Dissecting the journey and functions of barley smut fungus Ustilago hordei immunity-triggering effector UhAvr1 to reveal durable defense strategies in cereals

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Effector biology, the study of small secreted proteins from microbial pathogens, is a burgeoning research field in both human/animal and plant sciences. Studying effectors has already revealed insight into ways these proteins interact with and manipulate many basic host processes and signaling pathways. Effectors target development, protein degradation and gene regulation which include defense and resistance responses. Therefore, effectors teach us how pathogens cause disease but can also be used as probes to elucidate basic biological processes. Fungi have large suites of effectors, revealed by genome sequencing projects, but their mode of delivery, host cell targets and functions remain largely unknown. Having contributed to the generation and analyses of genome sequences of several cereal-infecting Basidiomycete pathogens (barley smut fungus Ustilago hordei and the related wheat leaf rust fungus Puccinia triticina), we can now focus on functions of the many predicted effectors. We recently identified in U. hordei an effector that triggers defense when infecting a barley cultivar with resistance gene Ruh1. This avirulence effector UhAVR1 is only expressed upon contact with the host, is secreted from the fungus, and its interaction with Ruh1 or its action leads to local host cell death and immunity to the infection. We genetically located Ruh1 on the short arm of barley chromosome 1 to a ~200-kb region. This proposal aims to shed light on 3 basic questions: 1. What are the mechanisms of UhAVR1 secretion and determinants for host cell uptake? 2. What is the function of UhAVR1 in barley cells, is it a real virulence factor, what host cell process or protein does it target? 3. How does UhAVR1 interact with Ruh1 and cause immunity and can we isolate Ruh1? Elucidating mechanisms of effector delivery, identification of their host target(s) and defense elicitation in this model system will aid studying difficult, yet economically more important cereal rust pathosystems also studied in our laboratory. Conserved fungal components essential for effector delivery are prime targets for designing specific control measures such as RNA silencing for which we have developed a strategy in wheat against rusts. An identified resistance gene can be used in breeding programs and host effector target genes can be modified by genome editing to obtain durable disease resistance in crop plants and less reliance on harmful pesticides.