ABSTRACT: Stripe rust, caused by Puccinia striiformis f. sp. tritici, is a destructive disease of wheat worldwide. Genetic resistance is the preferred method for controlling stripe rust, of which two major types are race-specific and race non-specific resistance. Race-specific resistance includes the qualitatively inherited all-stage resistance, controlled by single major resistance (R) genes. Conversely, adult-plant resistance is race non-specific, inherited quantitatively, and is durable. Previously, we characterized the gene expression signatures involved in Yr5-controlled all-stage resistance and Yr39-controlled adult-plant resistance using the Affymetrix Wheat GeneChip. For this study, we designed and constructed custom oligonucleotide microarrays containing probes for the 116 and 207 transcripts that we had found important for the Yr5 and Yr39 resistance responses, respectively. We used this custom microarray to profile the resistance responses of eight wheat genotypes with all-stage resistance (Yr1, Yr5, Yr7, Yr8, Yr9, Yr10, Yr15, and Yr17). The aim of this analysis was to identify common and unique gene expression signatures involved in race-specific resistance accross genotypes, which were used to infer information regarding the general pathways involved in all-stage resistance. Keywords: Stress response Eight wheat genotypes with defined single gene all-stage resistance to particular stripe rust (Pst) isolates were selected for this study, as well as the Pst susceptible genotype âAvocet Susceptibleâ (AVS). Near Isogenic Lines (NILs) for each of eight Yr resistance genes (Yr1, Yr5, Yr7, Yr8, Yr9, Yr10, Yr15 and Yr17) were developed at the Plant Breeding Institute, Sydney, Australia, by backcrossing the Yr gene donors with the recurrent susceptible spring wheat genotype (Triticum aestivum L.) AVS six times (AVS*6/Yr*) and selecting for the appropriate resistance at each generation. Both virulent and avirulent isolates of Pst were selected for each NIL, except for Yr15, for which no virulent isolate is currently known. Care was taken to select the fewest isolates needed to satisfy the spectrum of virulence/avirulence required, resulting in the use of six isolates identified as PST-17, PST-21, PST-43, PST-45, PST-78 and PST-AUS. Each isolate was selected and maintained on susceptible genotypes. For each of three biological replications, individual genotypes were planted in separate 25 X 42.5-cm flats using a potting mix (6 peat moss: 4 vermiculite with lime: 3 sand: 3 commercial potting mix: 2 perlite: 1.7 g/L lime: 3.3 g/L Osmocote: 2.2 g/L ammonium nitrate). Each flat consisted of three rows of six seedlings, with rows randomly assigned one of two harvest times (24 and 48 h post-inoculation). Seedlings from the 3rd row were used to monitor the expected disease responses to inoculation. Seedlings were grown to the second leaf stage (Feekes 1.2, emergence with second leaf unfolded, ~10 days after planting) in a greenhouse with a diurnal temperature cycle of 10ºC (2:00 am) to 25ºC (2:00 pm) and a 16 h light/8 h dark cycle. Inoculation was performed by misting the plants with sterile water and applying a 1:20 urediniospore:talc mixture to leaves with a sterile brush. Talc was used to aid in the spread and adhesion of spores over leaf surfaces. Control flats were treated the same way except for the absence of spores in the talc. All treatments for each biological replication were performed at 9 am Pacific Standard Time. To promote spore germination, all flats were transferred to a dew chamber (100% RH) operating at 10ºC in the dark for 24 h, before being placed in a growth chamber with a diurnal temperature cycle of 4ºC (2:00 am) to 20ºC (2:00 pm) and a 16 h light/8 h dark cycle. Rows of plants were harvested from all flats at the assigned times for RNA extraction.
