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Blocking the Metabolism of Starch Breakdown Products in Arabidopsis Leaves Triggers Chloroplast Degradation

Molecular Plant 2009年第6期2卷 1233-1246页

作者： Michaela Stettler simona eicke Tabea Mettler Gaelle Messerli Stefan Hortensteiner Samuel C. Zeeman Department of Biology ETH Zurich CH-8092 Zurich Switzerland Institute of Plant Biology University of Zurich CH-8008 Zurich Switzerland Present address: Max Planck Institute for Molecular Plant Physiology Potsdam-Golm 14476 Germany Present address: Johnson & Johnson Medical CH-8957 Spreitenbach Switzerland

In most plants, a large fraction of photo-assimilated carbon is stored in the chloroplasts during the day as starch and remobilized during the subsequent night to support metabolism. Mutations blocking either starch synthesis or starch breakdown in Arabidopsis thaliana reduce plant growth. Maltose is the major product of starch breakdown exported from the chloroplast at night. The maltose excess 1 mutant （mex1）, which lacks the chloroplast envelope maltose transporter, accumulates high levels of maltose and starch in chloroplasts and develops a distinctive but previously unexplained chlorotic phenotype as leaves mature. The introduction of additional mutations that prevent starch synthesis, or that block maltose production from starch, also prevent chlorosis of mex1. In contrast, introduction of mutations in disproportionating enzyme （DPE1） results in the accumulation of maltotriose in addition to maltose, and greatly increases chlorosis. These data suggest a link between maltose accumulation and chloroplast homeostasis. Microscopic analyses show that the mesophyll cells in chlorotic mex1 leaves have fewer than half the number of chloroplasts than wild-type cells. Transmission electron microscopy reveals autophagy-like chloroplast degradation in both mex1 and the dpe1/mex1 double mutant. Microarray analyses reveal substantial reprogramming of metabolic and cellular processes, suggesting that organellar protein turnover is increased in mex1, though leaf senescence and senescence-related chlorophyll catabolism are not induced. We propose that the accumulation of maltose and malto-oligosaccharides causes chloroplast dysfunction, which may by signaled via a form of retrograde signaling and trigger chloroplast degradation.

关键词： Carbohydrate metabolism photosynthesis senescence chloroplast biology Arabidopsis autophagy.

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