Biologists Find Cells’ Nuclear Surface More Complicated Than Expected

In an accidental discovery, however, researchers at North Carolina State University have found it’s not that simple, after all.

The researchers discovered that, far from being uniformly round spheres, plant cell nuclei can be deeply grooved and furrowed. They appear somewhat like human brains when viewed three-dimensionally.

The team, led by Dr. Nina Strömgren Allen, NC State professor of botany, and Dr. David Collings, a former postdoctoral fellow in her laboratory, has published its findings in the scientific journal, The Plant Cell.

Using green fluorescent protein (GFP) to highlight the structure of tobacco and onion skin cells under a confocal microscope, the researchers noticed deep groves on the surface of, and tunnels through, the cell nuclei. GFP is a new tool used by biologists to label cell organelles and molecules so that they can be imaged clearly at high resolution with a light microscope.

Although the scientists aren’t certain about the reason for the grooves and tunnels, they note that the structures greatly increase the nucleus’ surface area relative to the surrounding, jellylike cell cytoplasm.

That, they say, may assist in the communication of chemical messages and molecules between the nucleus and cell parts contained in the cytoplasm, because greater surface area may allow more molecules and chemical signals to pass quickly back and forth.

The nucleus contains the cell’s DNA, the genetic instruction manual that tells a cell how to operate and replicate itself. Chemical signals, molecules and ribosomes -- specialized cell particles that synthesize proteins -- continually move in and out of the cell nucleus.

The researchers also found that the tunnels and folds in the cell nucleus contain cytoplasm and bundles of actin filaments, strings of protein along which organelles were observed moving in the nuclear grooves.

"The implication of this discovery is that we need to look more closely at communications between the nucleus and the cytoplasm, and we need to understand why these grooves and tunnels are there," Allen said.

Although most plant cells the researchers examined contain nuclei with grooves and tunnels, Allen notes that not all do. It’s possible that only large cells, including plant surface cells, contain these structures.

She and her colleagues at NC State’s Cellular Molecular Imaging Facility, which she directs, made the unexpected discovery while working on another project: The examination of how the cells at the tip of plant roots are able to sense changes in the direction of gravity. As part of that project, they tested recently available GFP, originating from a jellyfish, to fluorescently label molecules and cellular structures under a confocal microscope, which can create three-dimensional images. As a test before using the GFP to look at the root cells, they used it on onion skill cells to highlight a tiny network called the endoplasmic reticulum, a highly folded membrane in the cell cytoplasm which has many functions, including protein creation. Using the GFP to highlight chemical sequences associated with the endoplasmic reticulum, the researchers found that deep folds and grooves indented the surface of the nucleus, and that the endoplasmic reticulum actually passed through the center of the nucleus.

"In science, the prepared mind sees things that one might not expect to see. That’s the fun part of science," Allen said.

Allen’s colleagues on the project were former post-doctoral fellows Collings, now at Sydney University in Australia, and Dr. Sarah Wyatt, now at Ohio University; former NC State graduate student Amie Scott; former NC State undergraduate Crystal Carter, now a medical student at East Carolina University; and Jochen Rink, a graduate student at the Max Planck Institute for Molecular Cell Biology and Genetics in Dresden, Germany.

The research was funded by the NASA Specialized Center of Research and Training at NC State University, and by the N.C. Agricultural Research Service at NC State.

As a result of their original project, the research team has published a paper in the journal Planta about the role of root tip cells in sensing gravity. They describe the discovery of a network of actin filaments in root tip cells, which may help a cell sense when the plant has been turned on its side. The researchers believe that when the plant is upended, the actin filaments may help communicate whether cell structures called amyloplasts have fallen toward the bottom of the cell, pulled there by gravity. Research on how gravity affects plant growth is important for agriculture and for future manned space flights, which will require plants to provide food, cleanse water, and turn carbon dioxide into breathable oxygen.