PT - JOURNAL ARTICLE AU - Lennart Wirthmueller AU - Shuta Asai AU - Ghanasyam Rallapalli AU - Jan Sklenar AU - Georgina Fabro AU - Dae Sung Kim AU - Ruth Lintermann AU - Pinja Jaspers AU - Michael Wrzaczek AU - Jaakko Kangasjärvi AU - Daniel MacLean AU - Frank L. H. Menke AU - Mark J. Banfield AU - Jonathan D. G. Jones TI - An oomycete effector protein induces shade avoidance in <em>Arabidopsis</em> and attenuates salicylate signaling by binding to host proteins of the RADICAL-INDUCED CELL DEATH1 family AID - 10.1101/137844 DP - 2017 Jan 01 TA - bioRxiv PG - 137844 4099 - http://biorxiv.org/content/early/2017/05/14/137844.short 4100 - http://biorxiv.org/content/early/2017/05/14/137844.full AB - The oomycete pathogen Hyaloperonospora arabidopsidis (Hpa) causes downy mildew disease on Arabidopsis. During infection, Hpa like other biotrophic pathogens, suppresses activation of plant innate immunity by translocating effector proteins into host cells. Some of these effectors localize to the host cell nucleus where they may manipulate transcriptional reprogramming of plant defense genes. Here we report that the nuclear-localized Hpa effector HaRxL106, when expressed in Arabidopsis, induces shade avoidance and attenuates the transcriptional response to the defense signaling molecule salicylic acid. HaRxL106 interacts with RADICAL-INDUCED CELL DEATH1 (RCD1) and loss of RCD1 function renders Arabidopsis resilient against HaRxL106-mediated suppression of immunity. To further characterize the molecular functions of RCD1 we solved a crystal structure of RCD1’s Poly-(ADP-ribose)-Polymerase (PARP) domain and, based on non-conservation of amino acids constituting the active site of canonical PARPs, conclude that RCD1 has no PARP activity. We report that RCD1-type proteins are phosphorylated and identified histone-modifying Mut9-like kinases (MLKs) as RCD1-interacting proteins. A mlk1,3,4 triple mutant exhibits stronger SA-induced defense marker gene expression compared to wild-type plants. Our data suggest that HaRxL106 suppresses Arabidopsis innate immunity by manipulating the function(s) of RCD1 in the host cell nucleus and point towards a role of RCD1 as a transcriptional co-regulator that integrates signals from light and pathogen sensors.