- Open Access
Pscroph, a parasitic plant EST database enriched for parasite associated transcripts
© Torres et al; licensee BioMed Central Ltd. 2005
Received: 28 July 2005
Accepted: 16 November 2005
Published: 16 November 2005
Parasitic plants in the Orobanchaceae develop invasive root haustoria upon contact with host roots or root factors. The development of haustoria can be visually monitored and is rapid, highly synchronous, and strongly dependent on host factor exposure; therefore it provides a tractable system for studying chemical communications between roots of different plants.
Triphysaria is a facultative parasitic plant that initiates haustorium development within minutes after contact with host plant roots, root exudates, or purified haustorium-inducing phenolics. In order to identify genes associated with host root identification and early haustorium development, we sequenced suppression subtractive libraries (SSH) enriched for transcripts regulated in Triphysaria roots within five hours of exposure to Arabidopsis roots or the purified haustorium-inducing factor 2,6 dimethoxybenzoquinone. The sequences of over nine thousand ESTs from three SSH libraries and their subsequent assemblies are available at the Pscroph database http://pscroph.ucdavis.edu. The web site also provides BLAST functions and allows keyword searches of functional annotations.
Libraries prepared from Triphysaria roots treated with host roots or haustorium inducing factors were enriched for transcripts predicted to function in stress responses, electron transport or protein metabolism. In addition to parasitic plant investigations, the Pscroph database provides a useful resource for investigations in rhizosphere interactions, chemical signaling between organisms, and plant development and evolution.
Parasitic plants directly invade and rob nutrients from host plants [1, 2]. The consequences can be devastating to the host plant and some of the world's most pernicious agricultural pests are parasitic weeds . The number of parasitic angiosperms is surprisingly large with over four thousand parasitic species identified in nineteen different plant families . Parasitic plants have a wide diversity of growth habits ranging from the tiny flowered mistletoes that live in the tops of trees to the enormously flowered and rootless Rafflesia whose entire vegetative body is endophytic . The degree to which parasites rely on host resources also varies. Some obligate parasites, like Rafflesia, have lost photosynthetic capabilities and are fully heterotrophic. Others, like Triphysaria, are facultative parasites that can mature without a host plant but will parasitize neighboring plants when available.
The single feature shared by all parasitic plants is the ability to invade host tissues via a haustorium . Haustoria of parasitic plants fulfill multiple functions including host attachment, penetration, and translocation of resources from host to parasite . Interestingly, the competence to develop haustoria has originated in autotrophic ancestors multiple times during the evolution of angiosperms . There are two general hypotheses for the evolutionary origins of haustoria. One hypothesis suggests that the genes encoding haustorium development are derived from non-plant organisms, such as bacteria or fungi, that are endophytic or which have transferred a set of genes required for haustorium formation into the parasite genome . The second is that genes encoding haustorium development are derived from those present in autotrophic angiosperms where they fulfill functions unrelated to parasitism. The identification of genes associated with haustorium development will provide insights into the evolutionary origins of plant parasitism. These genes will also elucidate the degree to which haustoria in different parasitic families are encoded by convergent or homologous genetic pathways.
Parasitic plants in the Orobanchaceae develop haustoria on their roots in response to contact with host roots. Several molecules, typically products of the phenylpropanoid pathway, have been identified that induce haustorium development when applied to Orobanchaceae roots in vitro [5, 8–10]. Early haustorium development in response to exogenous signal molecules is characterized by three visible phenotypes: temporary cessation in root elongation, isodiametric cortical swelling, and haustorial hair proliferation [11, 12].
Molecular phylogeny places the Orobanchaceae on a single phylogenetic clade of parasites distinct from the nearest non-parasitic relative . This suggests that the genetic mechanisms controlling haustorium development in the Orobanchaceae are likely similar. Triphysaria, formerly Orthocarpus, is an Orobanchaceae that grows as a common, springtime annual throughout the Pacific coast from Canada to Baja . Triphysaria is a small genus of five intercrossing diploid species that are amenable to classical genetic analyses . Triphysaria is closely related to the devastating agricultural weeds Striga and Orobanche; however, Triphysaria itself has no agricultural significance. Triphysaria are facultative parasites that can grow to maturity without host plants but will readily parasitize many host species when available, including Arabidopsis and maize. Triphysaria form haustoria within twelve hours of being exposed to Arabidopsis roots or root factors in vitro . The speed, synchrony, and dependence on exogenous inducer makes haustorium development in Orobanchaceae an excellent system for identifying transcripts associated with subterranean plant-plant communications.
