De novo comparative transcriptome analyses of strawberry (Fragaria vesca) genotypes in response to Phytophthora cactorum.

In this RNA-seq study, we compared the transcriptome of three Fragaria vesca genotypes in response to Phytophthora cactorum. The goal of our study was to dissect the resistance mechanism of the diploid strawberry (F. vesca) that are resistant to P. cactorum. A susceptible genotype (NCGR1218) and two resistant (NCGR1603 and Bukammen) F. vesca genotypes were used for the comparative transcriptome analyses. Plants were inoculated with P. cactorum zoospores (2mL of 2 × 105 spores/mL) in the crown (rhizome) and sampled 48 hours later. The appropriate controls for each genotype were i) samples wounded and inoculated with water and sampled 48 hours after the treatment and ii) untreated samples. Four biological replicates, each consisting of four individual test plants from each genotype were used for the transcriptome study. All the samples were collected from the crown, flash-frozen in liquid nitrogen and stored at -80 °C until RNA isolation. Total RNA was isolated using the SpectrumTM Plant Total RNA Kit (Sigma-Aldrich, USA) according to the manufacturer's instructions. For sequencing, the libraries were prepared using the TruSeqTM stranded total RNA library prep kit (Illumina, USA), indexed and pooled, and sequenced in four lanes using the Illumina HiSeq 3/4000 (2×150 bp) System by the Norwegian Sequencing Centre, Oslo, Norway. Raw reads were quality filtered, de novo assembled into transcripts and were analysed for differentially expressed genes between the inoculated and control samples.