Abstract
Loss-of-function of barley mildew locus o (Mlo) confers durable broad-spectrum penetration resistance to the barley powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh). Given the importance of mlo mutants in agriculture, surprisingly few molecular components have been identified to be required for this type of resistance in barley. With the aim to identify novel cellular factors contributing to mlo-based resistance, we devised a pharmacological inhibitor screen. Of the 41 rationally chosen compounds tested, five caused a partial suppression of mlo resistance in barley, indicated by increased levels of Bgh host cell entry. These chemicals comprise brefeldin A (BFA), 2′,3′-dideoxyadenosine (DDA), 2-deoxy-d-glucose, spermidine, and 1-aminobenzotriazole. Further inhibitor analysis corroborated a key role for both anterograde and retrograde endomembrane trafficking in mlo resistance. In addition, all four ribonucleosides, some ribonucleoside derivatives, two of the five nucleobases (guanine and uracil), some guanine derivatives as well as various polyamines partially suppress mlo resistance in barley via yet unknown mechanisms. Most of the chemicals identified to be effective in partially relieving mlo resistance in barley also to some extent compromised powdery mildew resistance in an Arabidopsis mlo2 mlo6 double mutant. In summary, our study identified novel suppressors of mlo resistance that may serve as valuable probes to unravel further the molecular processes underlying this unusual type of disease resistance.
Highlights
Powdery mildew is a common and widespread disease in angiosperm plants [1,2]
A screen for chemical suppressors of mlo-mediated resistance in barley identifies five compounds that result in increased Blumeria graminis f. sp. hordei (Bgh) host cell entry
With the aim of identifying additional components and/or pathways involved in mlo-mediated powdery mildew resistance, we selected 41 chemicals (Supplementary Table S1) that are known to inhibit key cellular pathways or to regulate physiological processes involved in plant defense and tested them for their impact on the Bgh infection phenotype in barley mlo mutant plants
Summary
Powdery mildew is a common and widespread disease in angiosperm plants [1,2] It is caused by obligate biotrophic fungal pathogens of the order Erysiphales (Ascomycetes; [3]). Barley (Hordeum vulgare) and Arabidopsis thaliana are monocotyledonous and dicotyledonous host plant species, respectively, for which the interaction with powdery mildew fungi has been studied extensively at the genetic, molecular, and cellular level [4,5,6]. In both instances, various plant immune pathways can limit the extent of fungal invasion. The barley Mlo gene encodes for a member of an evolutionary conserved type of integral membrane protein with yet unknown biochemical activity [12,13]
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