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BAX INHIBITOR-1 Is Required for Full Susceptibility of Barley to Powdery Mildew

¹Lehrstuhl für Phytopathologie, Technische Universität München, Emil-Ramann-Straße 2, 85350 Freising-Weihenstephan, Germany; ²Plant Pathology, Scottish Crop Research Institute (SCRI), Dundee DD2 5DA, U.K.; ³Science and Advice for Scottish Agriculture (SASA), Virology and Zoology section, Roddinglaw Road, Edinburgh, EH12 9FJ, U.K.; ⁴Institute of Plant Genetics and Crop Plant Research, Corrensstraße 3, 06466 Gatersleben, Germany

BAX INHIBITOR-1 (BI-1) is one of the few proteins known to have cross-kingdom conserved functions in negative control of programmed cell death. Additionally, barley BI-1 (HvBI-1) suppresses defense responses and basal resistance to the powdery mildew fungus Blumeria graminis f. sp. hordei and enhances resistance to cell death--provoking fungi when overexpressed in barley. Downregulation of HvBI-1 by transient-induced gene silencing or virus-induced gene silencing limited susceptibility to B. graminis f. sp. hordei, suggesting that HvBI-1 is a susceptibility factor toward powdery mildew. Transient silencing of BI-1 did not limit supersusceptibility induced by overexpression of MLO. Transgenic barley plants harboring an HvBI-1 RNA interference (RNAi) construct displayed lower levels of HvBI-1 transcripts and were less susceptible to powdery mildew than wild-type plants. At the cellular level, HvBI-1 RNAi plants had enhanced resistance to penetration by B. graminis f. sp. hordei. These data support a function of BI-1 in modulating cell-wall-associated defense and in establishing full compatibility of B. graminis f. sp. hordei with barley.