A problem for plants
There can be no doubt that seedlings know that their roots have to grow down and their stems upward. This awareness of gravity seems to be maintained even in older plants, for if a young tree was lying flat on the ground (perhaps as a result of a storm), but with its roots still anchored in the soil, its tip would slowly bend upward in the months to come. Animals possess gravity receptors, statocysts, ear-stones; if they lack them they use their eyes and perceive the light from above, but plants? Where are their gravity sensors and where are their “eyes”?
Well, the latter are replaced by their green leaves, which one might say function like eyes as they capture and absorb the light that reaches them. But why do plants that germinate in pitch dark caves also grow upward for a few weeks (until they wither and die) when there isn’t even any light at all? And why, did animals evolve to use retinal as part of their visual pigment rhodopsin and plants use chlorophyll to absorb light? That is still a hotly debated question, which, incidentally, hasn’t become any easier with discoveries that some bacteria contain bacteriorhodopsin, some Chlorophyceae (green algae) possess rhodopsin and the mesopelagic deep sea dragonfish Malacosteus niger enhances its sensitivity to the red part of the spectrum by using derivatives of chlorophyll just like it has recently been reported for some flatworms of the genus Dugesia in the journal Scientific Reports by Italian researchers.
The question of gravity receptors in plants seems solved (from what I remember of the Botany class I attended years ago as a young student): plants contain amyloplasts (little disc-like particles made of starch) in their roots’ apical cells. The ‘heavy’ starch particles rest at the bottom of the cells of the root tip and act like statoliths (earstones), providing the plant with information on gravity and causing the plant roots to exhibit positive and the stem negative geotropism. But plants have no brain and it is thought that plant hormones like ‘auxins’ in combination with the cells’ cytoskeletal elements like actin trigger cell growth and proliferation in the desired direction. And that’s something which really puzzled me when I walked past some stone walls on my way to or from work on Hachijojima in Japan.
Growing on the walls were flowering plants, which always grew downward: shoots, leaves, branches, all with positive geotropism, while the part of the plant anchoring it with its roots was located further above! Plants such as these cannot be ‘trained’ to grow upward, but they possess cytoskeletons and auxin hormones. Similarly puzzling to me was an observation I made diving over seagrass flats in Indonesian waters: gravity in water is strongly reduced and if these grasses (or other aquatic plants like those in my aquarium) also contained starchy amyloplasts, their ‘weight’ would create an incredibly weak force to detect gravity with.
So, do seagrasses have foregone gravity perception with starchy ‘statoliths’ in the form of amyloplasts in their root tips and (in common with invertebrates like waterfleas) use a “dorsal light response” to let them know the directions of up and down? I think this could be an interesting topic and one could try to germinate sea grasses on a sieve suspended in an aquarium illuminated only from below. My working hypothesis would be that the response to light would override any information from possible gravity receptors and the plants would grow in the opposite to the normal direction with stems exhibiting positive geotropism. Of course, one doesn’t necessarily need to have seagrass for this experiment: anyone willing to try this with aquarium plants or even peas or bean sprouts illuminated only from below, just give it a go.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
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