Plant tropisms

Gravitropism

Most plant organs are maintained at a particular angle with respect to gravity due to gravitropism. Gravitropism is one of the fundamental unique physiological processes of plants and despite intensive study for 150 years it is very poorly understood. At York, we have been studying various aspects of gravitropism, challenging dogmas and trying to build a more robust conceptual framework.
Some dogmas that we have challenged are:

• that shoots grow up and roots grow down. Although very young shoots and very young roots nearly always grow vertically up or down, most mature organs do not do so. Just look at a tree. Look at the branching of a root. Look at this Tradescantia. The very young nodes are growing up and the oldest nodes are growing down. So the same organ has changed its gravitropism such that when young it grew upwards if placed horizontally yet when older it grew downwards if placed horizontally. We explain this behaviour by proposing that plant organs possess the ability to orient themselves at any angle with respect to gravity. In the organs of very young seedlings the organs are showing extreme behaviour which does not account for all possibilities. Good models of gravitropism must be able to account for all gravitropic behaviour not just the extremes. More infomation?
• we have argued that the traditional hormonal control model for shoot gravitropism as defined by Went & Thimann is an inadequate explanation of gravitropism. There can be no argument but that the model is wrong in its original form:
• it has been shown by many workers (including us), at intervals over the last century, that transorgan movement of a substance cannot be important for gravitropism because semicylinders show a good gravitropic response
• it has also been shown by many workers (including us), again at intervals over the last century, that the tip of a coleoptile or hypocotyl plays no direct role in gravitropism - the coleoptile or hypcotyl can show a good gravitropic response even after the tip has been removed.

Phototopism

Some plant organs, when subjected to a light gradient, will show a growth response such that the orientation of the organ changes relative to that light gradient - phototropism. We have studied many aspects of phototropism at York, challenging existing dogmas by experiment and analysis. We try to ask simple, fundamental questions about the processes involved.

Some of the dogmas we have challenged are:

• that phototropism is the response to unilateral light and that plants bend towards the strongest light. In fact unilateral light is rarely encountered in the natural world and cannot have acted as a focus for the evolution of phototropism. It can be shown that phototropism occurs even when seedlings are exposed to opposing, unequal bilateral illumination. Indeed phototropism can occur to a midpoint between two equal light sources illuminating two flanks 90 degrees apart - in this case the plant does not bend towards either light but it vectors in response to the two light sources.
• we have argued that the traditional hormonal control model for shoot phototropism as defined by Went & Thimann is an inadequate explanation of phototropism. There can be no argument but that the model is wrong in its original form::
• it has also been shown by many workers (including us), again at intervals over the last century, that the tip of a coleoptile or hypocotyl plays no direct role in phototropism in response to the types of illumination likely to be received in the natural world - the coleoptile or hypcotyl can show a good gravitropic response even after the tip has been removed.

Autotropism

Plant organs that show differential flank elongation as part of the process of becoming curved during gravitropism or phototropism often show a reversed phase of differential flank elongation to straighten the organ again. This straightening has been called autotropism and we rediscovered the process about 20 years ago. To be honest we don't understand what is going on but neither does anyone else - indeed many people just ignore it for that reason.
If you want to see autotropism in action have a look at this time-lapse video of a sunflower hypocotyl showing gravitropism and then autotropism. There are marks on the hypocotyl to allow you to watch what happens at each point along the growing zone. If you watch the upper regions, near the cotyledons, you will see that they bend at first but about half way through the sequence they start to straighten again. That is autotropic straightening.

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