Physiological Changes in Barley Powdery Mildew Resistance Conditioned by Tandem Repeat Copy Number.

Wild barley accessions have evolved broad-spectrum defence against barley powdery mildew through recessive mutations. However, the defence response is associated with deleterious phenotypes with a cost to yield and fertility, with implications for natural fitness and agricultural productivity. This research elucidates the mechanism behind a novel allele, designated (), which has a milder phenotype compared to standard . Bisulphite sequencing and histone ChIP-seq analyses using near-isogenic lines showed pronounced repression of the promoter in standard compared to (), with repression governed by 24 nt heterochromatic small interfering RNAs. The () allele appears to largely reduce the physiological effects of while still endorsing a high level of powdery mildew resistance. RNA sequencing showed that this is achieved through only partly restricted expression of , allowing adequate temporal induction of defence genes during infection and expression close to wild-type levels in the absence of infection. The two alleles showed copy number proportionate oxidase and peroxidase expression levels during infection, but lower amino acid and aromatic compound biosynthesis compared to the null allele . Examination of highly expressed genes revealed a common WRKY W-box binding motif (consensus ACCCGGGACTAAAGG) and a transcription factor more highly expressed in resistance. In conclusion, () appears to significantly mitigate the trade-off between defence and normal gene expression.