Elicitor-induced Activation of Phospholipases Plays an Important Role for the Induction of Defense Responses in Suspension-cultured Rice Cells

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Abstract
Biphasic generation of reactive oxygen species (ROS) induced by N-acetylchitooligosaccharide elicitor in rice cells was associated with the activation of phopholipase C (PLC) and phospholipase D (PLD). The activation of both enzymes was observed for the first phase of ROS generation, but only the activation of PLD was evident for the second response. Activation of PLD was associated with its recruitment to the membrane. Enzymatic products of these phospholipases, diacylglycerol (DG) and phosphatidic acid (PA), could induce ROS generation by themselves. Moreover, the addition of these lipids compensated the inhibition of the second phase of ROS generation by cycloheximide, indicating the involvement of the synthesis of PLD or related proteins in the second phase of ROS generation. DG and PA also induced the expression of elicitor-responsive genes in the absence of the elicitor. They could not induce phytoalexin biosynthesis by themselves but greatly enhanced the elicitor-induced phytoalexin accumulation. Further, the inhibition of PLD by 1-butanol inhibited the elicitor-induced phytoalexin accumulation, indicating the involvement of PLD and its reaction product, PA, in the induction of phytoalexin biosynthesis. These results indicated the importance of phospholipid signaling, especially by PLD and its product PA, in plant defense responses.
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