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

Abstract

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.

The Sporisorium reilianum Effector Vag2 Promotes Head Smut Disease via Suppression of Plant Defense Responses

“…The next closest similarity is to Tin1-3 and Tin1-6 of U. maydis which, respectively, are most closely related to Vag2 and Sr10052.2 of S. reilianum [ 67 ]. In contrast to vag2 which was expressed throughout the biotrophic phase with the highest values in young ears ( Figure 3 A), Uhavr1 was expressed only in the early stages of the biotrophic phase [ 68 ] suggesting specific roles of the two proteins in their respective pathosystems. UhAvr1 was shown to lead to complete immunity in barley cultivars carrying the resistance gene Ruh1, which triggers hypersensitive cell death (HR).…”

“…UhAvr1 was shown to lead to complete immunity in barley cultivars carrying the resistance gene Ruh1, which triggers hypersensitive cell death (HR). In contrast, in susceptible plants, the protein contributes to fungal virulence by suppressing general defense responses including nonhost/PTI and ETI invoked by various pathogens [ 68 ].…”

Smuts to the Power of Three: Biotechnology, Biotrophy, and Basic Biology

“…Montenegro Alonso and coworkers. They found that the effector protein is required for virulence on susceptible barley, induced a hypersensitive reaction (HR) in resistant barley cultivars, was secreted via the ER–Golgi pathway, located to the cytosol when expressed in plant cells, and had a role in suppressing conserved plant basal immunity components [ 9 ]. Effectors of U. maydis were the object of study of J.R.L.…”

The Ustilago hordei–Barley Interaction is a Versatile System for Characterization of Fungal Effectors

“…However, heterologous expression of some effectors in planta can result in strong pleiotropic defects that compromise symptom evaluations. In another strategy, the type III secretion system (T3SS) of Pseudomonas syringae (for Arabidopsis) and Pseudomonas fluorescens or Pseudomonas atropurpurea (for wheat and barley) is used for functional characterization of several intracellular effectors from obligate biotrophs, such as rusts and powdery mildews [11][12][13][14][15]. Any growth promotion observed for Pseudomonas sp.…”

“…Any growth promotion observed for Pseudomonas sp. transformants, which deliver the desired effectors into the host cell during infection, is interpreted as a positive contribution to virulence [11][12][13][14][15]. However, the T3SS of Pseudomonas sp.…”