December 4, 2022

Microscopic view of the mitochondria in a root tip of Arabidopsis thaliana. The interior of the mitochondria (matrix) is marked by a fluorescent protein. Credit: University of Münster

Calcium is a very special nutrient. In the cells of most living things, calcium ions act as so-called second messengers to transmit important signals. The same applies equally to animal, plant and fungal cells. Through collaboration of several research institutes at national and international level, members of the “Plant Energy Biology” working group at the University of Münster, led by Prof. Markus Schwarzländer, and from the team led by Prof. Alex Costa at 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 an important role in their response to touch. The study is now published in the journal The plant cell.

How do the calcium ions get into the mitochondria?

“It’s amazing that such a simple ion can be so important for transmitting information,” says Markus Schwarzländer. “We assume that the calcium ions develop this potential through the exact place and time of their deployment.” It has been known since 1965 that mitochondria in plants can absorb calcium ions and in this way – presumably – can be involved in calcium signaling pathways. How exactly the transport is made possible, however, has been disputed for decades. The inner membrane of the mitochondria is impermeable to most ions, but certain proteins in the membrane can allow the calcium ions to pass through this partially permeable membrane and transmit signals in this cell organelle.

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In the case of animals, the question of the identity of the mitochondrial calcium channel was resolved in 2011 when researchers from Harvard and Padua Universities discovered the calcium channel MCU (mitochondrial calcium uniporter). This breakthrough paved the way for the discovery that plants also contain MCU genes. What was still unclear, however, was whether these genes also form calcium channels in the living cell, not least because the uptake of calcium ions in animal mitochondria shows markedly different patterns than in plant mitochondria.

Gene expression reveals the importance of calcium ion transport for cellular powerhouses

To clarify the role of 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 limited the capacity of the cellular machinery and thus, for the first time in a living plant, could observe the consequences to which this limitation leads. For this they used a fluorescent protein that indicates changes in the concentration of calcium ions in the mitochondria in the form of a light signal.

How Calcium Ions Get into Plants' Cellular Power Plants

Microscopic image of mitochondria (green) and chloroplasts (red) in mesophyll cells in a leaf of Arabidopsis thaliana. The interior of the mitochondria (matrix) is characterized by a fluorescent protein; in the chloroplasts the chlorophyll fluoresces. Credit: University of Münster

What could be seen was that because the MCU genes were deactivated, much less calcium ions got into the mitochondria. This means that the researchers not only showed that living plant cells – just like animal cells – transport their calcium ions to the mitochondria via the MCU channels. “We were also able to show,” says Markus Schwarzländer, “that this is by far the most important pathway for rapidly transporting calcium ions into the mitochondria. It means that we now have the ability to force signal transmission by calcium ions to the cellular level.” channels and, if possible, affect the encoded information.”

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After this groundbreaking observation, the team used plants with reduced mitochondrial calcium transport capacity to find out what role mitochondrial calcium plays for the plant and its fitness. In the case of animals, calcium ions in the mitochondria regulate energy production, but there was no evidence of a similar function in plants.

By analyzing the expression of the entire plant genome, the researchers were now able to show that the reduced transport capacity for calcium ions influences the regulation of the plant hormone jasmonic acid. Jasmonic acid is a defense hormone in plants that protects against herbivores by being activated when the plant is injured. Among other things, jasmonic acid also regulates aging – ie the regulated death of tissues – and also the reactions to mechanical stimuli such as touch.

The plants manipulated by the researchers showed a slightly delayed aging: in a dark environment the leaves lost their green pigmentation less quickly. They also showed a markedly weaker response to touch. “What is particularly surprising to us,” says Schwarzländer, “is that there is clearly a link between the transport of calcium ions to the mitochondria and the regulatory process driven by the jasmonic acid. The results show that molecular processes such as the uptake of calcium ions in the mitochondria, which have been conserved through evolution in animals and plants, can be used to perform new functions.”

Targeted reprogramming of mitochondrial calcium transport seems an interesting avenue, as controlling the response to touch could be useful, for example in agriculture, where plants are often planted close together.

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Studies with synthetic biosensors

One of the central methods used in the now published study was “in vivo biosensors.” Here, proteins are designed – using molecular biological and biotechnological methods – in such a way that they serve as synthetic measuring sensors in living organisms. When plants are genetically transformed, they themselves produce a sensor that provides live information about the status of cells in living plants. In addition, these biological sensors can be used for measurement purposes in specific parts of the cell. This is achieved by genetically placing them in a particular compartment of the cell. Doing this with traditional methods is difficult because such methods usually break the cell, resulting in the loss of all organization in the cell.

Control of a mitochondrial protection mechanism identified

More information:
Cristina Ruberti et al, MCU proteins dominate mitochondrial Ca . in vivo2+ inclusion in Arabidopsis roots, The plant cell (2022). DOI: 10.1093/plcell/koac242

Provided by the University of Münster

Quote: How Calcium Ions Get into Plant Cellular Power Stations (2022, Aug 8), retrieved Aug 8, 2022 from

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