UhAVR1, an HR-triggering avirulence effector of Ustilago hordei, is secreted via the ER-Golgi pathway to the cytosol of barley coleoptile cells and contributes to virulence early in infection.

The basidiomycete Ustilago hordei (Uh) causes covered smut disease of barley and oats. Virulence effectors that aid the infection process and support the pathogens lifestyle have been described for this fungus. Genetically, six avirulence genes are known and one codes for UhAVR1, the only proven avirulence effector identified in smut pathogens to date that triggers complete immunity in barley cultivars carrying the resistance gene Ruh1. A prerequisite for resistance breeding is understanding the host targets and molecular function of UhAVR1. Analysis of this effector upon natural infection of barley coleoptiles using teliospores showed that UhAVR1 is expressed during the early stages of fungal infection where it leads to HR triggering in resistant cultivars or performs its virulence function in susceptible cultivars. Fungal secretion of UhAVR1 is directed by its signal peptide and occurs via the BrefeldinA-sensitive ER-Golgi pathway, both in cell culture away from its host, and during barley interaction. Transient expression of this effector in barley and a heterologous host, Nicotiana benthamiana (Nb), supports a cytosolic localization. Delivery of UhAVR1 via foxtail mosaic virus, Pseudomonas species or Agrobacterium-mediated suppression of cell inducers in barley and Nb support a role in the suppression of a common component(s) of ETI and PTI which is conserved in both plant systems.