Towards the goal of identifying genes associated with plant parasitism, we sequenced cDNA libraries enriched by suppression subtractive hybridization (SSH)  for transcripts regulated in Triphysaria roots during haustorium development. To date we have sequenced approximately nine thousand ESTs from three SSH libraries generated after treating Triphysaria roots with either intact Arabidopsis roots or the chemical haustorium inducer 2,6-dimethoxybenzoquinone (DMBQ). DMBQ, first purified as a haustorium inducer from sorghum , induces high rates of haustorium development in Triphysaria; however its role in mediating haustorium formation in Triphysaria – Arabidopsis interactions is not known. The Pscroph database provides on-line access to these EST and assembly sequences and provides BLAST and keyword search functions . Comparative analysis with other transcriptomes will highlight genes and pathways associated with the origins of haustorium development and the evolution of heterotrophy in plants. These studies may provide insights into genetic strategies for developing crops resistant to parasitic weeds and into strategies for exploiting allelopathic interactions in agriculture generally.
Construction and content
Triphysaria versicolor seeds were collected from thousands of cross-pollinating plants growing in a grassland stand near Napa CA. They were surface sterilized in a solution of 2% sodium hypochlorite (50% household bleach) and 0.01% Triton X-1000, rinsed thoroughly with water and germinated at 16°C in 0.25X Hoagland's solution and 1% agar . After two to three weeks the seedlings were transferred along one edge of a square Petri dish containing the appropriate media and incubated at 25°C at a near vertical angle so that the roots grew down along the surface of the agar. Arabidopsis Columbia seeds were obtained from Lehle seeds (Round Rock, TX, USA), surface sterilized, and germinated in agar. Arabidopsis seedlings were then cultivated hydroponically for 40 days in liquid media (30 seedlings in 30 ml of 0.25X Hoagland's media in 250 ml flasks shaking at 50 rpm at 25°C under a 16 hours light/8 hours dark cycle).
Early haustorium development could be detected within 24 hr of host parasite contact as swollen, hairy knobs emerging just proximal to the Triphysaria root tips (Figure 1b,c,d). Triphysaria exposed to media without Arabidopsis or DMBQ did not develop haustoria.
Triphysaria roots were dissected and frozen in liquid nitrogen at various time intervals ranging from immediately after treatment to up to five hours later. Triphysaria RNA was prepared as described . Suppressive subtractive hybridization was used to generate cDNA libraries enriched for up or down regulated transcripts using a commercial kit (BD Sciences, Clontech, Mountain View, CA). The Host Forward (HF) library was enriched for transcripts upregulated in Triphysaria roots after contact with Arabidopsis and was made using mock-treated Triphysaria as the hybridization driver. The Host Reverse (HR) library was enriched for transcripts down regulated after host contact and was made using Arabidopsis exposed Triphysaria as driver. The EDIT library was enriched for transcripts upregulated in root tips within five hours of exposure to DMBQ as previously described .
EST sequencing and assembly
Subtracted cDNAs were ligated into pCR2.1-Topo (Invitrogen, Carlsbad, CA) and cloned in E. coli. Colonies were picked into 384 well microtiter plates and frozen at -80°C. Sequencing templates were prepared from the library using a rolling circle amplification technology with TempliPhi kit (Amersham Biosciences, Piscataway, NJ). Sequencing reactions were run using the BigDye terminator v3.1 (Applied Biosystems, Foster City, CA.) and the products were separated and detected using the ABI3730xl DNA Fragment Analyzer (Applied Biosystems, Foster City, Foster City, CA) at the UC Davis College of Agriculture and Environmental Sciences Genomics Facility.
DNA trace files were base-called using Phred (version 0.990722.g) and low quality sequences were removed based on a Phred p value ≤ 0.05 . The sequences were masked for the pCR 2.1-TOPO cloning vector, linker sequences, and repetitive sequences (excluding poly A and poly T) based on alignments generated by the BLASTN program as used by the PyMood Sequence Processor (Allometra, Davis, CA) (Alexander Kozik, pers. comm.). Sequences less than 100 nts were discarded from further analyses. Approximately five percent of the clones had linker sequences internal to an ORF sequence. These were determined to be chimeras generated by the ligation of multiple SSH fragments into a single plasmid. The chimeric sequences were computationally digested into independent ESTs. The finished ESTs were submitted to GenBank's dbEST repository .
EST and contig statistics of HF, HR and EDIT libraries
% with AT annotation1
Database of early haustorial transcripts
The EST sequences and assembly alignments are available at the Pscroph database . Data are stored in a MySQL database and made available on the web using a phpMyAdmin interface. The database is housed at the University of California-Davis Genome Center.
Proteins predicted to be encoded by the assemblies were annotated from the BLASTX reports comparing Triphysaria sequences to either all proteins in GenBank (rel145.fsa_aa release Dec 15, 2004) or to all predicted proteins in Arabidopsis (ATH1.pep_cm_20040228). These BLAST reports can be accessed at the web site as full text files or by keyword searches of protein annotations. The keyword search function reports the best three hits obtained from GenBank or TAIR databases with e values ≤ 10-8. Each best hit is hyperlinked to the corresponding report page at NCBI or TAIR. The web site also provides a BLAST function that allows homology searches against DNA or protein sequences in each or all libraries.
