The conserved protein complex known as Mediator conveys transcriptional signals by acting as an intermediary between transcription factors and RNA polymerase II. As a result, Mediator subunits play multiple roles in regulating developmental as well as abiotic and biotic stress pathways. In this report we identify the head domain subunits MEDIATOR18 and MEDIATOR20 as important susceptibility factors for Fusarium oxysporum infection in Arabidopsis thaliana. Mutants of MED18 and MED20 display down-regulation of genes associated with jasmonate signaling and biosynthesis while up-regulation of salicylic acid associated pathogenesis related genes and reactive oxygen producing and scavenging genes. We propose that MED18 and MED20 form a sub-domain within Mediator that controls the balance of salicylic acid and jasmonate associated defense pathways.

The Mediator complex is an evolutionarily conserved protein complex involved in transcriptional regulation. In plants, the Mediator complex plays an important role in regulating development as well as the response to abiotic and biotic stress. In this thesis, I investigated three subunits of the Arabidopsis thaliana Mediator complex MEDIATOR25, MEDIATOR18 and MEDIATOR20 (MED25, MED18 and MED20) in plant defence.MED25 has been shown to interact with several transcription factors (TFs) to control jasmonate (JA)-associated transcription. Here, I investigated these interactions further and characterized the minimum protein domains of the TFs needed for interaction and identified new proteins that interact with MED25. The results showed that the protein sequence corresponding to amino acids 138-218 of ETHYLENE RESPONSE FACTOR1 (ERF1) appears to be the minimum protein length for interaction with MED25 and contains the conserved motif (CMIX). Although the protein motif has been found in different proteins in A. thaliana that belong to a variety of families, only members from the IXc AP2/ERF family were found to interact with the MED25 ACID domain.Apart from MED25, I found that the MED18 and MED20 subunits interact within the Mediator complex and infection of the corresponding mutants with Fusarium oxysporum revealed that they display a high level of resistance. RNAseq analysis of F. oxysporum infected med18 and med20 roots showed that salicylic acid (SA)-associated PATHOGENESIS RELATED genes and (ROS) reactive oxygen producing genes were up-regulated while some JA-regulated genes were repressed.We also noticed induction and repression of genes involved in glucosinolate production. This may suggest that ROS and antifungal metabolites are induced in med18 and med20 mutants and could potentially contribute to the reduced colonization of the mutants by F. oxysporum. Overall, the results reveal a new role for MED18 and MED20 in susceptibility to F. oxysporum, most likely via their involvement in JA signalling whose up-regulation would compromise SA and ROS signalling pathways that can confer resistance against this hemibiotrophic pathogen.ii

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