Zinc-finger (ZiF) fold secreted effectors form a functionally diverse family across lineages of the blast fungus Magnaporthe oryzae

Abstract

Filamentous plant pathogens deliver effector proteins into host cells to suppress host defence responses and manipulate metabolic processes to support colonization. Understanding the evolution and molecular function of these effectors provides knowledge about pathogenesis and can suggest novel strategies to reduce damage caused by pathogens. However, effector proteins are highly variable, share weak sequence similarity and, although they can be grouped according to their structure, only a few structurally conserved effector families have been functionally characterized to date. Here, we demonstrate that Zinc-finger fold (ZiF) secreted proteins form a functionally diverse effector family in the blast fungus Magnaporthe oryzae . This family relies on the Zinc-finger motif for protein stability and is ubiquitously present, forming different effector tribes in blast fungus lineages infecting 13 different host species. Homologs of the canonical ZiF effector, AVR-Pii from rice infecting isolates, are present in multiple M. oryzae lineages, and the wheat infecting strains of the fungus, for example, possess an allele that also binds host Exo70 proteins and activates the immune receptor Pii. Furthermore, ZiF tribes vary in the host Exo70 proteins they bind, indicating functional diversification and an intricate effector/host interactome. Altogether, we uncovered a new effector family with a common protein fold that has functionally diversified in lineages of M. oryzae . This work expands our understanding of the diversity of M. oryzae effectors, the molecular basis of plant pathogenesis and may ultimately facilitate the development of new sources for pathogen resistance. Author Summary: Diseases caused by filamentous plant pathogens impact global food production, leading to severe economic and humanitarian consequences. These pathogens secrete hundreds of effectors inside the host to alter cellular processes and to promote infection and disease. Effector proteins have weak or no sequence similarity but can be grouped in structural families based on conserved protein folds. However, very few conserved effector families have been functionally characterized. We have identified a family of effectors with a shared Zinc-finger protein fold (ZiF) that is present in lineages of the blast fungus Magnaporthe oryzae that can, collectively, infect 13 different grasses. We characterized the binding of a sub-set of these proteins to putative Exo70 host targets and showed they can be recognized by the plant immune system. Furthermore, we show that other ZiF effectors do not bind Exo70 targets, suggesting functional specialization within this effector family for alternative interactors. These findings shed light on the diversity of effectors and their molecular functions, as well as potentially leading to the development of new sources of blast disease resistance in the future.