Viral protease NIa-Pro increases MEDIATOR SUBUNIT16 cleavage and plant susceptibility to turnip mosaic virus and its aphid vector Myzus persicae.

Plant viruses, such turnip mosaic virus (TuMV), both trigger and inhibit host plant defense responses, including defenses that target their insect vectors, such as aphids. TuMV infection and its protein, NIa-Pro (nuclear inclusion protease a), suppress aphid-induced plant defenses, however the mechanisms of this suppression are still largely unknown. In this study, we determined that NIa-Pros protease activity is required to increase aphid performance on host plants and that 40 transcripts with predicted NIa-Pro cleavage sequences are regulated in Arabidopsis plants challenged with aphids and/or virus compared to healthy controls. One of the candidates, MEDIATOR 16 (MED16), regulates the transcription of ethylene (ET)/jasmonic acid (JA)-dependent defense responses against necrotrophic pathogens. Using immunoblots, mutants, bioassays, and qRT-PCR, we show that a nuclear localization signal is cleaved from MED16 in virus-infected plants and in the presence of NIa-Pro along with the presence of aphids, suggesting MED16 functions in the nucleus may be impacted. Consistent with this, aphid induction of the MED16-dependent transcript of PLANT DEFENSIN 1.2 (PDF1.2), was reduced in virus-infected plants and in plants expressing NIa-Pro compared to controls, and NIa-Pros protease activity was required for this reduction. Finally, we show performance of both the virus and the aphid vector was enhanced on med16 mutant Arabidopsis compared to controls. Overall, this study demonstrates MED16 regulates defense responses against both the virus and the aphid and provides insights into the mechanism by which TuMV suppresses anti-virus and anti-herbivore defenses.