Uncovering the control of leaf starch degradation

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Plants feed themselves by converting carbon dioxide from the air into sugars, in the process of photosynthesis. These sugars provide the energy and building blocks for plant growth. Although photosynthesis can only happen in the light, most plants continue to grow at night. They are able to do this because some of the sugars made during the day are stored in leaves as starch. At night, the starch is broken down to release sugars that can be used for growth. We have discovered that plants control very precisely the rate at which starch is broken down to release sugars during the night, so that the supply of starch lasts precisely until the dawn when photosynthesis starts again. This control is very important, because mutant plants that run out of starch before dawn are unable to grow for the last part of the night. They are therefore less productive overall than normal plants. Remarkably, we have shown that the plant must be able to anticipate the length of the night and to do arithmetic in order to achieve this. When it gets dark, the plant measures the amount of starch it has, estimates the time until dawn, then divides the amount of starch by the length of time for which it must last in order to set the correct rate of breakdown to release sugars. We know that the length of time until dawn is measured by the plant's internal circadian clock, but we have very little knowledge of other aspects of the arithmetic division mechanism. The aim of this project is to discover more about how it works. To discover what genes and proteins are involved in the mechanism, we have produced plants that glow in the dark when they run out of starch and stop growing. This achievement means that we can now identify mutant plants that run out of starch at the wrong time; they will glow earlier or later during the night than normal plants. The mutant plants will have defects in genes needed for the arithmetic division mechanism. By identifying the defective genes we will be able to piece together how the arithmetic division mechanism works, and so understand how plants are able to grow all through the night.