Phospholipase D activation is an early component of the salicylic acid signaling pathway in Arabidopsis cell suspensions

Involvement of phospholipase D and phosphatidic acid in the light-dependent up-regulation of sorghum leaf phosphoenolpyruvate carboxylase-kinase

The photosynthetic phosphoenolpyruvate carboxylase (C(4)-PEPC) is regulated by phosphorylation by a phosphoenolpyruvate carboxylase kinase (PEPC-k). In Digitaria sanguinalis mesophyll protoplasts, this light-mediated transduction cascade principally requires a phosphoinositide-specific phospholipase C (PI-PLC) and a Ca(2+)-dependent step. The present study investigates the cascade components at the higher integrated level of Sorghum bicolor leaf discs and leaves. PEPC-k up-regulation required light and photosynthetic electron transport. However, the PI-PLC inhibitor U-73122 and inhibitors of calcium release from intracellular stores only partially blocked this process. Analysis of [(32)P]phosphate-labelled phospholipids showed a light-dependent increase in phospholipase D (PLD) activity. Treatment of leaf discs with n-butanol, which decreases the formation of phosphatidic acid (PA) by PLD, led to the partial inhibition of the C(4)-PEPC phosphorylation, suggesting the participation of PLD/PA in the signalling cascade. PPCK1 gene expression was strictly light-dependent. Addition of neomycin or n-butanol decreased, and a combination of both inhibitors markedly reduced PPCK1 expression and the concomitant rise in PEPC-k activity. The calcium/calmodulin antagonist W7 blocked the light-dependent up-regulation of PEPC-k, pointing to a Ca(2+)-dependent protein kinase (CDPK) integrating both second messengers, calcium and PA, which were shown to increase the activity of sorghum CDPK.

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

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 alpha/beta 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 His(6)-SOBER1 hydrolyzed PC but not PA or LysoPC in vitro indicating that the enzyme has phospholipase A(2) (PLA(2)) activity. Chemical inhibition of PLA(2) activity in leaves expressing SOBER1 resulted in HR in response to Pst DC3000 AvrBsT. These data are consistent with the model that SOBER1 PLA(2) 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.