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Mechanism of control of size and number of chloroplasts

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Posted June 3, 2015
This news or article is intended for readers with certain scientific or professional knowledge in the field.

Researchers at the University of Tokyo and the National Institute of Genetics have found that the lipid phosphatidylinositol 4-phosphate (PI4P) binds solely to a region of a protein in the division apparatus responsible for the division of chloroplasts in plant cells.

Changes in chloroplasts on addition of an agent that inhibits phosphatidylinositol 4-phosphate (PI4P) synthesis. In contrast to chloroplasts in untreated plant cells (A), the number of chloroplasts per cell increases but their size is reduced when treated with an inhibitor of PI4P synthesis (B, C). Treatment with an inhibitor of synthesis of PI3P, another kind of phosphoinositide, has almost no effect on the size and number of chloroplasts. White scale bar = 10 micrometers. Image credit: Kumiko Okazaki

Changes in chloroplasts on addition of an agent that inhibits phosphatidylinositol 4-phosphate (PI4P) synthesis. In contrast to chloroplasts in untreated plant cells (A), the number of chloroplasts per cell increases but their size is reduced when treated with an inhibitor of PI4P synthesis (B, C). Treatment with an inhibitor of synthesis of PI3P, another kind of phosphoinositide, has almost no effect on the size and number of chloroplasts. White scale bar = 10 micrometers. Image credit: Kumiko Okazaki

Chloroplasts are essential plant organelles responsible for photosynthesis and many fundamental metabolic reactions. Chloroplasts are not formed newly within the cell, but multiply only by division of existing chloroplasts. It is well known that PI4P, a phosphorylated derivative of phosphatidylinositol, modulates fundamental cellular processes. Although the existence of PI4P in chloroplasts has been reported, little is known about its function or whether it is involved in chloroplast division.

The research group of Research Fellow Kumiko Okazaki and Professor Hajime Wada at the University of Tokyo Graduate School of Arts and Sciences and Professor Shinya Miyagishima of the National Institute of Genetics found that artificially decreasing the level of PI4P in chloroplasts in Arabidopsis accelerated chloroplast division resulting in an increased number of chloroplasts per cell, indicating that PI4P inhibits chloroplast division. Analyses of mutant Arabidopsis plants demonstrating irregularities in chloroplast division revealed that the binding of PI4P to PDV1, a chloroplast division protein, changes the interaction between PDV1 and DRP5B, another component of the chloroplast division machinery. This suggests that PI4P is a regulator of chloroplast division in a PDV1- and DRP5B-dependent manner.

The current research is a significant contribution to our understanding the basic mechanisms of plant growth and may lead to techniques for controlling the number and size of chloroplasts, enabling the development of crop varieties with varying sizes of starch granules.

Source: University of Tokyo

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