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identified over 17,000 unigenes. The compara-
tive analysis of this unigene dataset with other
plant genomes or ESTs revealed that approxi-
mately 80% of the unigenes had homologes in
other dicotyledonous plants including
Arabidop-
sis
, poplar, and grape. Interestingly they found
589 unigenes that were conserved in the hemi-
parasitic
Triphysaria
species, a close relative of
Striga
, but not in other plant species. Further-
more they also identified 1,445 putative SSRs
in the
S
.
hermonthica
unigene dataset. These
recently developed extensive set of molecular
resources using advanced molecular tools will
help in studying
S
.
hermonthica
for genome
annotation, gene discovery, functional analysis,
molecular breeding, comparative mapping with
different plant genomes, epidemiological stud-
ies, and studies of plant evolution. The recently
started Parasitic Plant Genome Project (West-
wood et al. 2012) aims to develop new tools
for understanding the biology of
Orobanche
and
Striga
. This project had sequenced tran-
scripts from three parasitic species and a non-
parasitic relative in the
Orobanchaceae
with the
goal of understanding genetic changes associ-
ated with parasitism. The species studied span
the trophic spectrum from free-living nonpar-
asite to obligate holoparasite. Parasitic species
used included:
Triphysaria versicolor
, a photo-
synthetically competent species that opportunis-
tically parasitizes roots of neighboring plants;
Striga hermonthica
, a hemiparasite that has
an obligate need for a host such as sorghum;
and
Orobanche aegyptiaca
, a holoparasite with
absolute nutritional dependence on a host.
Tri-
physaria
is a genus of five hemiparasitic species
that grow as common annuals throughout the
Pacific Coast of the western United States (Hick-
man 1993).
Triphysaria
has a broad host range
that includes maize, rice, and Arabidopsis, and
is closely related to the agricultural pests
Striga
and
Orobanche
.
Triphysaria
has no agricul-
tural significance and so can be grown without
quarantine restrictions (Goldwasser et al. 2002).
Triphysaria
flowers are amenable to classical
genetic manipulations and genomic resources
are being developed, making
Triphysaria
ause-
ful model species for parasite studies (Torres
et al. 2005). For the genome project, tissues
for transcriptome sequencing from each plant
were gathered to identify expressed genes for
key life stages from seed conditioning through
anthesis. Importance of this project lies in that
the two of the species studied,
S. hermonthica
and
O. aegyptiaca
, are economically important
weeds and the data generated by this project are
expected to aid in research and control of these
species and their relatives. The sequences gen-
erated through this project will provide an abun-
dant molecular resource for understanding pop-
ulation dynamics, as well as provide insight into
the biology of parasitism and advance progress
toward understanding parasite virulence and
host resistance mechanisms. In addition, the
sequences provide important information on tar-
get sites for herbicide action or other novel
control strategies such as trans-specific gene
silencing.
RNA interference (RNAi), or post-
transcriptional gene silencing (PTGS), is a
conserved mechanism in eukaryotes by which
double-stranded RNA molecules (dsRNA),
formed either by complementary base pairing of
transgenic sequences or by fold-back of endoge-
nous noncoding sequences, are processed by
Dicer-like nucleases into short 21-24 nt interfer-
ing RNAs (siRNA) or micro-RNAs (miRNAs).
These small RNAs are then incorporated, along
with Argonaute-like proteins, into RNA-induced
silencing complexes that direct the degradation
of endogenous RNAs that are homologous to the
siRNAs (Bartel 2004; Baulcombe 2004). When
siRNAs are introduced into specific tissues of
a plant by biolistics or agroinfection, siRNA
moves through plasmodesmata into other tissues
in a non-cell-autonomous fashion (Voinnet
2005). RNA-dependent RNA polymerase
amplifies the primary siRNA, allowing further
spread of the silencing signal (Himber et al.
2003).
RNAi signals can also enter the phloem and
spread systemically throughout a plant, and even
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