Telling the time: what makes the plant clock tick?

A section of our new Understanding Plants area demonstrates how plants tell the time and why...
The circadian beds in the Understanding Plants areaThe circadian beds in the Understanding Plants area
New this year in the Understanding Plants area, are two curved beds bordered by trellising, positioned to the south side of Bateson Walk. Growing in one, are plants with flowers that open in the morning, such as Eschscholzia californica ‘Alba’ and in the other, flowers which open in the evening, or give their scent out at night like Nicotiana sylvestris.

Both these beds showcase Professor Alex Webb’s research group’s work on plant circadian rhythm. Plants have an internal 24 hour clock, and like us they use this clock to tell what time of day it is.

Professor Webb and his colleagues are investigating just what makes the plant clock tick. We may be awake in the day, asleep at night and get hungry before meal times, but why should plants need to know what time it is? As with animals, telling the time is a basic survival mechanism. Plants need to understand their environment and know what time it is in order to carry out essential functions at the most suitable time - like when to grow, flower, set seed and lose leaves.

Scientists in Professor Webb’s lab have discovered that sugars made during photosynthesis, together with light levels, and temperature, trigger plant clock genes. Three genes make three proteins bearing the same name: CCA1, LHY and TOC1 (after the ticking of a clock). These keep the plant clock ticking to a 24 hour rhythm.

Circadian clock genes act as timekeepers, making proteins that act like cogs, turning the plant clock hands. At sunrise, light kick-starts photosynthesis to make sugars and encourages two proteins CCA1 and LHY to be made. As the day progresses, these proteins are destroyed, allowing another protein - TOC1 to be produced. At dusk TOC1 levels are highest and the sugars made during the day are stored and used by the plant to survive overnight. The TOC1 protein is destroyed before sunrise and as the sugar levels drop below a certain threshold, the plant clock is reset each morning.

Plants don’t just react to the planet’s day-night cycle, they anticipate it. The amount of sugar in the plant cells provides feedback – so that the plant clock can be reset every morning on the anticipation of dawn. Plants keep to the 24 hour clock and adjust it as sunrise alters with the changing seasons.

So why is the plant clock so important to us? As plants are our primary food source we rely on this to organise our harvests and gain the most from our crops. Understanding these rhythmic activities might enable scientist to adjust plant circadian clocks. This will help farmers grow plants in regions where day lengths are different from their native habitats and will also allow them to stagger their harvests.

As the Botanic Garden is closed at dawn and dusk, it is not possible to see many of the flowers in action. However, we have captured time-lapse film of some of the flowers in our beds opening and closing at dawn and dusk and visa-versa. This can be viewed on the Cambridge University Museum's blogpost by clicking the link to the left-hand side.

Keep an eye on the squashes in the cucurbitaceae bed in the Systematic Beds and the Schools Garden. They don’t just appear to have grown overnight, they really have grown while the Garden has been shut. They like all plants, have been using up stored sugars to grow during the hours of darkness.

To read more about how and why the Understanding Plants area was made - click on the link to the left.

(This article originally appeared in the University of Cambridge Museum's blogpost.)
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