (IS15401), and Segue-
tana CZ and CMDT45 - have been identified and,
as has been observed by Tabo et al. (2006) and
ICRISAT (2009), these can be integrated with
available crop management options to enhance
productivity.
Success has been limited because Striga
impacts host development shortly after attach-
ing to the root, long before the parasite emerges
above ground, and yield losses are not lin-
early related to parasite biomass (Gurney et al.
1999). However, a lack of knowledge of the
genetic factors controlling host-parasite sig-
nalling at different stages of the life cycle, the
paucity of resistant host germplasm, the poly-
genic nature of resistance coupled with complex
genotype
Development of Bioassays and
Dissecting Striga Resistance
Mechanisms
Additionally, such combinations would offer
more buffering capacity to host plant population
against building virulence in Striga populations,
as it could only result from multiple mutations
to overcome these obstacles resulting in durable
host-plant resistance. To date, five specific Striga
resistance mechanisms had been described,
which include resistance associated with low ger-
mination stimulant ( LGS ) production, low pro-
duction of the haustorial initiation factor ( LHF ),
Germination inhibitors ( GI ), hypersensitive
response ( HR ), and the incompatible response
( IR ) to parasitic invasion of host genotypes
(Ejeta et al. 2000). Assessing these resistance
mechanisms across several lines individually is
very difficult, as most of times their effects are
confounding and are difficult to separate. In this
scenario, breeding for Striga resistance would
be greatly assisted by in vitro methods that allow
inspection of pre-attachment and early post-
attachment phases of Striga interaction with host
root systems. Such observations could reveal
underlying resistance mechanisms in source
germplasm and allow selection for specific
resistance mechanism, alone or in combination,
in breeding populations for future exploitation.
Recent approaches such as RNA interference
and microRNA assays to further character-
ize host-parasite interactions at nucleotide
level offer new avenues for improving Striga
resistance, but require more specific assays.
In case of S triga , it is particularly important
that control is expressed early in the parasitic life
cycle since severe consequences on host health
are manifested within the first few days after par-
asitic attachment (Gurney et al. 1999). Tradition-
ally, breeding for most of complex traits such as
Striga resistance had been based on field selec-
tion/phenotypic performance of germplasm, and
selection efficiency is based on a well-defined
trait phenotype such as emerged parasite num-
ber or severity scores in artificially infested
E) interactions,
and insufficient knowledge of parasite race struc-
ture impede progress. Genomic approaches offer
new opportunities to dissect polygenic resis-
tance traits into their underlying genetic com-
ponents (quantitative trait loci [QTL]), allow-
ing breeders to utilize marker-assisted selection
(MAS) for the transfer and pyramiding of use-
ful alleles into locally adapted cultivars. Fur-
ther, sequence information and genomic tools
for forward and reverse genetics open the way
for identification of key genes controlling par-
asitism (in both host and parasite) and their
genetic manipulation. Here, we critically assess
the current and future roles of genomic platforms
such as New Generation Sequencing tools, RNA
interference (RNAi) technologies, in advanc-
ing knowledge of resistance and susceptibil-
ity to Striga and molecular marker technol-
ogy in accelerating the development of Striga -
resistant cultivars of sorghum. Recent status of
development of Striga -resistant cultivars is also
discussed.
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