Plant Cell Biology 2022
Stromules are protrusions formed by plastids under certain conditions. First discovered in the 19th century, they were later forgotten and "rediscoved" in the 1990ies. They were thought to form a network of plastids within the cell, but this was then disproven by the elegant experiments of Martin Schattat and Jaideep Mathur using photoswitchable fluorescent proteins. We want to understand, what stromules are doing and we can show that they are inducible by the jasmonates (plant stress hormones). Since the plastids are the main chemical factories of plants, change of their shape might lead different channelling of metabolic pathways. In other words: form rules matter. Can we test this idea experimentally. We use the terpenoid pathway as paradigm - it exists in two versions, using different precursors, in cytoplasm versus plastid. For these precursors, we have fluorescent derivatives, so that we can watch metabolism under the microscope. How does it change, when stromules are induced by methyl jasmonate? Scriptum with protocols . Bachelor thesis on stromules .
Mikrotubuli und Form
How where microtubules actually discovered?
Microtubules are generally known as building blocks of the division spindle. Less known is, however, that at first they were not recognised. Microtubules were discovered in plant cells - 1963 by Ledbetter and Porter. Curiously, they were predicted, one year earlier, by the biophysicist Paul Green (1962). He adopted a physical viewpoint upon the growth of plant cells and stated that the uptake of water into the vacuole should make plant cells to widen rather than to elongate. From this, he concluded that the cell wall must differ in their extensibility depending on direction, they should be stiffer in the transverse compared to the longitudinal axis of the cell. Therefore, the cell must be endowed with a mechanism to steer the direction of cellulose fibres in the cell wall. This mechanism, he reasoned, should depend on minute tubules ("micro-tubules"). These "micro-tubules " were then, in fact, discovered one year later using the emerging technology of electron microscopy. Only years later, it turned out that also the division spindle consists of microtubules.
Environment and Shape: Microtubules as Bridge
Since plants cannot run away, if they do not like their environment, they have only the way out to adapt. As part of this adaptation, they can change shape. How strong signals from the environment bear on plant shape can be seen, when potatoes are forgotten in the barn. These long and thin, so called etiolated, seedlings invest their entire energy into length growth. The biological purpose is of course to reach the light. Once arrived in the light, elongation is stopped immediately. Light is the signal that acts most rapidly on microtubules. They respond by changing their orientation from transverse into longitudinal. Since microtubules act as tracks for enzymes producing cellulose, the cellulose fibres will also be oriented longitudinal. This will constrain elongation, and the cell widens instead. In the dark, microtubules are transverse and reinforce etiolated growth.
What are we working on?
Microtubules, thus, are tools to change shape in response to the environment. Around three decades ago, we could show this for the first time, using phototropism as case study. A light-induced reorientation of microtubules was the cause for the bending of plants towards the light (Nick et al. 1990). Later, we investigated the responses of microtbubules to different envirnomental signals such as light, gravity, or mechanic stress and also addressed the question, to what extent these responses were mediated by plant hormones, and how they are integrated into the biology of the plant. Since plant form is of great agricultural impact, we are progressively interested to use our knowledge for the improvement of crop plants.