Garden Tool Set

Phototropism - or how plants lean towards the light

Many plants have a tendency to lean towards the light. Until Charles Darwin and his son performed what is now a famous experiment in botany, sparking detailed investigations into how plants grow towards the light, spanning three centuries.

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:

Some of the many experimental modifications to test for a reaction to light stimulus.

A - The coleoptile with no modifications: A bend towards the light can be seen.
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:

1. 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. 
2. This also showed that while it was the tip that perceived the directional light, the reaction happened further down the stem. 

This led Darwin to posit that there was an 'influence' that moved from the site of perception to the site of reaction. 

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