Abstract
BackgroundSunflower downy mildew is a major disease caused by the obligatory biotrophic oomycete Plasmopara halstedii. Little is known about the molecular mechanisms underlying its pathogenicity. In this study we used a genomics approach to gain a first insight into the transcriptome of P. halstedii.ResultsTo identify genes from the obligatory biotrophic oomycete Plasmopara halstedii that are expressed during infection in sunflower (Helianthus annuus L.) we employed the suppression subtraction hybridization (SSH) method from sunflower seedlings infected by P. halstedii. Using this method and random sequencing of clones, a total of 602 expressed sequence tags (ESTs) corresponding to 230 unique sequence sets were identified. To determine the origin of the unisequences, PCR primers were designed to amplify these gene fragments from genomic DNA isolated either from P. halstedii sporangia or from Helianthus annuus. Only 145 nonredundant ESTs which correspond to a total of 373 ESTs (67.7%) proved to be derived from P. halstedii genes and that are expressed during infection in sunflower. A set of 87 nonredundant sequences were identified as showing matches to sequences deposited in public databases. Nevertheless, about 7% of the ESTs seem to be unique to P. halstedii without any homolog in any public database.ConclusionA summary of the assignment of nonredundant ESTs to functional categories as well as their relative abundance is listed and discussed. Annotation of the ESTs revealed a number of genes that could function in virulence. We provide a first glimpse into the gene content of P. halstedii. These resources should accelerate research on this important pathogen.
Highlights
Sunflower downy mildew is a major disease caused by the obligatory biotrophic oomycete Plasmopara halstedii
Expressed sequence tags analysis After two rounds of subtraction hybridization, cDNAs were cloned into pGemT-easy vector and the bacteria arrayed in 96 well plates
All the 230 primer pairs tested amplified fragments either from P. halstedii DNA or from Helianthus annuus DNA; 145 primer pairs amplified fragments only from P. halstedii DNA, indicating that these expressed sequence tags (ESTs) were derived from P. halstedii genes that are expressed during infection in sunflower
Summary
Sunflower downy mildew is a major disease caused by the obligatory biotrophic oomycete Plasmopara halstedii. Sunflower downy mildew is a major disease caused by the Oomycete Plasmopara halstedii (Farl.) Berl et de Toni. The first physiological race of this obligate parasitic oomycete has been identified by Zimmer in North America and Europe [1]. Both in compatible and incompatible interactions, host penetration occurs at the lower part of the hypocotyl [2]. Molecular analysis showed that the resistance could be associated with an unusual delayed hypersensitive reaction and a systemic acquired response that take place inside the hypocotyls with the seedlings showing no apparent symptoms [3]
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