A unique resistance mechanism is associated with RBgh2 barley powdery mildew adult plant resistance

Abstract

Gene expression at the RBgh2 locus indicates involvement in cAMP/G-protein-coupled signalling and innate immunity in barley powdery mildew adult plant resistance. Barley powdery mildew is a globally significant disease, responsible for reduced grain yield and quality. A major effect adult plant resistance gene, RBgh2, was previously found in a landrace from Azerbaijan. The atypical phenotype suggested different underlying genetic factors compared to conventional resistance genes and to investigate this, genome-wide gene expression was compared between sets of heterogeneous doubled haploids. RBgh2 resistance is recessive and induces both temporary genome-wide gene expression changes during powdery mildew infection together with constitutive changes, principally at the RBgh2 locus. Defence-related genes significantly induced included homologues of genes associated with innate immunity and pathogen recognition. Intriguingly, RBgh2 resistance does not appear to be dependent on salicylic acid signalling, a key pathway in plant resistance to biotrophs. Constitutive co-expression of resistance gene homologues was evident at the 7HS RBgh2 locus, while no expression was evident for a 6-transmembrane gene, predicted in silico to contain both G-protein- and calmodulin-binding domains. The gene was disrupted at the 5' end, and G-protein-binding activity was suppressed. RBgh2 appears to operate through a unique mechanism that co-opts elements of innate immunity.

Fig. 1

Powdery mildew disease phenotypes of RBgh2 F₁ progeny. Symptoms are shown for F₁ individuals from crosses between cv. Baudin and the RBgh2-containing DH line Ba69_012 at 14 dpi. Parental lines are shown on the left (panels a and b) and F₁ individuals in panels c–f

Fig. 2

Gene expression and GO term differences between RBgh2 barley powdery mildew-resistant and -susceptible DH lines. a An overview of the RNA experimental design and grouped sample comparisons. b Venn diagram of significantly differentially expressed genes (SDEGs). Groups I and II represent Bgh-inoculated APR and susceptible lines relative to their respective control samples, and group III represents APR Bgh-inoculated relative to susceptible Bgh-inoculated samples. c Plots for significant GO term enrichments for SDEGs. Biological processes, cellular components and molecular functions are shown from left to right. The y-axis displays the ratio of significantly DEGs/total number of genes annotated for a GO term (in the barley genome) for each group comparison (x-axis), with the same comparison groups as in b. GO terms represented by induced genes have positive values on the y-axis and suppressed genes by negative values on the y-axis

Fig. 3

Graphical representation of gene expression around the RBgh2 locus. Average normalised expression is shown for genes within approximately 2 Mbp of closest RBgh2 genetic marker, based on an adjusted p value ≤ 0.05 and absolute log₂ FC> 5. a Doubled haploid (DH) line gene expression. Bgh-inoculated samples are indicated in purple and controls in blue. APR RBgh2 DH lines are shown in green and susceptible lines in red. b Gene expression in mlo near-isogenic lines (NILs) from Ge et al. (2020). NILs are S2 (mlo-11(cnv2), in green), S12 (mlo-11, in red) and P22 (mlo-5, in cyan). Recurrent parent control lines are S0 (cv. Baudin, for mlo-11(cnv2) and mlo-11, in tan) and P18 (cv. Pallas, for mlo-5, in light purple) (colour figure online)