Transcriptome characterization and expression profiling in chestnut cultivars resistant or susceptible to the gall wasp Dryocosmus kuriphilus.

The oriental gall wasp Dryocosmus kuriphilus represents a limiting pest for the European Chestnut (Castanea sativa, Fagaceae) as it creates severe yield losses. The European Chestnut is a deciduous tree, having major social, economic and environmental importance in Southern Europe, covering an area of 2.53 million hectares, including 75,000 ha devoted to fruit production. Cultivars show different susceptibility and very few are resistant to gall wasp. To deeply investigate the plant response and understand which factors can lead the plant to develop or not the gall, the study of transcriptome is basic (fundamental). To date, little transcriptomic information are available for C. sativa species. Hence, we present a de novo assembly of the chestnut transcriptome of the resistant Euro-Japanese hybrid 'Bouche de Bétizac' (BB) and the susceptible cultivar 'Madonna' (M), collecting RNA from buds at different stages of budburst. The two transcriptomes were assembled into 34,081 (BB) and 30,605 (M) unigenes, respectively. The former was used as a reference sequence for further characterization analyses, highlighting the presence of 1444 putative resistance gene analogs (RGAs) and about 1135 unigenes, as putative MiRNA targets. A global quantitative transcriptome profiling comparing the resistant and the susceptible cultivars, in the presence or not of the gall wasp, revealed some GO enrichments as "response to stimulus" (GO:0050896), and "developmental processes" (e.g., post-embryonic development, GO:0009791). Many up-regulated genes appeared to be transcription factors (e.g., RAV1, AP2/ERF, WRKY33) or protein regulators (e.g., RAPTOR1B) and storage proteins (e.g., LEA D29) involved in "post-embryonic development". Our analysis was able to provide a large amount of information, including 7k simple sequence repeat (SSR) and 335k single-nucleotide polymorphism (SNP)/INDEL markers, and generated the first reference unigene catalog for the European Chestnut. The transcriptome data for C. sativa will contribute to understand the genetic basis of the resistance to gall wasp and will provide useful information for next molecular genetic studies of this species and its relatives.