A negative regulator of ribosomal RNA processing specifies R-gene-independent cell death in barley - powdery mildew interactions
Programmed Cell Death (PCD) plays a pivotal role in plant development and defense. Pathogen dependent cell-death mutants were used to investigate the complex regulatory pathways between PCD and R-gene mediated resistance. Time-course expression profiles of Blumeria graminis f. sp. hordei (Bgh) challenged C.I. 16151 (harboring the Mla6 powdery mildew resistance allele) and its fast-neutron-derived "Bgh-induced tip cell death1" mutant, bcd1, were compared using the 22K Barley1 GeneChip. Contrasts were made to identify genes associated with the cell death phenotype as opposed to R-gene mediated resistance. One hundred eighty-two genes were found in the intersection of these contrasts at a threshold p value < 0.001 (equivalent false discovery rate < 5%). One hundred forty-seven of these 182 genes were found to be constitutively overexpressed in the bcd1 mutant. GO annotation indicates that these genes are mainly involved in metabolism, showing common physiological process with other stressed-induced genes. Six deleted genes that co-segregated with the cell death phenotype mapped to the same region on chromosome 7 (5H), and are highly syntenous with rice. F2 segregation analysis of crosses between genotypes harboring bcd1 and Mla6 x Bcd1 and mla6 demonstrated that the tip cell death was independent of R-gene mediated resistance. Virus induced gene silencing (VIGS) of one of the six deleted genes, CA031190, phenocopied bcd1-mediated tip cell death. These findings suggest that Bcd1 mediates metabolism involved in cell death progression as a result of signaling during the barley-powdery mildew interaction, but is independent of gene-for-gene resistance.