Wheat plants were grown in the presence (Si+) or absence (Si-) of Si and inoculated (B+) or not (B-) with Bgt for a total of four treatments: Si-B-, Si+B-, Si-B+, and Si+B+. These treatments were applied in the following manner. One week after sowing, distilled water was replaced by a Hoagland solution amended or not with potassium silicate (Kasil 6, PQ Corp etc) at a concentration of 1.7 mM (Si+ Hoagland). Twice a week, Si was added to the Si+ nutrient solution in order to maintain Si concentration at 1.7 mM. Every other week, both Si- and Si+ Hoagland solutions were renewed and the pH adjusted to 5.8 in all systems at each Si addition or nutrient solution renewal. Four weeks after sowing (i.e. three weeks after Si amendment started), half the plants were inoculated with Bgt as described previously. Briefly, one day prior to inoculation, reservoir wheat plants heavily infected with Bgt were gently shaken to remove old spores and to stimulate the production of fresh ones. The inoculation was performed by shaking the heavily infected wheat plants over the experimental plants.
Hydroponic systems were used to precisely control Si feeding of plants and to reduce external Si contamination. Seeds of wheat cultivar AC Drummond, chosen for its known high susceptibility to Bgt, were sown in 9-cm pots in a nylon bed consisting of nylon stockings cut into small ribbons. Hydroponic systems were set up to immerse roots for 15 minutes every 30 minutes. The plants were grown in a greenhouse (16 h light at 22°C and 8h dark at 18°C, 80% humidity). Plants were immersed in distilled water only during the first week following sowing.
Plants were individually harvested, flash frozen in liquid nitrogen and kept at -80°C until used. Leaves of each plant were separately ground with a pestle and mortar in liquid nitrogen. Total RNA was extracted with RNeasy plant kit (Qiagen, Hilden, Germany). Concentration and quality of RNA were assessed on a Nanodrop spectrophotometer (Nanodrop, Wilmington, De, USA) and 2100 Bioanalyser (Agilent, Palo Alto, Ca, USA).
Biotin-labeled cRNAs were synthesized using MessageAmp II-Biotin Enhanced Kit (Ambion, Austin, Tx, USA) following the manufacturer’s instructions using 1 µg total RNA. Labelled cRNA was then fragmented using 5X Array Fragmentation Buffer supplied with the MessageAmp II-Biotin Enhanced Kit.
The labelled samples were added to an Affymetrix GeneChip® Wheat Genome Array (Affymetrix, Santa Clara, CA, USA), according to the manufacturer’s instructions. The chips were hybridized for 16 h at 45 °C in a rotisserie oven at 60 rpm
Following hybridization, the arrays were washed and stained in an Affymetrix Fluidics Station 450, according to the standard protocol from Affymetrix, and scanned using an Affymetrix Scanner 3000.
The R 2.6 software and Bioconductor were used to perform data analysis using dedicated packages Limma , Affy , and AffyPLM available on the Bioconductor website (http://www.bioconductor.org/). Data quality assessment was performed as described. These packages were also used to correct for background (RMA method), normalize the microarray data (quantile method) and generate lists of differentially expressed genes according to the fold change and t-test p-values.