Uncovering the control of leaf starch degradation

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.

At night, maintenance and growth processes in the Arabidopsis plant are sustained by mobilisation of starch reserves acquired by photosynthesis during the previous day. Precise control of the rate of starch degradation, and its coordination with the demands of metabolism and growth, are essential for normal productivity. Using experimental and modelling approaches we have established that mobilisation is controlled by a mechanism that effectively arithmetically divides the time until dawn by the starch level to compute a linear rate that permits almost complete utilisation of reserves by dawn. We know that measurement of time until dawn is a function of the circadian clock, that starch granule numbers and properties are important for the mechanism, and that control operates on the pathway of starch degradation at a post-translational level, but most of the components of the mechanism are unknown. The aim of this project is to discover these components, using genetic, biochemical and metabolic approaches. We will:

-Discover novel mutants defective in the control of starch degradation, by screening a "starvation reporter" mutant population. Plants emit luminescence when starch reserves have been exhausted. In initial work, this screen has already yielded mutants with novel starch degradation phenotypes. Identification of the mutated genes will reveal new components of the arithmetic division apparatus.

-Further characterise selected existing mutants with interesting starch degradation phenotypes, as a basis for discovering new components of the arithmetic division mechanism.

Characterisation of novel and existing mutants will include complementation of mutant plants with tagged, wild-type versions of the protein (for identification of interacting proteins), and with versions of the protein lacking key domains, and detailed analysis of starch granule numbers, sizes shapes and surface and matrix properties.

Beneficiaries of this work are as follows:

Crop breeders and biotechnologists will have access to 1) new information about factors that influence productivity, and link plant carbohydrate status to productivity; 2) genes crucial for normal productivity. This access will be through existing PI contacts, BBSRC-sponsored forums, activities with the JIC KEC team including annual reviews and a follow-up with biotech companies in year 3, and the stakeholder platform of a large EU consortium.

Plant scientists will gain new insights into outputs of the circadian clock, sugar- and starvation signalling, pathways of carbohydrate catabolism, the control of plant growth in relation to carbohydrate availability, and the relationship between photosynthesis and patterns of growth. These insights will come from publications, from invited seminars, from formal and informal meetings associated with the PI's BBSRC and EU grants, and from attendance of major national and international meetings by the PI and PDRA.

Biologists interested in the control of reserve utilisation in organisms that undergo predictable periods of fasting (e.g. hibernation and migration) will benefit from new understanding of the underlying mechanisms in plants. They will have access to the outputs of the project through press releases, publication in non-specialist journals, and seminars at meetings with broad scientific remits.

Non-scientists will appreciate the intriguing findings that plants can both anticipate the length of the night and carry out arithmetic division to prevent night-time starvation. They will have access to our findings through 1) releases through standard and social media channels at the time of publication of new results, 2) talks by the PI and PD to a range of audiences including school students, teachers and the public, 3) science fairs and local shows, 4) discussion groups, for example Norfolk farmers and Friends of the John Innes Centre.

Early career scientists will benefit from the training opportunities provided by the project. The project is expected to provide experience of research for several undergraduate and school students on short placements.