Virulence strategies of an insect herbivore and oomycete plant pathogen converge on a host E3 SUMO ligase

Plant parasites must colonise and reproduce on plants to survive. In most cases, active immune responses, triggered by (conserved) microbe-encoded molecules keep invaders at bay. Post-translational modifications (PTMs) of proteins are vital for contextual regulation and integration of plant immune responses. Pathogens and pests secrete proteins (effectors) to interfere with plant immunity through modification of host target functions and disruption of immune signalling networks. Importantly, molecular virulence strategies of distinct pathogens converge on a small set of regulators with central roles in plant immunity. The extent of convergence between pathogen and herbivorous insect virulence strategies is largely unexplored. Here we report that effectors from the oomycete pathogen, Phytophthora capsici, and the major aphid pest, Myzus persicae target the host immune regulator SIZ1, an E3 SUMO ligase. SIZ1-regulated immunity in Arabidopsis against bacterial pathogens is known to require the resistance protein SNC1, and signalling components PAD4 and EDS1. We show that SIZ1 functions as a negative regulator of plant immunity to aphids and an oomycete pathogen. However, this immune regulation is independent of SNC1, PAD4 and EDS1-signalling pointing to the presence of a novel SIZ1-mediated immune signalling route. Our results suggest convergence of distinct pathogen and pest virulence strategies on an E3 SUMO ligase that negatively regulates plant immunity.