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
Montenegro Alonso, A.P., Ali, S., Song, X., Linning, R., Bakkeren, G. (2020). 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. Journal of Fungi, [online] 6(3), 1-32. http://dx.doi.org/10.3390/jof6030178
Plain language summary
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.
The basidiomycete Ustilago hordei causes covered smut disease of barley and oats. Virulence effectors promoting infection and supporting pathogen 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 smuts to date that triggers complete immunity in barley cultivars carrying 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 in cell culture away from its host. Transient in planta expression of UhAVR1 in barley and a nonhost, Nicotiana benthamiana, supports a cytosolic localization. Delivery of UhAVR1 via foxtail mosaic virus or Pseudomonas species in both barley and N. benthamiana reveals a role in suppressing components common to both plant systems of Effector-and Pattern-Triggered Immunity, including necrosis triggered by Agrobacterium-delivered cell death inducers.