UhAVR1, an HR-Triggering Avirulence Effector of Ustilago hordei, Is Secreted via the ER�Golgi Pathway, Localizes to the Cytosol of Barley Cells during in Planta-Expression, and Contributes to Virulence Early in Infection

Ustilago hordei is a basidiomycete fungus which causes covered smut disease of barley and oats. This fungus, like many pathogens, secretes small proteins as virulence effectors in the host environment to promote infection and support the pathogen’s lifestyle. In some barley cultivars, resistance genes have been identified which can recognize such effectors and cause arrest of the pathogen. We identified previously such an effector, named UhAvr1 in this fungus which causes such avirulence when infecting barley cultivar ‘Hannchen’ which has the corresponding resistance gene Ruh1. In this study, we further investigate this effector and show it is secreted using a general pathway. By linking it to fluorescent proteins and using confocal microscopy, we can express and detect it inside barley cells, and also in cells of a nonhost plant, Nicotiana benthamiana. We show that this effector is produced only when encountering the barley host very early during infection. We show that in cultivar ‘Odessa’ which does not have this resistance gene, this effector has a function in suppressing the defense response so it can infect better. However, in cultivar ‘Hannchen’, it causes cell death very early during infection, a mechanism which the plant evolved to stop the fungus. Understanding such interactions and defense mechanism at the molecular level will assist with breeding for resistance.