Molecular basis for differential host response of sorghum against different formae speciales of Sporisorium reilianum

Sporisorium reilianum is a biotrophic fungus causing head smut in maize and sorghum. The fungus exists in two formae speciales, S. reilianum f. sp. zeae (SRZ) and S. reilianum f. sp. reilianum (SRS), which can infect maize and sorghum, respectively. SRZ is not able to systemically spread in sorghum, presumably because it is challenged by several plant defense responses. The strongest visual defense response of sorghum against SRZ is the production of phytoalexins that lead to a red coloration of infected leaf tissues. We hypothesize that phytoalexin biosynthesis is induced by SRZ-specific secreted effector proteins. Effectors are classified as apoplastic or cytoplasmic based on their localization in the extracellular space or inside the plant cells, respectively. Identification of factors from SRZ that induce phytoalexin biosynthesis will lead to the identification of host-specificity factors of Sporisorium reilianum. To find out which factor of SRZ induces phytoalexin formation in sorghum, 77 predicted SRZ-specific effectors were cloned in an expression vector under control of a strong constitutive plant promoter. Of these, 40 are high-priority effector candidates as predicted by EffectorP. Potential apoplastic effectors with high cysteine content were cloned with signal peptide and potential cytoplasmic effectors were cloned without signal peptide to ensure proper localization after expression in plant cells. The candidate effectors are being tested for their ability to induce phytoalexin formation by particle bombardment of sorghum leaves. This analysis will show whether any of the cloned candidate effectors is able to induce phytoalexin formation in sorghum and thus determines host selection of S. reilianum.