Multiple pairs of allelic MLA immune receptor-powdery mildew AVR _A effectors argue for a direct recognition mechanism

Abstract

Nucleotide-binding domain and leucine-rich repeat (NLR)-containing proteins in plants and animals mediate intracellular pathogen sensing. Plant NLRs typically detect strain-specific pathogen effectors and trigger immune responses often linked to localized host cell death. The barley Mla disease resistance locus has undergone extensive functional diversification in the host population and encodes numerous allelic NLRs each detecting a matching isolate-specific avirulence effector (AVR _A) of the fungal pathogen Blumeria graminis f. sp. hordei ( Bgh). We report here the isolation of Bgh AVR _a7 , AVR _a9 , AVR _a10 , and AVR _a22 , which encode small secreted proteins recognized by allelic MLA7, MLA9, MLA10, and MLA22 receptors, respectively. These effectors are sequence-unrelated, except for allelic AVR _a10 and AVR _a22 that are co-maintained in pathogen populations in the form of a balanced polymorphism. Contrary to numerous examples of indirect recognition of bacterial effectors by plant NLRs, co-expression experiments with matching Mla-AVR _a pairs indicate direct detection of the sequence-unrelated fungal effectors by MLA receptors.

eLife digest

Powdery mildews are fungal diseases that affect many plants, including important crops such as barley. The fungi behind these diseases deliver molecules known as effectors inside plant cells, which manipulate the plants' biology and help the fungus to invade the plants' tissues. In response, some plants have evolved immune receptors encoded by so-called R genes (short for resistance genes) that detect the effectors inside the plant cell and trigger an immune response. The response often kills the plant cell and those nearby to limit the spread of the fungus. Effectors that are recognized by host immune receptors are termed avirulence effectors (or AVRs for short).

Scientists tend to assume that most effectors do not bind directly to their immune receptors. Instead, it is thought that the immune receptors are more likely to be detecting a change in some other plant protein that is caused by the effectors' activities.

In barley populations, one R gene that protects against powdery mildew encodes an immune receptor known as MLA. Different plants can carry subtly different versions of this R gene meaning that they make similar but different variants of the same receptor. Each MLA variant confers immunity only to strains of powdery mildew that carry the matching AVR effector. A few AVR effectors from powdery mildews have been identified, but most AVR effectors from powdery mildews remain unknown.

Saur et al. looked for new AVR effectors from powdery mildew fungi collected in the field, and found four that were recognized by barley plants carrying MLA variants. Two of these new effectors were fairly similar to each other, but they were all unlike those that had been identified previously.

When Saur et al. engineered barley cells to make these new AVRs alongside their matching MLA receptors, the cells died – which is consistent with the expected immune response. Similar experiments with distantly related tobacco plants agave the same results. This suggested that the immune receptors did not need any other barley proteins to recognize the effectors, indicating that the interaction between the two may be direct. Indeed, two other techniques that test for direct protein-protein interactions, – one that involved extracts from tobacco leaves, and another that involved yeast, – gave results consistent with a direct interaction between the MLA receptor variants and the fungal effectors.

Plant disease is still a major cause of loss of yield in crops. Transferring an R gene from one plant species to another is a potentially powerful approach to help crops resist disease. The discovery that multiple variants of the same resistance gene can bind to dissimilar effectors from a disease-causing fungus in distantly related plant species underlines the potential of this approach.