Suppression of Arabidopsis Mediator Subunit-Encoding MED18 Confers Broad Resistance Against DNA and RNA Viruses While MED25 Is Required for Virus Defense Nasser K. Hussein 1,2 * , Layla J. Sabr 2 , Edina Lobo 1 , James Booth 1 , Emily Ariens 1 , Swaminathan Detchanamurthy 1 and Peer M. Schenk 1 1 Plant-Microbe Interactions Laboratory, School of Agriculture and Food Sciences, the University of Queensland, Brisbane, QLD, Australia, 2 Plant Protection Department, College of Agriculture, University of Baghdad, Baghdad, Iraq Mediator subunits play key roles in numerous physiological pathways and developmental processes in plants. Arabidopsis Mediator subunits, MED18 and MED25, have previously been shown to modulate disease resistance against fungal and bacterial pathogens through their role in jasmonic acid (JA) signaling. In this study, Arabidopsis mutant plants of the two Mediator subunits, med18 and med25, were tested against three ssRNA viruses and one dsDNA virus belonging to four different families: Turnip mosaic virus (TuMV), Cauli flower mosaic virus (CaMV), Alternanthera mosaic virus (AltMV), and Cucumber mosaic virus (CMV). Although both subunits are utilized in JA signaling, they occupy different positions (Head and Tail domain, respectively) in the Mediator complex and their absence affected virus infection differently. Arabidopsis med18 plants displayed increased resistance to RNA viral infection and a trend against the DNA virus, while med25 mutants displayed increased susceptibility to all viruses tested at 2 and 14 days post inoculations. Defense marker gene expression profiling of mock- and virus-inoculated plants showed that med18 and med25 mutants exhibited an upregulated SA pathway upon virus infection at 2 dpi for all viruses tested. JA signaling was also suppressed in med18 plants after virus infection, independent of which virus infected the plants. The upregulation of SA signaling and suppression of JA signaling in med18 may have led to more targeted oxidative burst and programmed cell death to control viruses. However, the susceptibility exhibited by med25 mutants suggests that other factors, such as a weakened RNAi pathway, might play a role in the observed susceptibility. We conclude that MED18 and MED25 have clear and opposite effects on accumulation of plant viruses. MED18 is required for normal virus infection, while MED25 is important for defense against virus infection. Results from this study provide a better understanding of the role of Mediator subunits during plant-virus interactions, viral disease progression and strategies to develop virus resistant plants. Keywords: Alternanthera mosaic virus, Arabidopsis thaliana, Cauliflower mosaic virus, Cucumber mosaic virus, mediator subunit, plant virus resistance, Turnip mosaic virus Frontiers in Plant Science | www.frontiersin.org March 2020 | Volume 11 | Article 162 1 Edited by: Aardra Kachroo, University of Kentucky, United States Reviewed by: Rae-Dong Jeong, Chonnam National University, South Korea Richard S. Nelson, Oklahoma State University, United States *Correspondence: Nasser K. Hussein n.hussein@uq.edu.au Specialty section: This article was submitted to Virology, a section of the journal Frontiers in Plant Science Received: 17 October 2019 Accepted: 03 February 2020 Published: 04 March 2020 Citation: Hussein NK, Sabr LJ, Lobo E, Booth J, Ariens E, Detchanamurthy S and Schenk PM (2020) Suppression of Arabidopsis Mediator Subunit-Encoding MED18 Confers Broad Resistance Against DNA and RNA Viruses While MED25 Is Required for Virus Defense. Front. Plant Sci. 11:162. doi: 10.3389/fpls.2020.00162 ORIGINAL RESEARCH published: 04 March 2020 doi: 10.3389/fpls.2020.00162