Gene expression on different carbon sources in response to hxt1 deletion in Ustilago maydis

Plant pathogenic fungi cause massive crop losses and therefore, severe economic deficits. The smut Ustilago maydis, a ubiquitous pest of corn, is highly adapted to its host to parasitize on its organic carbon sources. Recently, we identified the U. maydis sucrose transporter Srt1 to be of crucial importance for biotrophic development. Here we report the identification of Hxt1, a further member of the U. maydis sugar transporter family, which is of importance for full fungal virulence. Hxt1 mainly utilizes the hexoses glucose, fructose and mannose, but with lower affinity also secondary carbon sources like galactose and xylose. Deletion of hxt1 in U. maydis reduces virulence and growth on hexoses, in contrast growth on the secondary carbon sources is enhanced. Expression analysis revealed that monosaccharide-dependent regulation of transcription is hampered in hxt1 deletion mutants, leading to the expression of genes involved in the metabolism of secondary sugars and in the initiation of pathogenicity. Thus, we propose that Hxt1 has a dual function as monosaccharide-transporter and -sensor. While Hxt1 aids the initiation of pathogenicity by sensing starvation conditions on the plant surface as a receptor, it feeds the fungus in planta as a transporter. To analyze expression changes of SG200 and SG200∆hxt1 both strains were grown in glutamine minimal array media supplemented with 1% glucose or 1 %xylose to an OD600 of 1.0 for 6 h, respectively. For each experiment two independent replicates were conducted.