Analogies between plant and animal immune systems, and hijacking of ADP-ribosylation signaling by pathogen effectors.

ADP ribosylation reactions are relevant either in animals and plants challenged by pathogens and in their immune response. Pathogens intracellularly or through introduction of effectors alter the signalling pathways and the protein hubs at various levels. SirTM-type are ADP-ribosylating sirtuins present in pathogenic organisms such as Staphylococcus aureus and Streptococcus pyogenes (Rack et al., 2015), in strains of Clostridiaceae, Enterococcaceae, Spirochaetaceae , in the fungi Aspergillus, Candida, Entamoeba, and in Fusarium and Phytophthora plant pathogens. Legionella pneumophila modulates cellular and innate metazoan processes (as in humans, drosophila, and C. elegans), interfering with TLR2 signalling, and immunity. L. pneumophila Sde protein is a mono-ADP-ribosyltransferase protein that converts ubiquitin to ADP-ribosylated Ub (ADPR-Ub), processes ADPR-Ub to phosphoribosyl-Ub (PR-Ub), and conjugates PR-Ub to a serine residue in substrates, transferring ubiquitin to endoplasmic reticulum-associated Rab-family GTPases (i.e. Rab33b). In this way, bacteria and fungi promote inactivation of vesicular trafficking, autophagy and phagocytosis. Also plant pathogens contain effectors with ADP-ribosylating activity (HopU1, HopF2, AvrRpmi1); HopM1 interacts and induce degradation of an ADP ribosylation factor-guanine nucleotide exchange factor (ARF-GEF) involved in vesicle trafficking. In plants, PARP-like domain proteins are Radical Cell Death 1 (RCD-1) and Similar to RCD One (SRO). PARylation of protein targets occurs during Pattern Triggered immunity: one ADP-ribosylated protein is the Forkhead Associated (FHA) protein Dawdle (DDL), a transcriptional activator. RCD1-type proteins are phosphorylated, and interact with Mut9-like kinases (MLKs). An mlk1,3,4 triple mutant exhibits stronger SA-induced defense marker gene expression, suggesting that MLKs also affect transcriptional regulation of SA signaling. The effector HaRxL106 from Hyaloperonospora arabidopsidis suppresses transcriptional activation of salicylic acid (SA)-induced defense genes, probably by converting RCD1 into a transcriptional co-repressor.