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Phosphatidylinositol-3-Kinase in Tomato (Solanum lycopersicum. L) Fruit and Its Role in Ethylene Signal Transduction and Senescence

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Title: Phosphatidylinositol-3-Kinase in Tomato (Solanum lycopersicum. L) Fruit and Its Role in Ethylene Signal Transduction and Senescence
Author: Pak Dek, Mohd Sabri
Department: Department of Plant Agriculture
Program: Plant Agriculture
Advisor: Paliyath, GopinadhanSubramanian, Jayasankar
Abstract: The ripening process is initiated by ethylene through a signal transduction cascade leads to the expression of ripening-related genes and catabolism of membrane, cell wall, and storage components. One of the minor components in membrane phospholipids is phosphatidylinositol (PI). Phosphatidylinositol-3-kinase (PIK) is an enzyme that phosphorylates PI at the 3-OH position of inositol head group to produce phosphatidylinositol 3-phosphate (PI3P). Phosphorylation of PI may be an early event in the ethylene signal transduction pathway that generates negatively charged domains on the plasma membrane. PI3P domains may potentially serve as a docking site for phospholipase D (PLD) after ethylene stimulation. It is hypothesized that ethylene stimulation may activate PI3K resulting in enhanced level of phosphorylated phosphatidylinositol. However, the properties and function of PI3K is not well understood in plants. In the present study, the effect of PI3K inhibition during tomato fruit ripening was evaluated. This study demonstrated that PI3K activity is required for normal ripening process of the fruit. Inhibition of PI3K activity using wortmannin significantly reduced tomato ripening process. The inhibitory effect of wortmannin was similar in magnitude to that obtained with 1-methylcyclopropene (1-MCP), an ethylene receptor blocker. This observation was further supported by genetic modification of PI3K gene expression in tobacco via stable transformation. PI3K overexpression (OX), and downregulation of PI3K with short hairpin antisense (sh) constructs, affected ethylene biosynthesis in flowers. PI3K OX resulted in increased levels of ethylene production and accelerated flower senescence. Meanwhile, down regulation of PI3K inhibited ethylene biosynthesis and prolonged the flower shelf life. Subcellular localization by GFP fluorescence in transgenic plants showed that PI3K was preferentially localized at the plasma membrane, nucleolus and inner part of stomata, based on the tissue. Further characterization of PI3K C2 domain revealed that this protein could bind to all phosphoinositides including PI, and its own phosphorylated products. This suggests that PI3K C2 domain enables the translocation of PI3K from cytosol to plasma membrane, and potentially other endomembranes. In conclusion, PI3K activity is required for fruit ripening, potentially acting via membrane phospholipid catabolism in conjunction with PLD, and releasing the downstream signal transduction process from suppression by CTR
Date: 2015-06
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