OpenFlowers

OpenFlowers

By Nicolas Silva, Clara Haas, Margaux Bieuville and Amélie Bouissou

The main goal of the biosensor weeks is to compare biological and electronical sensors. To do so we decided to use a phototransistor as an electronic sensor for light. For the biological sensor, our first idea was to study the opening of flowers depending on the intensity of luminosity. The flower with the most amazing movement reacting to light is the nympheas as you can see here: https://www.youtube.com/watch?v=dem8ZDXycR4. The Oxalis triangularis has also impressive leaf movement with variation of light: https://www.youtube.com/watch?v=7mSBTkKqqOU.

Sadly, we are in January so finding plants with flowers sensitive to different luminosities is not really easy.  

We therefore decided to change the scale of our experiment and study the movement of chloroplasts in cells at different light intensities. Chloroplasts are little compartments in plant cells that allow photosynthesis to happen, therefore they are sensitive to different types and intensities of light. When there is little or no intense light, the chloroplasts migrate to the surface of the cell to capture the most light in order to do photosynthesis. On the contrary when there is too much light the chloroplasts migrate against the side cell walls to avoid photodamage. You can read more about this phenomenon in this Nature article: “Chloroplast avoidance movement reduces photodamage in plants” http://www.nature.com/nature/journal/v420/n6917/full/nature01213.html 

We compared the reactions of a biological sensor in the form of chloroplasts and an electronic sensor, a phototransistor, in different light intensities.  We put leaf cells on a microscope slide and put them for 20 minutes under either maximum light, medium intensity light or dark.  Simultaneously, we were measuring this light intensity with the phototransistor, an electronic component connected to an Arduino board then to a computer.  We then observed the leaf cells under the microscope.  Since chloroplasts contain chlorophyll, a fluorescent pigment, we were able to distinguish the chloroplasts thanks to fluorescence microscopy.  We took pictures and counted how many chloroplasts were touching the side cell walls.  You can see our experimental protocol on the following image:

Visual protocol of our experiment and data analysis

 

    Our results were very interesting since they allowed us to evaluate the efficiency and precision of both sensors.  In the dark, 4 chloroplasts on average touch the side membranes whereas in medium intensity light approximately 8 chloroplasts are touching and in maximum light, 11 chloroplasts were touching on average.  Similarly to what we expected, we observed that the more light the leaf cells were exposed to, the more chloroplasts touched the side cell membranes, as if to avoid being in the center of the cell.  This matches the previous research done on the topic as you can read in the Nature article.  The phototransistor, the electric component, returned non-linear values, meaning that the value that it measured did not correspond to the intensity sent.  For the intensity measured in the dark or in full light, the values were pretty accurate but for medium intensities, the value measured was not proportional to the current sent.

    Through these two experiments, we learned that though chloroplast layout is a good indicator of light intensity, it is not very precise.  From one cell to another, the number of chloroplasts can vary and their response to light will not be identical.  The response time, meaning the time from initial light exposition to the reading of the result, is also very long since we have to prepare the leaf cell sample, expose it for 20 minutes, observe the cells on the microscope then count the number of chloroplasts.  On the other hand, the phototransistor is connected to a computer, and the measured value can be seen almost instantly.  However, the phototransistor is not fully reliable since there were several problems.

Plant cell magnification x63 with mRFP filter after 20 minutes exposition with maximum intensity light

References:

Kasahara, Masahiro et al. "Chloroplast Avoidance Movement Reduces Photodamage in Plants." Nature.com. Letters to Nature, 19 Dec. 2002. Web. 19 Jan. 2016.

Islam, Sayeedul, and Shingo Takagi. "." NCBI. Plant Signaling and Behavior, 5 Feb. 2010. Web. 19 Jan. 2016. Co-localization of Mitochondria with Chloroplasts Is a Light-dependent Reversible Response

Chloroplast Photorelocation Movement: A Sophisticated Strategy for Chloroplasts to Perform Efficient Photosynthesis, Noriyuki Suetsugu et al. (2012)

The Noun Project: https://thenounproject.com/