The nuclear protein Ros1 contributes to virulence and is essential for spore formation.

(A) Cellular localization of Ros1. FB1_PotefRos1mCherry-P_nup107Nup107eGFP expressing a Ros1mCherry fusion protein and the nuclear envelope marker Nup107eGFP were grown to mid exponential phase in YEPSL and analyzed by fluorescence microscopy. The green and red fluorescing signals corresponding to Nup107GFP and Ros1mCherry are shown in the left and middle panels, respectively. The right panel shows an overlay of both signals. The size marker corresponds to 10 μM. (B) The deletion of ros1 attenuates virulence. Wild type strains FB1 and FB2, the ros1 deletion strains FB1Δros1 and FB2Δros1 and the complementation strains FB1Δros1-Ros1 and FB2Δros1-Ros1 were mixed in the indicated combinations and injected into maize seedlings. Disease symptoms were scored 12 days after infection according to [10]. Colors used for disease scores are indicated on the right side. Three independent experiments were performed and the average values are expressed as a percentage of the total number of infected plants (n) given above each column. Error bars indicate standard deviation. Statistically significant differences between FB1 x FB2 and ros1 deletion strains FB1 x FB2 and the complementation strains are indicated by black stars. Statistically significant differences between ros1 deletion strains and the complementation strains are indicated by white stars (one-way ANOVA applying the Tukey-Kramer test[36]) (C) Representative tumors formed after infection of maize seedlings with the indicated combinations of strains are shown 12 days after infection (top panel). In the examples chosen, stem-bending is induced. The ros1 deletion strains induce tumors, but these lack dark coloration typical for tumors containing mature spores. Lower panel: dispersed tumor tissue of the indicated strains was analyzed by light microscopy. Mature teliospores are absent from tumors induced by the ros1 deletion strains. The size bar corresponds to 20 μM.