Molecular Breeding for Striga Resistance in Sorghum - Translational Genomics for Crop Breeding: Biotic Stress - page 80

plots over multiple years and locations (Omanya
et al. 2004). However, these filed evaluation
measures of Striga resistance fail to distinguish
between resistance/tolerant traits and the asso-
ciated interactive events between host and para-
site. A potential host plant may have any of sev-
eral defense responses to Striga infection (Ejeta
2007; Joel et al. 2007; Rich and Ejeta 2007). Tol-
erance to the ill effects of Striga is also known,
whereby the crop is able to produce acceptable
yields despite Striga infestation (Rodenburg et al.
2006).
Adequate genetic variation for a target trait
and availability of effective selection tools are
essential requisites for successful plant-breeding
efforts, and unfortunately selection methods that
work well for improving other desirable crop
traits have not operated at the same level of effi-
ciency for Striga resistance (Ejeta and Butler
1993). Similar to other major complex trait, field
selection for Striga resistance had not been suc-
cessful because of the difficulty in clearly identi-
fying resistant variants, lack of understanding on
the genetic control of field resistance, and the dif-
ficulty of establishing uniform infestation of the
parasite population under varying environmental
conditions. Lack of rapid and efficient screen-
ing techniques had been a major constraint in
the progress of Striga resistance breeding, and
hence development of bioassays received pri-
mary focus and consideration across all Striga
resistance breeding programs.
Rhizotrons and other in vitro growth sys-
tems of various designs have been described
for co-culture of weedy root parasites and their
hosts (Rubiales et al. 2006). An agar-based sys-
tem and its modifications had been quite use-
ful in identifying sorghum seedlings with resis-
tance based on low Striga germination stimulant
production (Omanya et al. 2004), and sorghum
accessions had been useful for their ability to
trigger formation of the haustorium or appres-
sorium in S. asiatica (Rich et al. 2004) and
to observe early post-attachment reactions on
sorghum expressed as hypersensitive response
to Striga (Mohamed et al. 2003). These methods

had inherent limitation of short time for obser-
vations and inconsistency. Co-culture of Striga
on sorghum in Petri dishes stood on end and
containing moistened paper topped with glass
fiber with a hole to accommodate host shoot
growth (Arnaud et al. 1999) allowed observa-
tions of parasitic associations over several weeks,
but attachment frequency in these rhizotrons is
also low if the growth medium is too wet. Larger
rhizotrons using sand (Gurney et al. 2003) or
rockwool (Gurney et al. 2006; Yoshida and Shi-
rasu 2009) sandwiched between plastic plates
have been well suited to co-culture of S. her-
monthica on cereals. A fundamental problem
with all the aforementioned methods, however,
is that newly attached Striga are so small (mil-
limeters or less) that a microscope capable of
at least 10 magnifications is required to view

newly attached parasites. This means that this
entire activity becomes very cumbersome, low
throughput, resource intensive, and time con-
suming. To be useful in a breeding program, in
vitro methods should mimic natural conditions,
occupy little space (particularly for containment
facilities), and be low cost and relatively easy
to set up and maintain. They should allow non-
destructive and progressive observations, prefer-
ably at multiple times during co-culture, consis-
tent, repeatable, and with high heritability.
A sand-based rhizotron for monitoring Striga
parasitism with the aid of a scanner dur-
ing the critical attachment and early post-
attachment phases was recently developed
(Amusan et al. 2011). The sand-packed titer
plate assay (SPTPA) was used to examine Striga-
susceptible and -resistant maize (Amusan et al.
2008) and sorghum, previously identified in field
trials, to pinpoint the stage at which Striga
seedlings stop growing or died on the host roots.
These modifications and the ones to follow will
help dissect the different Striga resistance mech-
anisms into the component traits, which in turn
will allow for easier selection and introgression
in breeding programs. Use of these assays had
enhanced the ability more systematically to eval-
uate and exploit sorghum germplasm as sources

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