Global transcriptional profiling of SG200 and the SA sensing mutant SG200∆rss1 during biotrophic growth

The phenolic compound salicylic acid (SA) is a key signaling molecule regulating local and systemic plant defense responses against biotrophs. Many microbial organisms, including pathogens, share the ability to degrade SA. However, the mechanism by which they perceive SA is unknown. We showed that Ustilago maydis, the causal agent of corn smut disease, employs a so far uncharacterized SA sensing mechanism. We identified and characterized the novel SA sensing regulator, Rss1, a binuclear zinc cluster protein with dual functions as putative SA receptor and transcriptional activator. To determine global transcriptional changes between the rss1 deletion mutant SG200∆rss1 and its progenitor strain SG200 during biotrophic growth and to identify potential Rss1 target genes, we conducted a microarray analysis at four days post infection. We identified only nine genes that were differentially regulated (adjusted p-value < 0.05), six of those were significantly repressed while three were significantly induced in SG200∆rss1. This set includes genes important for SA and tryptophan degradation indicating that Rss1 could play a role in the regulation of respective metabolic processes. Maize plants (Early Golden Bantam) were infected with SG200 or SG200Δrss1. Four days post infection, samples were collected in three independently conducted experiments by sampling 12 plants per experiment. After RNA extraction Agilent microarrays were performed.