Molecular Breeding for Striga Resistance in Sorghum - Translational Genomics for Crop Breeding: Biotic Stress - page 78

of field crops are infested with Striga annually,
especially S. hermonthica and S. asiatica . These
species infest maize, sorghum, pearl millet, fin-
ger millet, and upland rice, causing severe stunt-
ing. Yield loss attributable to Striga is acute, per-
haps even exacerbated, ranging from 30% to 90%
(Kroschel et al. 1999; van Ast et al. 2005; Ejeta
2007; Joel et al. 2007). Impacts are greatest on
infertile soils, and the poorest subsistence farm-
ers are the most severely affected. According
to The Food and Agricultural Organization of
the United Nations (FAO) ( http://www.fao.org/),
Striga infests over 40% of the cereal produc-
ing areas of sub-Saharan Africa, is continuing to
spread, and accounts for US$7.4 billion in lost
crops annually (Sharma 2006), with significant
negative impact on the food supply of several
million people. Effective control of Striga has
been difficult to achieve through conventional
agronomic practices, since the parasite exerts
its greatest damage before its emergence above
ground. Estimates on the extent of crop damage
in a country or region in the African continent
vary depending on the crop cultivar and degree
of infestation (Parker and Riches 1993). How-
ever, Striga infestation is most severe in east-
ern Africa, especially in Ethiopia, where over
50% of sorghum production field are infested by
several Striga species, and the invasion by the
parasite is expanding at an alarming rate, often
resulting in total crop losses annually on many
farms. An expansion of Striga infestation is also
occurring in West Africa. The impact of Striga
in these regions is compounded by its predilec-
tion for attacking crops already under moisture
and nutrient stress, conditions very acute in these
regions and getting to be prevalent in much of the
semiarid tropics.
Despite its agricultural importance, the
molecular mechanisms controlling the establish-
ment of parasitism are not well understood. The
major species affecting sorghum in East Africa
is S. hermonthica , and its life cycle is unique
and well adapted to its parasitic lifestyle. The
seeds need to be exposed to germination stim-
ulants exudated from the host roots, such as

strigolactones and ethylene; otherwise they can
remain dormant in the soil for several decades
(Bouwmeester et al. 2007). The seeds are tiny
and possess limited amounts of nutrients, and
this restricts their growth without a host connec-
tion. When a potential host is recognized through
the sensing of strigolactones or other germina-
tion stimulants, the seeds that are close to the
host roots (within 5 mm) can germinate. The
germinated seedlings form haustoria—round-
shaped organs specialized in host attachment
and penetration (Yoder 2001). The formation of
haustoria also requires host-derived signal com-
pounds. The haustoria penetrate the host roots
and finally connect with the vasculature to rob the
host plant of water and nutrients. This dramatic
developmental transition from an autotrophic
to a heterotrophic lifestyle occurs within

several days.
Intensive efforts in the scientific community,
mainly in the United States during the 1960s,
lead to the identification of some germination
stimulants. This was followed by the develop-
ment of a "suicidal germination" strategy to erad-
icate Striga weeds (Rispail et al. 2007). By this
strategy, a germination stimulant (in this case
ethylene) was mixed in the soil to trigger germi-
nation in the absence of the hosts. This approach
was used successfully to eradicate Striga asiat-
ica in North Carolina. The suicidal germination
approach was not applicable for African farmers
due to the high cost of the strategy and the much
larger scale of infestation.
Integrated Striga management packages have
been designed that include: Striga resistant vari-
eties (Rodenburg et al. 2006); judicious and
appropriate timing and application of fertiliz-
ers in combination with organic fertilizers, suit-
able crop rotations, and trap cropping (van Ast
et al. 2005; Oswald 2005; Joel et al. 2007);
intercropping with forage legume Desmodium
uncinatum (Khan et al. 2007) and seed coating
with amino acids, fusarium spps, and herbi-
cides (Kanampiu et al. 2003); and water conser-
vation measures. Striga management will con-
tinue to require cultural and chemical treatments,