Study : Transcriptome analysis of a stem rust resistance locus on wheat chromosome 7AL
Transcriptome analysis of a stem rust resistance locus on wheat chromosome 7AL
Aim:To characterise a recently discovered stem rust resistance locus on wheat chromosome 7AL. Transcriptome analysis by RNA-sequencing, in association with microscopic observations, was used to compare responses to the Puccinia graminis f. sp. tritici pathogen of the susceptible line Columbus, and two independent backcrossed resistant lines containing the locus, Columbus-NS765 and Columbus-NS766. Results: Microscopic observations of infected leaves revealed that the resistance conferred by the 7AL resistance locus was initiated by two days post-inoculation, upon the entry of the stem rust fungus into the plant through the stoma. Death of guard and epidermal cells adjacent to the fungal points of entry was observed to be clearly more frequent in resistant lines than in the susceptible genotype, suggesting that the resistance response is similar in all genotypes, but enhanced in the resistant lines. Transcriptomic analysis, combined with assignment of genes to wheat chromosomes, revealed a disporportionately high number of differentially expressed genes were located on chromosomes 7AL and 6A. A number of genes annotated as cysteine-rich receptor-like kinases were located on chromosome 7AL. Closer investigation indicated that the encoded proteins were in fact putative receptor-like cytoplasmic kinases (RLCKs). One of the putative RLCK genes contained a SNP marker previously shown to co-segregate with the 7AL resistance locus. The large number of differentially expressed genes on chromosome 6A indicated the presence of a large introgression on this chromosome that co-segregated with stem rust resistance in the two independent resistant lines, but its role in the resistance response is currently unclear. Conclusions: This study represents the first transcriptome analysis of responses to stem rust in wheat, and the first investigation of the resistance conferred by the newly-discovered wheat 7AL stem rust resistance locus. Microscopy showed the resistance response was associated with pre-haustorial cell death. Results of the RNA-seq, which has the resolution to discriminate between homeologous wheat genes, along with assignment of differentially expressed genes to wheat chromosomes, suggested putative receptor-like cytoplasmic kinases linked to the 7AL locus as candidate resistance genes for further investigation. Overall design: Illumina Sequencing was performed for 3 biological replicates of seedling leaf RNA from 3 genotypes of Triticum aestivum (1 susceptible and 2 resistant) at 3 time points after inoculation with Puccinia graminis f. sp. tritici, total 27 samples.