Physiological Changes in Barley mlo-11 Powdery Mildew Resistance Conditioned by Tandem Repeat Copy Number

The development of pleiotropic phenotypes in powdery mildew-resistant barley and Arabidopsis thaliana mlo mutants is linked to nitrogen availability

Powdery mildew-resistant barley (Hordeum vulgare) and Arabidopsis thaliana mlo mutant plants exhibit pleiotropic phenotypes such as the spontaneous formation of callose-rich cell wall appositions and early leaf chlorosis and necrosis, indicative of premature leaf senescence. The exogenous factors governing the occurrence of these undesired side effects remain poorly understood. Here, we characterised the formation of these symptoms in detail. Ultrastructural analysis revealed that the callose-rich cell wall depositions spontaneously formed in A. thaliana mlo mutants are indistinguishable from those induced by the bacterial pattern epitope, flagellin 22 (flg22). We further found that increased plant densities during culturing enhance the extent of the leaf senescence syndrome in A. thaliana mlo mutants. Application of a liquid fertiliser rescued the occurrence of leaf chlorosis and necrosis in both A. thaliana and barley mlo mutant plants. Controlled fertilisation experiments uncovered nitrogen as the macronutrient whose deficiency promotes the extent of pleiotropic phenotypes in A. thaliana mlo mutants. Light intensity and temperature had a modulatory impact on the incidence of leaf necrosis in the case of barley mlo mutant plants. Collectively, our data indicate that the development of pleiotropic phenotypes associated with mlo mutants is governed by various exogenous factors.

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

KEY MESSAGE

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.