Molecular and mechanistic role of Arabidopsis PAD4 protein in defense against the green peach aphid

Aphids are important pests of plants that cause substantial loss in plant productivity. In contrast to the chewing herbivorous insects, aphids utilize their slender stylet to feed continuously from the sieve element of plants for extended periods of time. We have utilized the interaction between the model plant Arabidopsis thaliana and the green peach aphid (GPA; Myzus persicae Sülzer) to study plant defenses against aphids. Previously, we had demonstrated that PHYTOALEXIN DEFICIENT4 (PAD4) is required for antixenosis (feeding deterrence) and antibiosis (affect aphid fecundity) mediated resistance against GPA. The PAD4 protein sequence contains the Serine, Aspartic Acid and Histidine (Ser-Asp-His) catalytic triad, which is found in many α/β fold acyl hydrolases. The Arabidopsis plants expressing mutant versions of PAD4 [PAD4(S118A) and PAD4(D178A)] supported higher numbers of GPA as compared to wild type (WT) in no-choice tests. However, these mutations did not impact antixenosis activity of PAD4 as determined by choice test experiments. Furthermore, the PAD4-modulated expression of SENESCENCE ASSOCIATED GENES in response to GPA infestation was also not impacted by these mutant alleles. The PAD4(H229A) mutation did not adversely impact PAD4 function. Furthermore, Electrical penetration graph (EPG) technique revealed that S118 residue of PAD4 is critical in limiting GPA feeding from its sieve element. Additionally, the redundancy between ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and SA in not required for limiting GPA infestation on Arabidopsis, which further supports our previous conclusion that PAD4 is functioning independent of its signaling partner EDS1, in mediating defense against GPA.