An oomycete effector protein induces shade avoidance inArabidopsisand attenuates salicylate signaling by binding to host proteins of the RADICAL-INDUCED CELL DEATH1 family

AbstractThe oomycete pathogenHyaloperonospora arabidopsidis(Hpa) causes downy mildew disease onArabidopsis. During infection,Hpalike 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-localizedHpaeffector HaRxL106, when expressed inArabidopsis, 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 rendersArabidopsisresilient 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. Amlk1,3,4triple mutant exhibits stronger SA-induced defense marker gene expression compared to wild-type plants. Our data suggest that HaRxL106 suppressesArabidopsisinnate 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.