The twilight zone: the initiation of starch degradation in leaves

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. At the end of the day, as it gets dark, plants must switch from photosynthesis to starch breakdown as their source of sugars for growth. We know that this switch happens gradually during twilight. As the light fades and the supply of sugars from photosynthesis goes down, the supply of sugars from starch gradually increases. However, almost nothing is known about how plants bring about this transition from photosynthesis to starch breakdown as the source of sugars.

We have discovered that starch breakdown is triggered by falling light levels at the end of the day. Surprisingly, however, the same fall in light levels does not trigger starch breakdown in the middle of the day. This discovery implies that plants have a sophisticated mechanism that anticipates when the night will come, and triggers starch breakdown in response to falling light levels only at that point. During this project, we will investigate how falling light levels trigger starch breakdown, and whether the timing of this process is controlled by the plant's internal clock (the circadian clock). The research will make use of mutant plants that have either a reduced ability to perceive light levels, or alterations in the functioning of their internal clock. The outcomes will be new information about how plants maintain their growth throughout the day night cycle, and insights into how different light regimes may affect plant productivity.

Plants undergo a huge metabolic shift at the boundary between day and night, acquiring sugars for growth via photosynthesis in the light, but via degradation of stored starch in the dark. Plant scientists often grow material in square-wave light regimes, with instantaneous onset of darkness. This situation imposes transient carbon starvation at the start of the night. It is not representative of the real world, in which light levels decline gradually. Older physiological studies show that natural twilight is accompanied by a gradual transition from photosynthesis to starch degradation as the source of sucrose for growth. Almost nothing is known about this transition. We have found that starch degradation is initiated specifically in response to declining light levels at the end of the light period. The same decline in light levels in the middle of the light period does not initiate starch degradation. These results imply that starch degradation is initiated either directly by changes in light levels or indirectly by a decline in the rate of photosynthesis, and that this response is gated by a mechanism sensitive to the time of day.
To discover the mechanisms involved we will examine whether degradation is initiated by light or by declining photosynthesis, employing mutants defective in light perception and techniques that reduce the rate of photosynthesis in constant light. We will investigate whether the gating of initiation of starch degradation is a function of the circadian clock, using mutants defective in clock function. To obtain further information about the mechanisms involved, we will analyse changes in the primary metabolome, the transcriptome and the phosphoproteome in response to declining light levels at various points in light period, and look for natural variation in this response that could permit discovery of the genes involved. Finally, we will compare the productivity and fitness of plants grown in square-wave and more natural light regimes.

In the short to medium term, companies growing crops in controlled environments will benefit from a better understanding of the impact of different kinds of light regimes on plant productivity. Developers and manufacturers of controlled environment rooms and of lighting systems for commercial horticulture will benefit from new knowledge about the relationship between light regimes and plant productivity. In the longer term, crop breeders and crop biotechnologists will have access to new information about genetic and environmental factors that influence plant productivity. Communication to these communities will be through existing PI contacts, BBSRC-sponsored forums, JIC KEC activities, information on the JIC website, and articles in relevant trade journals.

Information from our research could potentially provide societal and environmental benefits by reducing the energy costs of greenhouse-based horticulture.

A wide range of non-scientists will appreciate the intriguing finding that plants can anticipate the time of onset of night and use this information to ensure a smooth transition from photosynthesis to starch breakdown as the source of sugars for growth. Our findings will be communicated to these communities through press releases, talks and discussions with audiences including school students, teachers, local shows and science fairs, Norfolk farmers and Friends of the John Innes Centre. We will also produce a YouTube video to illustrate the purpose and findings of our research in a manner accessible to lay people.