Molecular Dissection of Maize-Sporisorium reilianum Interactions: Host Developmental Changes and Pathogen Effectors

Sporisorium reilianum causes head smut disease on maize. Symptoms of S. reilianum become obvious when the fungus forms spores and induces phyllody in the inflorescences. This study aimed to provide an understanding of how S. reilianum changes the developmental program of maize, and to identify and characterize symptom and virulence determinants of the pathogen. The maize-S. reilianum interaction was approached from both sides. From the side of maize, the induced alterations on morphology, transcription, hormone level and concentration of reactive oxygen species (ROS) were investigated. Morphological analysis showed that S. reilianum infection promoted outgrowth of subapical ears suggesting that fungal presence suppressed apical dominance. S. reilianum colonization also led to an increase in the total cytokinin and auxin content of the inflorescence and an accumulation of ROS that could explain suppression of apical dominance. Furthermore, S. reilianum infection triggered loss of floral organ and meristem identities, and loss of meristem determinacy. Microarray analysis showed that these developmental changes were accompanied with transcriptional regulation of genes known or proposed to regulate floral organ and meristem identities, and meristem determinacy in maize. From the side of S. reilianum, genome comparison with the closely related smuts Ustilago maydis and Ustilago hordei was performed to predict potential candidate genes with function in symptom formation or virulence on maize. Functional characterization of the candidates showed that deletion of dicer did not lead to changes in virulence or in the mutant behavior in response to different stresses. In addition, genes with avirulence functions (sr10057, sr10060, sruni1 and sruni2) and those with virulence functions (clusters 19A, cluster 5-1uni and sruni5) were identified. Cluster 19A3 was suggested to encode leaf senescence inhibitors, since its deletion triggered leaf tip death. Additionally, vag1 fulfilled avirulence and virulence functions in leaves and nodes, respectively. Two genes, sad1 and sad2 showed involvement in suppression of apical dominance. SAD1 and VAG1 were degraded in a leaf-specific manner. SAD1 localized to the nucleus and the cytoplasm when expressed in plant cells and, interacted with the effector MIG1 and plant proteins involved in ubiquitination, nuclear functions and signaling. These interaction partners could shape the mechanism of tissue-specific degradation of SAD1 and the role of SAD1 in suppression of apical dominance. Hence, a molecular link could be established between specific effectors of S. reilianum and induced developmental changes in the host