SOBER1 phospholipase activity suppresses phosphatidic acid accumulation and plant immunity in response to bacterial effector AvrBsT

• Published in: Proceedings of the National Academy of Sciences - PNAS 2009-12, Vol.106 (48), p.20532-20537
• Main Authors: Kirik, Angela, Mudgett, Mary Beth
• Format: Article
• Language: English
• Subjects: Arabidopsis; Arabidopsis Proteins - genetics; Arabidopsis Proteins - immunology; Arabidopsis Proteins - metabolism; Arabidopsis thaliana; AvrBsT protein; Bacteria; bacterial diseases of plants; bacterial proteins; Bacterial Proteins - immunology; Bacterial Proteins - metabolism; Biological Sciences; carboxylic ester hydrolases; Carboxylic Ester Hydrolases - genetics; Carboxylic Ester Hydrolases - immunology; Carboxylic Ester Hydrolases - metabolism; Comparative analysis; Disease resistance; enzyme activity; Enzymes; Flowers & plants; host-pathogen relationships; hypersensitive response; Immunity; Leaves; Lipids; Lipids - analysis; Lysophosphatidylcholines - metabolism; Mass Spectrometry; Models, Biological; Mutation; Pathogens; phosphatidic acid; Phosphatidic acids; Phosphatidic Acids - metabolism; phosphatidylcholines; Phosphatidylcholines - metabolism; phospholipase; phospholipase A2; phospholipase D; Phospholipases A2 - metabolism; Phospholipids; Plant cells; Plant Diseases - immunology; Plant Diseases - microbiology; Plant immunity; Plant Leaves - metabolism; Pseudomonas syringae - immunology; Pseudomonas syringae pv. tomato; Substrate Specificity; Tomatoes
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0903859106

Arabidopsis thaliana ecotype Pi-0 is resistant to Pseudomonas syringae pathovar tomato (Pst) strain DC3000 expressing the T3S effector protein AvrBsT. Resistance is due to a loss of function mutation (sober1-1) in a conserved α/β hydrolase, SOBER1 (Suppressor of AvrBsT Elicited Resistance1). Members of this superfamily possess phospholipase and carboxylesterase activity with diverse substrate specificity. The nature of SOBER1 enzymatic activity and substrate specificity was not known. SOBER1-dependent suppression of the hypersensitive response (HR) in Pi-0 suggested that it might hydrolyze a plant lipid or precursor required for HR induction. Here, we show that Pi-0 leaves infected with Pst DC3000 expressing AvrBsT accumulated higher levels of phosphatidic acid (PA) compared to leaves infected with Pst DC3000. Phospholipase D (PLD) activity was required for high PA levels and AvrBsT-dependent HR in Pi-0. Overexpression of SOBER1 in Pi-0 reduced PA levels and inhibited HR. These data implicated PA, phosphatidylcholine (PC) and lysophosphatidylcholine (LysoPC) as potential SOBER1 substrates. Recombinant His₆-SOBER1 hydrolyzed PC but not PA or LysoPC in vitro indicating that the enzyme has phospholipase A₂ (PLA₂) activity. Chemical inhibition of PLA₂ activity in leaves expressing SOBER1 resulted in HR in response to Pst DC3000 AvrBsT. These data are consistent with the model that SOBER1 PLA₂ activity suppresses PLD-dependent production of PA in response to AvrBsT elicitation. This work highlights an important role for SOBER1 in the regulation of PA levels generated in plants in response to biotic stress.