Molecular Breeding for Striga Resistance in Sorghum - Translational Genomics for Crop Breeding: Biotic Stress - page 82

(Pscroph) ( http://pscroph.ucdavis.edu/) (T orres
et al. 2005) has been developed representing
transcripts differentially regulated in Triphysaria
before and after contact with host roots, in
response to host root exudates and to the HIF
2,6-dimethoxybenzoquinone (DMBQ) (Yodler
et al. 2007). Approximately 40,000 ESTs from
the different suppressive subtraction hybridiza-
tion (SSH) libraries have been sequenced
and annotated ( http://www.plantgdb.org/prj/
ESTCluster/progress.php) . This approach is
being extended to produce EST collections
from Striga and Orobanche facilitating identi-
fication of potentially essential genes for ger-
mination stimulant perception, haustorial for-
mation, and parasite development. Stable and
transient transformation systems for Triphysaria
(Tomilov et al. 2007) and transformation of
Striga have also been successful, allowing the
function of parasite genes to be tested by
silencing or overexpression. These approaches
may allow to design novel control strategies,
for example, by utilizing gene-silencing vec-
tors designed against essential parasite specific
genes. If such vectors were transformed into
cereals and the RNAi signal moved from host
to parasite, silencing of the parasite gene could
be lethal or inhibit parasite development. Post-
transcriptional gene silencing (PTGS) has been
demonstrated to work against viruses and nema-
todes, but its applicability for Striga control is
unproven. Preliminary evidence suggests that
this approach may work for the holoparasitic
Orobanche where mRNA levels encoding the
enzyme mannose-6-phosphate were reduced by
60-80% in the parasite when grown on trans-
genic tomato plants containing the appropriate
silencing construct (Rady 2007). However, a
key difference between Orobanche and Striga
is that only the former establishes direct phloem
connections with the host as the silencing sig-
nal is thought to be phloem mobile (Tournier
et al. 2006).
Once the haustorium has attached to the host,
Striga penetrates the cortex and endodermis to
form a direct link with host xylem vessels.

Identifying host genotypes that block penetra-
tion and identification of the genes involved is
a major focus of Striga research. There are a
few cultivated and many more wild sorghums
that exhibit partial post-attachment resistance;
several sorghum cultivars and a wild accession
P47121 exhibit a hypersensitive response phe-
notype when infected with an ecotype of S. asi-
atica (Mohamed et al. 2003; Rich et al. 2004),
and the sorghum cultivar N13 appears to exhibit
“mechanical” resistance at the root endodermal
barrier (Haussmann et al. 2004). The molecular
basis and signalling pathways underlying resis-
tance or susceptibility in plant-plant interactions
are obscure, and rice offers the opportunity for
molecular dissection of these traits. Preliminary
analyses of changes in gene expression in suscep-
tible and resistant cultivars using rice microar-
rays revealed that many of the genes upregulated
in Nipponbare are those classically associated
with defense responses to microbial pathogens,
for example, pathogenesis related (PR) genes,
cytochrome P450s, and WRKY transcription
factors (Scholes et al. 2007).
Understanding the mechanistic basis of dif-
ferent types of resistance to Striga will facilitate
pyramiding of resistance genes, via genetic engi-
neering strategies or MAS with aim of provid-
ing durable polygenic resistance. Relatively few
transcriptomic or proteomic studies to identify
susceptibility or resistance genes in Striga are
known to be under way. Increasing availability of
microarrays for sorghum and newer expression-
profiling tools such as RNAseq would address
this in the near future.

Striga Diversity, Racial
Differentiation, and its
Implications on Striga
Resistance Breeding

The issue of genetic diversity among Striga
species and within species populations structured
by geography, host crop, and other environmen-
tal factors affecting adoption plays an important
role in host-plant interaction and expression of