A tale of two Marys, a coenocyte and the blending of techniques

 
Two Marys and Virginia with their poster at ASB.
Two Marys and Virginia with their poster at ASB.

Imagine a cell, green and glistening, and as big as a hen’s egg. Imagine a cell that can regenerate new cells from a fragment of its own cytoplasm. Imagine a cell that can control its internal pressure as finely as the operator of a hot air balloon. If you think this is science fiction, think again. These cells have the unflattering name Ventricaria ventricosa but they’ve been around since the days of the dinosaurs, and they must be doing something right. Members of the Plant Membrane Biophysics Group, leader Mary Beilby, postdoc Virginia Shepherd, PhD student Chris Cherry-Gaedt, Visiting Professor Alan Walker and fascinated vacation students, have been wondering about this organism for quite some time now. How is it put together? How can it regulate its internal pressure? How does this relate to the electrical properties of its membranes and to the transport of ions into and out of each green gleaming orb?

conductance-voltage profile of Ventricaria cell
Figure 1 The conductance-voltage profile of Ventricaria cell with the internal pressure clamped at 0.05 MPa (trace 1a) and 0.1 MPa (trace 2a), in light (1b) and dark (2b) and without (1c) and with 10 M DCMU (2c)

The highlight of the year was the visit by Professor Mary Bisson of the State University of New York at Buffalo, who came to work with Mary Beilby on the electrophysiology of Ventricaria. She stayed for a fruitful month of experiments, field trips and talking about science. Mary Bisson obtained a travel grant from NSF (National Science Foundation) and she arrived in the last week of second session armed with her pressure probe. The pressure probe allows us to insert a silicone oil-filled microelectrode into Ventricaria, and measure the cell’s internal pressure with a pressure sensor. What’s more, the cell’s internal pressure can be controlled and changed with a micrometer screw that changes the internal probe volume. The pressure probe electrodes have to be quite large (up to 40 microns) otherwise they block. The two Marys were eager to find out if the cells can survive impalement with 3 electrodes; the PD (potential difference) measuring microelectrode, current-injecting electrode and the pressure probe electrode.

Our group worked very hard to incorporate the pressure probe into the existing experimental set up. Visiting Prof. Alan Walker organised on-line data-logging of the pressure probe output and the cell resting PD. Controlling the cell’s internal pressure allowed us to distinguish the pressure effect on membrane transporters from the effect of changing salinity. We also looked at the effect of light and darkness and metabolic inhibitors. The experiments were very successful. Particularly exciting are the findings that decreasing the cell’s internal pressure increased its electrical conductance by activating the potassium pump. The conductance was diminished by exposure to darkness and photosynthesis inhibitor DCMU, suggesting that photosynthesis-generated ATP powers the pump (see Figure 1 above).

Meanwhile postdoc Virginia Shepherd found that the nuclei of Ventricaria are not randomly arranged in space. Instead, they maximise their spacing in whole cells, and become clumped when fragments of cytoplasm are in the process of regenerating new cells. This led to the idea that the cytoplasm of Ventricaria is organised into many structured domains each containing a nucleus that can each regenerate a new organism. Towards the end of Mary Bisson’s visit the group attended the 26th Meeting of the Australian Society for Biophysics (ASB), where they presented posters, and finished a paper (submitted to the journal Protoplasma) about the curious cytoplasmic structure and the electrophysiology of one of the world’s oldest and weirdest organisms.

Mary Beilby and Virginia Shepherd

 


 

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