The actin cytoskeleton network has an important role in plant cell growth, division, and stress response. Actin-depolymerizing factors (ADFs) are a group of actin-binding proteins that contribute to reorganization of the actin network. Here, we show that the Arabidopsis (Arabidopsis thaliana) ADF3 is required in the phloem for controlling infestation by Myzus persicae Sülzer, commonly known as the green peach aphid (GPA), which is an important phloem sap-consuming pest of more than fifty plant families. In agreement with a role for the actin-depolymerizing function of ADF3 in defense against the GPA, we show that resistance in adf3 was restored by overexpression of the related ADF4 and the actin cytoskeleton destabilizers, cytochalasin D and latrunculin B. Electrical monitoring of the GPA feeding behavior indicates that the GPA stylets found sieve elements faster when feeding on the adf3 mutant compared to the wild-type plant. In addition, once they found the sieve elements, the GPA fed for a more prolonged period from sieve elements of adf3 compared to the wild-type plant. The longer feeding period correlated with an increase in fecundity and population size of the GPA and a parallel reduction in callose deposition in the adf3 mutant. The adf3-conferred susceptibility to GPA was overcome by expression of the ADF3 coding sequence from the phloem-specific SUC2 promoter, thus confirming the importance of ADF3 function in the phloem. We further demonstrate that the ADF3-dependent defense mechanism is linked to the transcriptional up-regulation of PHYTOALEXIN-DEFICIENT4, which is an important regulator of defenses against the GPA.

Plants have evolved mechanisms to protect themselves against pathogenic microbes and insect pests. In Arabidopsis, the immune regulator PAD4 functions with its cognate partner EDS1 to limit pathogen growth. PAD4, independently of EDS1, reduces infestation by green peach aphid (GPA). How PAD4 regulates these defense outputs is unclear. By expressing the N-terminal PAD4 lipase-like domain (PAD4LLD) without its C-terminal EDS1-PAD4 (EP) domain, we interrogated PAD4 functions in plant defense. Here, we show that transgenic expression of PAD4LLD in Arabidopsis is sufficient for limiting GPA infestation but not for conferring basal and effector-triggered pathogen immunity. This suggests that the C-terminal PAD4 EP domain is necessary for EDS1-dependent immune functions but is dispensable for aphid resistance. Moreover, PAD4LLD is not sufficient to interact with EDS1, indicating the PAD4-EP domain is required for stable heterodimerization. These data provide molecular evidence that PAD4 has domain-specific functions.

SUMMARY Arabidopsis pathogen effector‐triggered immunity (ETI) is controlled by a family of three lipase‐like proteins (EDS1, PAD4, and SAG101) and two subfamilies of HET‐S/LOB‐B (HeLo)‐domain “helper” nucleotide‐binding/leucine‐rich repeats (ADR1s and NRG1s). EDS1‐PAD4 dimers cooperate with ADR1s, and EDS1‐SAG101 dimers with NRG1s, in two separate defense‐promoting modules. EDS1‐PAD4‐ADR1 and EDS1‐SAG101‐NRG1 complexes were detected in immune‐activated leaf extracts but the molecular determinants for specific complex formation and function remain unknown. EDS1 signaling is mediated by a C‐terminal EP domain (EPD) surface surrounding a cavity formed by the heterodimer. Here we investigated whether the EPDs of PAD4 and SAG101 contribute to EDS1 dimer functions. Using a structure‐guided approach, we undertook a comprehensive mutational analysis of Arabidopsis PAD4. We identify two conserved residues (Arg314 and Lys380) lining the PAD4 EPD cavity that are essential for EDS1‐PAD4–mediated pathogen resistance, but are dispensable for the PAD4‐mediated restriction of green peach aphid infestation. Positionally equivalent Met304 and Arg373 at the SAG101 EPD cavity are required for EDS1‐SAG101 promotion of ETI‐related cell death. In a PAD4 and SAG101 interactome analysis of ETI‐activated tissues, PAD4R314A and SAG101M304R EPD variants maintain interaction with EDS1 but lose association, respectively, with helper nucleotide‐binding/leucine‐rich repeats ADR1‐L1 and NRG1.1, and other immune‐related proteins. Our data reveal a fundamental contribution of similar but non‐identical PAD4 and SAG101 EPD surfaces to specific EDS1 dimer protein interactions and pathogen immunity.

Plants have evolved mechanisms to attract beneficial microbes and insects while protecting themselves against pathogenic microbes and pests. In Arabidopsis, the immune regulator PAD4 functions with its cognate partner EDS1 to limit pathogen growth. PAD4, independently of EDS1, reduces infestation by Green Peach Aphid (GPA). How PAD4 regulates these defense outputs is unclear. By expressing the N-terminal PAD4-lipase-like domain (LLD) without its C-terminal ‘EDS1-PAD4’ (EP) domain, we interrogated PAD4 functions in plant defense. Here we show that transgenic expression of PAD4LLD in Arabidopsis is sufficient for limiting GPA infestation, but not for conferring basal and effector-triggered pathogen immunity. This suggests that the C-terminal PAD4-EP domain is necessary for EDS1-dependent immune functions. Moreover, PAD4LLD is not sufficient to interact with EDS1, indicating the PAD4-EP domain is required for heterodimerisation. These data provide molecular evidence that PAD4 has domain specific functions.

MYZUS PERSICAE-INDUCED LIPASE1 (MPL1) encodes a lipase in Arabidopsis thaliana that is required for controlling infestation by the green peach aphid (GPA; Myzus persicae), an important phloem sap-consuming insect pest. Previously, we demonstrated that MPL1 expression was upregulated in response to GPA infestation, and GPA fecundity was higher on the mpl1 mutant, compared to the wild-type (WT), and lower on 35S:MPL1 plants that constitutively expressed MPL1 from the 35S promoter. Here, we show that the MPL1 promoter is active in the phloem and expression of the MPL1 coding sequence from the phloem-specific SUC2 promoter is sufficient to restore resistance to the GPA in the mpl1 mutant. The GPA infestation-associated upregulation of MPL1 requires CYCLOPHILIN 20-3 (CYP20-3), which encodes a 12-oxo-phytodienoic acid (OPDA)-binding protein that is involved in OPDA signaling and is required for controlling GPA infestation. OPDA promotes MPL1 expression to limit GPA fecundity, a process that requires CYP20-3 function. These results along with our observation that constitutive expression of MPL1 from the 35S promoter restores resistance to the GPA in the cyp20-3 mutant, and MPL1 feedbacks to limit OPDA levels in GPA-infested plants, suggest that an interplay between MPL1, OPDA, and CYP20-3 contributes to resistance to the GPA