Discrimination of Arabidopsis PAD4 Activities in Defense against Green Peach Aphid and Pathogens

The Arabidopsis ( Arabidopsis thaliana ) lipase-like protein PHYTOALEXIN DEFICIENT4 (PAD4) is essential for defense against green peach aphid (GPA; Myzus persicae ) and the pathogens Pseudomonas syringae and Hyaloperonospora arabidopsidis . In basal resistance to virulent strains of P. syringae and H. arabidopsidis , PAD4 functions together with its interacting partner ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) to promote salicylic acid (SA)-dependent and SA-independent defenses. By contrast, dissociated forms of PAD4 and EDS1 signal effector-triggered immunity to avirulent strains of these pathogens. PAD4-controlled defense against GPA requires neither EDS1 nor SA. Here, we show that resistance to GPA is unaltered in an eds1 salicylic acid induction deficient2 ( sid2 ) double mutant, indicating that redundancy between EDS1 and SID2 -dependent SA, previously reported for effector-triggered immunity conditioned by certain nucleotide-binding-leucine-rich repeat receptors, does not explain the dispensability of EDS1 and SID2 in defense against GPA. Mutation of a conserved serine (S118) in the predicted lipase catalytic triad of PAD4 abolished PAD4-conditioned antibiosis and deterrence against GPA feeding, but S118 was dispensable for deterring GPA settling and promoting senescence in GPA-infested plants as well as for pathogen resistance. These results highlight distinct molecular activities of PAD4 determining particular aspects of defense against aphids and pathogens.