Plants have Body Clocks too!

06 July 2008

Interview with

Dr Harriet McWatters, Oxford University

Chris - With us now is Dr Harriet McWatters, she's from the department of plant sciences at Oxford University.  It will probably amaze a lot of people that plants also have a body clock in the sense of time.

A sunflowerHarriet - Yes, plants have a body clock which is set up in some ways very much like yours or mine because they have the same problem that you or I do.  They live on a planet with a 24hr day with day and night.  They need to know what time it is so they can coordinate aspects of their daily life such as petal opening with the rest of the world.  Russell has already mentioned bees.  Bees fertilise flowers so a flower which had its petals shut when a bee's visiting will not be pollinated.  It will not reproduce.

Chris - Do the plants do this in the same way as the brain cells Russell was talking about or do they use a totally different sort of clock machinery?

Harriet - The answer to that is both.  Each plant cell contains the machinery for a clock very much in the same way as a clock Russell described in animals.  There is a feedback loop where a protein eventually causes the switching off of its own production.  The actual genes and proteins involved in plants are completely different from the ones involved in animals clocks.

Chris - We all evolved ultimately from some common ancestor: us and plants.  We share quite a lot of our genes.  I share 60% of my DNA with a banana and I'm not that exceptional.  Why do plants have their own genes and we have different ones?

Harriet - The simple answer to that is we don't know.  We do know that it appears that clocks are extremely important in that almost every multi-cellular organism we've looked at has a clock.  In the four systems that we know best which are the plant, the animal, the fungal clock and also the blue-green algae.  The actual mechanisms are completely different meaning that clocks are important enough to have evolved four times in different lineages.

Chris - It must give a massive advantage.

Harriet - It does because it allows you to predict what is going to happen later on.  You're not caught by surprise every time the sun goes up.  If you are a day-active animal you can wake up and start to hunt before the day arrives.  If you are something which is going to be eaten by something which has then come out you can get back to your hiding place.  You can anticipate these events rather than always having to respond to them.

Chris - Plants also have a season because they have to do everything they need to do over a scope of a year which is to make leaves, they need to grow, make some blossom, make fruit - effectively have sex to make fruit, reproduce.  How does a plant coordinate time through the whole plant?

Harriet - Again there are the clocks in the leaves which appear to be detecting and measuring photoperiod.  Some signal is transferred from the leaf to the part of the plant which is going to produce a flower and that tells it hang on, it's going to stop producing these and start producing reproductive tissues such as flowers.  This is happening in an individual plant and it's also happening in a population of plants.  The clock in a plant leaf is detecting photoperiod in the world and then transmitting the information through the plant to the reproductive axis.

Chris - Does this mean then that if we understand how that works we could exploit it to say, make plants think they've got less time to grow then they have?  Then the put more effort into growing quicker so we get more yield from a crop, for example?

Harriet - Yes, potentially.  One of the big goals in agriculture is to reduce energy expenditure.  Removing the photoperiodic from plant growth is actually very important.  For example, with chrysanthemum which are extremely sensitive to photoperiod and also need to be produced at a particular time of year: in November and then again at Christmas time.  You need to produce plants using huge amounts of energy.  If you could reduce the photoperiodic component of a response they would grow regardless of the daylight if you gave them the right conditions.

Chris - If I take a plant which is well adapted to growing in one part of the world it's used to having a certain length of day, say the equator, because time doesn't really change whether it's winter or summer there and I put it in Scotland where it's going to have very long bright summers and very dark winters would that make a difference?

Harriet - It would make a difference.  It might make less of a difference than if you went that way from Scotland down to the equator.  Tropical plants are used to growing with very equitable day lengths.  Scottish plants require very much more light in the summer time.  In my garden at home I have a horse chestnut seedling which grew from a conker I collected in my mother-in-law's garden in Scotland.  This poor tree sets leaf several weeks later than the local horse chestnuts in Oxford and sheds its leaves early in the season.

Chris - It can't adapt?  One thing Russell was saying is that our brains can retune our body clock.  You've done the equivalent of a transatlantic flight for a conker but from Scotland down south.  The conker can't reset, why?

Harriet - It doesn't seem to be able to reset so it's got an idea in its little cells what a critical day length is for producing leaves or shedding its leaves.  It can't in one generation get over this.  The timing of leaf set is set in the genes and it's local to a particular population.

Chris - Scientists have shown that if you give a plant a really hard life for a while then the progeny it produces express genes differently than if you give it a good life.  It's almost like it's patterning, it's telling its offspring look, you're gonna have a hard life - you need to be optimised to grow in a bad environment.  Do you think the parent wrote this [into its genes]?

Harriet - I think the parent wrote this in.  We don't know form a single example whether it is maternal imprinting such as you are suggesting or a simple change in the genetic instruction such that you need sixteen hours of night or fourteen hours of light to produce your seeds.  It could be either.

Chris - President Bush has suggested he wants to see people colonising Mars in the next 50 years.  We know that plants are the best way to get energy from the sun and turn it into chemical energy that we can use.  That means we need to take plants on space missions.  Given what you've just said - what are the implications for taking plants from Earth, into space and then also farther afield?

Harriet - There's two things to consider.  One is the different arrangement of wavelengths that you get on Mars.  Earth plants are adapted to a range of wavelengths that we have here and are very different to a Martian atmosphere.  This is one of the reasons Mars looks red.  Also the day length is different on Mars so you would need to look into the effect upon day length of growing your plants.

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