Utility and discussion
We previously published a sequence characterization of 246 cDNAs from the EDIT library . At that time we sequenced clones that had been selected by colony hybridizations for transcripts most differentially abundant in the forward hybridization reaction compared to the reverse. This is suggested by the manufacturer to reduce the number of false positives. The colonies not analyzed fell into two, roughly equal sized groups; those that hybridized to both forward and reverse probes and those that hybridized to neither. While this step reduces the number of false positives, it also may eliminate interesting transcripts. In particular, colonies that hybridized with neither probe likely represented weakly expressed, low abundance transcripts . Furthermore, colonies that hybridized to probes from multiple libraries may contain conserved domains in otherwise distinct proteins. Therefore we sequenced additional, unselected clones from the EDIT library and eliminated the hybridization in constructing the HF and HR libraries.
Localization of SSH products to gene regions 1
Only 5' non-coding
Only 3' non-coding
Both 5' and 3' non-coding
Total SSH products analyzed 2
Depending on the library, from 34 % to 62% of the Triphysaria sequences were predicted to include non-coding sequences; one to ten percent of the cDNAs included both 5' and 3' non-coding sequences (Table 2). There were more 3' than 5' non-coding sequences in all libraries; there were eight times more 3' sequences in the HR library. The 3' non coding regions recovered in the SSH libraries were also longer than those predicted for the 5' (Figure 2). The 3' bias likely results from the initial cDNA synthesis reaction that is primed with poly-T. Depending on the library, between ten and twenty percent of the SSH products had poly-A tracts. The predominance of ORF encoded sequences in the libraries demonstrates that these libraries were less biased towards 3' sequences than would be expected without the Rsa1 digestion.
Occurrence of transcripts in different libraries 1
Total # assemblies
# assemblies in category
% category specific
The unpredictably high rate of cross-library hybridizing transcripts was not a function of the assembly because a BLASTN analysis of EST sequences before assembly gave similar results (data not shown). Approximately half of the cross-hybridizing sequences had multiple sequence polymorphisms, suggesting these are alleles of co-expressed genes or domains. The levels of expression of cross-hybridizing sequences were estimated from the Arabidopsis MPSS database to determine if they are particularly highly expressed in roots . The Arabidopsis homologs to cross-hybridizing Triphysaria sequences ranged in their root expression between six and over three thousand transcripts per trillion. An ANOVA analysis indicated no significant differences between the predicted expression levels of library specific sequences from false positives present in both forward and subtracted libraries (data not shown).
One possible explanation for the unpredictably large number of false positive clones following SSH procedures is miss-priming at the first or second PCR reactions. cDNAs that were not selected during hybridization would be similarly amplified from both libraries if they have sufficient homology to the 22-mers used in the final amplifications prior to cloning.
BLASTX was used to assign putative functions to virtual translations of each library specific assembly. Roughly 75–80% of the library specific sequences had homologies in the Arabidopsis protein database at an e value ≤ 10-8. Using the AT number of the best Arabidopsis hits, the putative Triphysaria proteins were placed into functional categories using the Gene Ontology at TAIR . The GO terms obtained for each library are summarized in supplemental table 8. The TAIR output included multiple GO terms for most assemblies so there are more GO descriptors than transcripts.
Functional classification of library specific sequences 1
HF vs HR
HF vs EDIT
HR vs EDIT
cell organization and biogenesis
DNA or RNA metabolism
electron transport or energy pathways
response to abiotic or biotic stimulus
response to stress
other biological processes
other cellular processes
other metabolic processes
other physiological processes
biological process unknown
Transcripts associated with the metabolism of nucleic acids and proteins were significantly less abundant in the HF and EDIT compared to the HR libraries. The down regulation of DNA metabolism genes is consistent with the earlier observations that cell division and DNA synthesis is rapidly terminated in Striga upon contact with DMBQ . There were also fewer transcripts predicted to encode protein metabolism functions in the HF and EDIT libraries. While changes in protein profiles have been observed in Striga following DMBQ treatment, the overall reduction in the proportion of transcripts encoding protein metabolism genes was not expected [34, 35].
Parasitic plants provide an excellent system for studying genetic mechanisms of chemical signaling between plants. In addition, parasitic weeds are among the world's most destructive agricultural pests against which few genetic resistances are available. Genetic suppression of parasite development at early stages in parasitism is a promising approach for engineering resistance against parasitic weeds but requires knowledge of the genetic factors regulating parasite development. The Pscroph database contains parasitic plant transcripts regulated by host encoded factors; these provide potential points for engineering parasite resistance. More generally, the identification of regulatory elements induced by the presence of other plants provides the potential for genetic weed control strategies.
Availability and requirements
The Pscroph database can be accessed at http://pscroph.ucdavis.edu.
The authors recognize and sincerely appreciate the contributions of Marta Matvienko, Alexander Kozik, and Steve Edberg to the development of this database. The database is housed at the University of California-Davis Genome Center. This work was supported by NSF grant # 0236545. M. J. Torres was a recipient of an NIH Biotechnology Training Grant Award.
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