Gene expression of Ustilago maydis SG200, SG200∆hxt1 and the avirulent SG200∆hxt1 hxt1(R164K) on maize leaf surface

Hxt1 is a high affinity hexose transporter important for virulence of the phytopathogenic basidiomycete Ustilago maydis on its host plant maize. Hxt1 shows the highest similarities to the glucose sensors Rgt2 and Snf3 from S. cereviciae (42% and 39% identity on amino acid level, respectively). In these sensors, the substitution of a highly conserved arginine to lysine leads to a constitutive active signal, resulting in expression of several glucose induced genes. Introduction of an analogous mutation in Hxt1 leads to loss of transport activity in the resulting Hxt1(R164K) protein. Expression of Hxt1(R164K) in hxt1 deletion strains results in a completely avirulent phenotype. Pathogenic developement of ∆hxt1 hxt1(R164K) strains is blocked immediately after plant penetration. Expression analysis of ∆hxt1 hxt1(R164K) cells 24 hours post inoculation revealed downregulation of a set of genes involved in carbohydrate metabolism indicating a role of Hxt1 in carbohydrate signaling during initiation of the pathogenic stage. To analyze expression changes of SG200 and SG200∆hxt1 and SG200∆hxt1 hxt1(R164K) cells were harvested from the surface of maize leaves 24 hour post inoculation. For each strain three independent replicates were conducted.