The twilight zone: the initiation of starch degradation in leaves

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We will investigate the changes in leaf primary metabolism that occur at the boundary between day and night. Plants undergo a huge metabolic shift at this boundary, 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 onset of darkness. 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 established 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. Finally, we will compare the productivity and fitness of plants grown in square-wave and more natural light regimes.

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