Biology Reference
In-Depth Information
of
Striga
resistance by focusing on each stage of
the parasitic process individually. The bioassays
had also provided further insights to the inter-
active biological processes between
Striga
and
the roots of host plants during the early stages of
the infection process, giving an increased under-
standing of the specific mechanisms of resistance
associated with each source of host genotype
(Cai et al. 1993). Furthermore, defense responses
triggered by infection could also be monitored
and exploited via these assays. Hence, disrupt-
ing these interactions offers unique opportuni-
ties for controlling
Striga
through identification
of genetic variants with single or multiple inter-
ventions at key critical stages throughout the life
cycle, which are likely to be simply inherited
and therefore easy to manipulate through con-
ventional breeding or via other biotechnologi-
cal approaches for the development of
Striga-
resistant crop cultivars.
breeding. Characterization and dissection of
Striga
resistance into specific mechanisms based
on a series of host-parasite signal exchanges
should be the central focus and premise for any
research approach and effort aimed at developing
Striga-
resistant cultivars (Ejeta et al. 1991).
The life cycle of
Striga
is intimately linked
to that of its host and depends on a com-
plex exchange of chemical signals; this poses a
challenge and an opportunity for control, both
before and after attachment of
Striga
to the
host root. Understanding host-parasite biology
at each life cycle stage is essential for the design
of novel control strategies. The first commit-
ted step of the
Striga
life cycle is germina-
tion, which occurs only in response to specific
secondary metabolites in the root exudates of
host and some non-host plants. There are sev-
eral classes of germination stimulants such as
strigolactones (Bouwmeester et al. 2003), most
commonly present in the exudates of many
cereals species (Awad et al. 2006). Some low-
germination-stimulant-producing sorghum cul-
tivars have been identified and often perform
well when used as part of an integrated control
program (Joel et al. 2007). Manipulation of the
production of germination stimulants requires
further knowledge of their biosynthetic path-
ways. Forward and reverse genetic approaches
may identify key genes involved in strigolactone
synthesis such as in maize (Bouwmeester et al.
2003), and the use of maize mutants revealed that
strigolactones are derived from the carotenoid
pathway (Matusova et al. 2005; Bouwmeester
et al. 2007) and can facilitate the identification
of genes involved in the synthesis and regulation
of strigolactones (Sun et al. 2007) in rice and
sorghum as model crops.
Following germination, haustorial inducing
factors (HIF) are required to trigger differenti-
ation of the parasite haustorium (Keyes et al.
2000; Keyes et al. 2007). To initiate the identi-
fication of parasite genes specifically involved
in haustoriogenesis,
Triphysaria versicolor
,a
facultative parasite closely related to
Striga
,is
being used as a model system. An EST database
Understanding Host-Parasite
Biology: Exploring Pathway
Stages as Entry Points for
Breeding Resistance to
Striga
Genetic resistance is a vital component of an
effective integrated
Striga
management program
(Ejeta 2007). Durable resistance for a trait
like S
triga
is most securely based on multiple
traits that block the establishment of weedy root
parasites on their hosts (Rispail et al. 2007). This
involves several metabolic pathways, both in
host and parasite, that play a very dynamic role
during expression of resistance or susceptibility
for host and of virulence or avirulance for para-
site. Successful parasitism is therefore a series of
interactive processes between host and parasite
conditioned by a large number of genetic and
physiological events, each possibly influenced
by additional array of environmental factors.
Host plant resistance based on observation of
emergence above ground of parasitic seedlings
and level of infestation, therefore, is a complex,
quantitatively inherited trait that is difficult to
select for using conventional approaches of plant
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