Arabidopsis phospholipase Dα1 and Dδ oppositely modulate EDS1-and SA-independent basal resistance against adapted powdery mildew

Plants use a tightly regulated immune system to fight off various pathogens. Phospholipase D (PLD) and its product, phosphatidic acid, have been shown to influence plant immunity; however, the underlying mechanisms remain unclear. Here, we show that the Arabidopsis mutants pld alpha 1 and pld delta, respectively, exhibited enhanced resistance and enhanced susceptibility to both well-adapted and poorly adapted powdery mildew pathogens, and a virulent oomycete pathogen, indicating that PLD alpha 1 negatively while PLD delta positively modulates post-penetration resistance. The pld alpha 1 delta double mutant showed a similar infection phenotype to pld alpha 1, genetically placing PLD alpha 1 downstream of PLD delta. Detailed genetic analyses of pldd with mutations in genes for salicylic acid (SA) synthesis (SID2) and/or signaling (EDS1 and PAD4), measurement of SA and jasmonic acid (JA) levels, and expression of their respective reporter genes indicate that PLD delta contributes to basal resistance independent of EDS1/PAD4, SA, and JA signaling. Interestingly, while PLD alpha 1-enhanced green fluorescent protein (eGFP) was mainly found in the tonoplast before and after haustorium invasion, PLD delta-eGFP's focal accumulation to the plasma membrane around the fungal penetration site appeared to be suppressed by adapted powdery mildew. Together, our results demonstrate that PLD alpha 1 and PLD delta oppositely modulate basal, post-penetration resistance against powdery mildew through a non-canonical mechanism that is independent of EDS1/PAD4, SA, and JA.