The potato cyst nematode effector RHA1B is a ubiquitin ligase and uses two distinct mechanisms to suppress plant immune signaling.

Plant pathogens, such as bacteria, fungi, oomycetes and nematodes, rely on wide range of virulent effectors delivered into host cells to suppress plant immunity. Although phytobacterial effectors have been intensively investigated, little is known about the function of effectors of plant-parasitic nematodes, such as Globodera pallida, a cyst nematode responsible for vast losses in the potato and tomato industries. Here, we demonstrate using in vivo and in vitro ubiquitination assays the potato cyst nematode (Globodera pallida) effector RHA1B is an E3 ubiquitin ligase that employs multiple host plant E2 ubiquitin conjugation enzymes to catalyze ubiquitination. RHA1B was able to suppress effector-triggered immunity (ETI), as manifested by suppression of hypersensitive response (HR) mediated by a broad range of nucleotide-binding leucine-rich repeat (NB-LRR) immune receptors, presumably via E3-dependent degradation of the NB-LRR receptors. RHA1B also blocked the flg22-triggered expression of Acre31 and WRKY22, marker genes of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), but this did not require the E3 activity of RHA1B. Moreover, transgenic potato overexpressing the RHA1B transgene exhibited enhanced susceptibility to G. pallida. Thus, our data suggest RHA1B facilitates nematode parasitism not only by triggering degradation of NB-LRR immune receptors to block ETI signaling but also by suppressing PTI signaling via an as yet unknown E3-independent mechanism. Author summary Globodera pallida is a plant-parasitic cyst nematode that causes vast losses in economically important crops such as potato and tomato. To successfully parasitize host plants, G. pallida produces proteins called effectors to overcome plant defenses. Here, we report identification of a novel G. pallida effector RHA1B as an E3 ubiquitin ligase, which is responsible for ubiquitin-proteasome-mediated protein degradation in general. We found that RHA1B can suppress plant defense signaling via both E3-dependent and -independent manners. In particular, it promotes degradation of a broad range of NB-LRR immune receptors. In addition, expression of RHA1B in potato plants made the plants more susceptible to G. pallida infection, indicating that RHA1B acts as an effector that aids parasitism. Overall, we found RHA1B as the first effector with ubiquitin ligase activity identified from eukaryotic pathogen infecting plants or animals. Our data suggest nematode uses RHA1B as a powerful weapon to manipulate host cellular signaling pathways, thereby interfering with plant immunity for successful parasitism.