Phycomyces blakesleeanus’ everyday fight:
Gravitropism VS Phototropism
By Lara Narbona, Elena Calamand, Tanguy Chotel
Phycomyces blakesleeanus is a fungus (mushroom) sensible to light, gravity and way more things like touch and wind. For example, when it senses a nearby object, P. blakesleeanus will change its growth direction as well as its speed in order to avoid it.
In this project, we decided to focus on its reaction to light and gravity, also known as phototropism and gravitropism. Phototropism can be defined as the ability of an organism to develop towards light while gravitropism as its ability to develop according to gravity. In the case of P. blakesleeanus, it does positive phototropism and negative gravitropism, meaning that it develops towards light but against gravity.
The exact purpose of the project was to find a threshold from which phototropism would have more impact to Phycomyces than gravitropism. So, from which intensity of red light, Phycomyces would mostly grow towards the light than against gravity. We chose red light because we found that blue enhances geotropism.
But why ? why would this fungus be attracted to light and not grow in the opposite direction of gravity ? Well from what we found and read, the "sporangiophores", the sexual parts of this organism needs to leave the ground and reach the surface to spread it's spores. In order to reach the surface, it must follow the light AND go in the opposite direction of gravity. This seems to be the most plausible explanation that we found for this mechanism.
But why ? why would this fungus be attracted to light and not grow in the opposite direction of gravity ? Well from what we found and read, the "sporangiophores", the sexual parts of this organism needs to leave the ground and reach the surface to spread it's spores. In order to reach the surface, it must follow the light AND go in the opposite direction of gravity. This seems to be the most plausible explanation that we found for this mechanism.
Protocol
To test it, we made our fungi grow on different petri dishes, boxes commonly used in microbiology to grow organisms in controlled environments. We attached them to the wall and put different intensities of LED above them: gravity would make them grow up and LEDs grow down. Different intensities were 20%, 40%, 60% and 80% of total LED intensity. To be sure that they were isolated from other lights except from the LEDs, we put opaque tape all around petri dishes and LEDs and pierced it to let oxygen go inside.
To be sure that light and gravity actually had an impact on the growth of our organisms, we did what we call a ‘control’: one for the maximal exposition to the condition and another for the minimal. And you can wonder: on what does that help? It actually gives us two extreme organism responses: the one with no light (we expect no-response) and the one with 100% light (we expect the organism being completely attracted by it). From this two responses we will know if our organism reacts to the condition we are changing (what we expect will happen) or not (both controls will grow similarly). To do the controls of gravity, we put the plates vertically on the wall (we expect the fungi to grow up) and horizontally on the table (we expect them not to have a priority of growing up or down).
Results
After 2 days, we took pictures of the growth of our organism. Because we taped almost-hermetically the petri dishes, we feared that our organisms wouldn’t have enough oxygen to grow properly. Also, we didn’t let the organisms grow for enough time to develop their reproductive parts (what is actually sensible to gravity and light).
Defying our expectations we were glad to discover that many of the boxes contained beautifully grown fungi, as you can see:
To compare how many of the fungus had grown towards the light and how many against gravity, we drew a parallel-to-the-floor line. We supposed that what had grown under the line would have been attracted by light and what had grown over the line would have been attracted by gravity.
From this, we decided to do two different approaches to analyse the growing of our fungus:
First, we chose to compare the area under and the one over the line: this would give us an idea of which factor of attraction would have been the strongest according to light intensity. Unfortunately for us, the organisms did not grow as planned:
We cannot see any clear tendency on this graph: the areas were quite similar on the two sides of the plate (even for the controls!).
Therefore, after seeing that our first approach might not have been the best, we thought about comparing densities of development at each part of the line. We wanted to see if the density was higher when exposed to light rather than gravity. Again, we had quite a surprise :
The density of development in both zones, above and below the line seems to be the same! It looks like the development was made without taking in account the influence of either light or gravity.
Bias
How can we explain such results ? Well, obviously we had a lot of bias during our experiment: the lack of oxygen, the fact that some light could have gone through the tape, and more importantly the fact that Phycomyces blakesleeanus would only develop its reproductive part (sensor for light and gravity) after 5 days… And our experiment only lasted 2 days.
Conclusion
As a conclusion, we can say that despite a lack of conclusive results, we analyzed precisely the parts of the experiment that could have gone wrong and are pretty sure that if we were to redo the experiment, we could be almost certain that results would be as described in the literature !
Bibliography
- « Agar papa dextrosa ». Wikipedia, la enciclopedia libre, 3 janvier 2017. https://es.wikipedia.org/w/index.php?title=Agar_papa_dextrosa&oldid=95995361.
- Barlow, P. W. « An introduction to gravity perception in plants and fungi — A multiplicity of mechanisms ». Advances in Space Research, Life and Gravity: Physiological and Morphological Responses, 17, no 6–7 (1996): 69‑72. doi:10.1016/0273-1177(95)00613-J.
- Grolig, Franz, Peter Eibel, Christine Schimek, Tanja Schapat, David S. Dennison, et Paul A. Galland. « Interaction between Gravitropism and Phototropism in Sporangiophores of Phycomyces Blakesleeanus ». Plant Physiology 123, no 2 (6 janvier 2000): 765‑76. doi:10.1104/pp.123.2.765.
- Corrochano, Luis M. « Sensory perception in the fungus Phycomyces blakesleeanus: a model organism for space research? », Vol. 41, 2016. http://adsabs.harvard.edu/abs/2016cosp...41E.376C.
- Dennison, David S. « The Effect of Light on the Geotropic Responses of Phycomyces Sporangiophores ». The Journal of General Physiology 47, no 4 (1 mars 1964): 651‑65. doi:10.1085/jgp.47.4.651.
- Galland, P. « The Sporangiophore of Phycomyces Blakesleeanus: A Tool to Investigate Fungal Gravireception and Graviresponses ». Plant Biology (Stuttgart, Germany) 16 Suppl 1 (janvier 2014): 58‑68. doi:10.1111/plb.12108.
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