1880 - The Darwin Experiments
Most famous for his seminal book, On the Origin of Species, Charles Darwin's work on botany was just as important. Darwin and his son Francis wrote a whole book observations regarding how plants respond to stimuli, The Power of Movements in Plants. They thought that plants could grow differentially and therefore in the direction of the light
The Darwin experiments, used oat coleoptiles. The experiment modified the growing conditions of these coleoptiles so that the response could be observed:
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| Some of the many experimental modifications to test for a reaction to light stimulus. |
B - The coleoptile with tip cut off: No response to light.
C - The coleoptile with tip covered with opaque cover: No response to light.
D - The coleoptile with tip covered with transparent cover: A bend towards the light can be seen.
E - The coleoptile with base covered with opaque cover. A bend towards the light can be seen.
They demonstrated the following:
- Not only did they find that the oat coleoptiles do bend towards a light stimulus, but that it was the tip of the plant which is active in initiating this response.
- This also showed that while it was the tip that perceived the directional light, the reaction happened further down the stem.
However, it wasn't until the following century that scientists were able to find out what this influence was.
1920s and 1930s - The Cholodny-Went Model
The experiments of Nicolai Cholodny, who worked with grass roots, and Fritz Went who worked with grass coleoptiles, progressed the knowledge provided by the Darwin experiments. Just as Darwin and Wallace discovered Natural Selection independently, Cholodny and Went independently made the same hypothesis which gave a name to Darwin's 'influence'.
They found that an asymmetric accumulation of auxin occurs in the stem as a response to the unidirectional light.
This means that there is differential growth in the plant stem. This happens when the transport of the plant growth regulator auxin accumulates on the side of the stem furthest from the light source - making those cells grow more rapidly. The auxin is transported from the side of the stem that is closest to, or receiving, the light - whose cells grow more slowly. With one side of the stem growing faster than the other, a bend is created that is directed towards the light source.
2013 - Research by Technische Universitaet Muenchen (TUM) and Université de Lausanne (UNIL)
The Cholodny-Went model, while popular was not the only hypothesis. It was also noted that plants with known defects in auxin transport still responded perfectly well to unidirectional light stimulus. This story has only this year seemed to reach a conclusion to this important, yet seemingly simple, question.
New research has confirmed that the Cholodny-Went model is correct. Scientists from UNIL inactivated several PIN transporters, important for proper cellular coordination, while scientists from TUM demonstrated the function of the D6 protein kinase, important for the regulation of PIN-mediated auxin transport.
They found that without these transporters and kinase components the plant was unresponsive to light signals that would have previously triggered phototropism.
Therefore we now know that Darwin's 'influence' - the auxin discovered by Cholodny and Went - is definitely the substance that the plant uses to mobilise the bending towards the light in response to blue light.
To read more about this new research, click to visit Science Daily or the original press release from TUM.
References
Willige B.C., Ahlers S., Zourelidou M., Barbosa I.C.R., Demarsy E., Trevisan M., Davis P.A., Roelfsema M.R.G., Hangarter R. & Fankhauser C. & (2013). D6PK AGCVIII kinases are required for auxin transport and phototropic hypocotyl bending in Arabidopsis., The Plant cell, PMID: 23709629
This is very interesting. It takes me back to my A level Botany days :o)
ReplyDeleteThanks for commenting. If only A levels on Botany still existed - I'd definitely sign up! :)
DeleteGreat Blog, I remember learning at about this at Burnley (Melbourne's best horticultural college). But I also just learnt something more, didn't know that it was the tip that initiates this.
ReplyDeletePlease put up more good blogs like this.
Hi Sandi, thanks for your comment. Really glad that you liked reading my post. You might like this previous post I wrote: http://notesofnature.blogspot.co.uk/2012/03/gibberellins-and-changes-in-ivy.html
Deletethe historical work by Darwin and Went et al is still on the AQA a-level biology syllabus. Some of us still uphold the flag for Botany. Come to think of it IAA and its effects are on the OCR 21st Century science GCSE syllabus.
ReplyDeleteThat's interesting to know. It's a shame that Botany isn't a stand alone subject anymore. I remember an RHS article last year lamenting the end of Botany degrees.
DeleteVery interesting! One of my buddleia is doing exactly that, bending away from a larger shrub towards the light. I was feeling irritated with the poor plant, but now I feel I should be more patient.
ReplyDeleteI know what you mean! We tend to expect the plant to act how we want it to, rather than what it is programmed to do - but it always tend to come good in the end. Thanks for your comment :)
DeleteAbsolutely fascinating ! A process that can be observed in many plants, and it always amazes me how much Darwin contributed to our understanding of nature beyond the Origin of Species
ReplyDeleteI've just started reading a book about how Darwin used his garden for experiments!
DeleteI knew that he once sectioned off some of his lawn to see which plant species would prosper without being tended or mown - so looking forward to finding out what else he did.
Thanks for commenting Tom.