Transport route of this important signal transmitter

Calcium is a very special nutrient. In the cells of most living things, calcium ions function as second messengers to transmit important signals. The same is true for animal, plant and fungal cells. Thanks to the collaboration of several national and worldwide research institutes, the members of the “Plant Power Biology” working group of the University of Münster, led by Prof. Markus Schwarzländer, and the team led by Prof. Alex Costa of the University of Milan have now identified the molecular machinery that allows calcium ions to be taken up into the mitochondria of plant cells – and that this form of transport plays a significant role in their response to touch. The study has now been published in the journal “The Plant Cell”.

“It is amazing that such an uncomplicated ion can be so crucial for the transmission of information”, says Markus Schwarzländer. “We assume that the calcium ions develop this potential through the exact location and timing of their deployment.” It has already been known since 1965 that plant mitochondria can capture calcium ions and in this way, presumably, be involved in calcium signaling pathways. Exactly how transportation is made attainable, however, has been disputed for decades. For most ions, the inner membrane of mitochondria is impermeable, but certain membrane proteins can ensure the passage of calcium ions through this partially permeable membrane and thus allow the transmission of signals in this cell organelle.

In the case of animals, the question of the identity of the mitochondrial calcium channel was resolved in 1965 when researchers from the universities of Harvard and Padua discovered the MCU (mitochondrial calcium uniporter) calcium channel. This breakthrough paved the way for the discovery that plants also contain MCU genes. What was not yet clear, however, was whether these genes also form calcium channels in the living cell, not least because calcium ion uptake in animal mitochondria shows markedly different patterns than in mitochondria. plants.

Gene expression reveals importance of calcium ion transport for cellular powerhouses

To clarify the role played by MCUs in plant cells, the Münster researchers had to simultaneously deactivate three of the six MCU genes in the model plant Arabidopsis thaliana. As a result, they restricted the capacity of the cellular machinery and were thus able, for the first time, to observe the consequences of this constraint in a living plant. For this, they used a fluorescent protein which indicates changes in calcium ion focus in the mitochondria in the form of a light signal. What we could see was that due to the deactivation of the MCU genes, a much lower number of calcium ions entered the mitochondria. This means that the researchers not only demonstrated that living plant cells – in the same way as animal cells – transport their calcium ions into the mitochondria by means of MCU channels. “We were also able,” explains Markus Schwarzländer, “to show that this is by far the most important pathway for rapidly transporting calcium ions into the mitochondria. This means that we now have the possibility to control the transmission of the signal by the calcium ions in the cellular power. stations and thus possibly influence the information coded.

After this pioneering observation, the team used plants with impaired mitochondrial calcium transport capacity to try to determine the role what mitochondrial calcium plays for the plant and its fitness. In the case of animals, calcium ions in mitochondria regulate energy production – but there was no indication of a similar function in plants. By analyzing the expression of the plant’s entire genome, the researchers were able to demonstrate that the reduced transport capacity of calcium ions has an impact on the regulation of the plant hormone jasmonic acid. Jasmonic acid is a plant defense hormone that provides protection against herbivores by activating if the plant is injured. Among other things, jasmonic acid also controls senescence – that is, regulated tissue death – as well as responses to mechanical stimuli such as touch. The plants manipulated by the researchers showed a slightly delayed senescence: in a dark environment, the leaves lose their green pigmentation less quickly. They also displayed a noticeably weaker response to touch. “What is particularly surprising to us,” says Schwarzländer, “is that there is obviously a link between the transport of calcium ions into the mitochondria and the regulatory process driven by jasmonic acid. The results show that molecular processes such as the uptake of calcium ions in mitochondria, which have been conserved in animals and plants during evolution, can be used to fulfill new functions.” Targeted reprogramming of mitochondrial calcium transport seems to be an interesting avenue, as controlling the response to touch could be useful, for example in agriculture, where plants are often planted closely together.

Surveys using synthetic biosensors

One of the main methods used in the study published today was “in vivo biosensitivity”. Here, proteins are engineered – using molecular biology and biotechnology methods – in such a way that they serve as synthetic measurement sensors in living organisms. When plants are genetically transformed, they themselves produce a sensor that provides live information about the state of living plant cells. Additionally, these biological sensors can be used for measurement purposes in specific areas of the cell. This is achieved by genetically placing them in a specific compartment of the cell. Doing this using traditional methods is difficult because in such methods the cell is usually broken resulting in the loss of all organization within the cell.

International collaboration leads to success

The work was made possible by a strategic pooling of international expertise. The working groups led by Markus Schwarzländer and Alex Costa constitute, with equal sections, the heart of the study. The lead author, Dr. Cristina Ruberti, first carried out research in Münster for two years before joining the Università degli Studi in Milan. The in-depth study of the reactions of plants to defective transport of calcium ions has been made possible thanks to complementary national and international collaborations. Partners included teams from Halle, Heidelberg, Bonn, Lund (Sweden), Antwerp (Belgium) and Viçosa (Brazil).

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