Carbon nanotubes (CNTs) have already been explored as drug carriers into mammalian cells. Compared to nanoparticles, CNTs have a larger inner volume which allows more drug molecules to be encapsulated, and this volume is more easily accessible because the end caps can be easily removed, and they have distinct inner and outer surfaces for functionalization (see Nanotechnology's 'magic bullet' for more details).

In addition to nanomedicine applications, plant science research focusing on investigation of plant genomics and gene function as well as improvement of crop species has become a nanotechnology frontier.

To what degree nanotechnology materials can be employed in delivering payloads into plant cells is a subject that has not yet been explored very well although there appears to be demand from plant cell biologists to take advantage of nanomaterials. Plant cells differ from animal cells in several aspects, a major one being that, in addition to the cell membrane, they possess a wall surrounding them to provide mechanical and structural support. The plant cell wall is generally made up of polysaccharides and cellulose, which provides a stiff and rigid environment for the cell to live in, but also means that there is an extra barrier to overcome with regard to delivering molecules into the plant cell. For example, many intracellular imaging reagents (e.g. calcium dye) which are widely used in mammalian cells can not be applied to intact plant cells.

In a new research report, scientists in China have investigated the capability of single-walled carbon nanotubes (SWCNTs) to penetrate the cell wall and cell membrane of intact plant cells.

Overlay of confocal fluorescence image and DIC (differential interference contrast) optical image of the cells incubated with SWCNT/fluorescein isothiocyanate. (Image: Dr. Fang, Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS)


"In the study of mammalian cells, carbon nanotubes have shown their ability to easily traverse across biological barriers such as cell membranes, and even blood-brain barrier, with little cytotoxicity," Xiaohong Fang tells Nanowerk. "Especially SWCNTs hold great promise as nanovectors to deliver various molecules into living mammalian cells. In our recent work, we investigated whether carbon nanotubes can pass through the plant cell wall and be used as molecular transporters for plant cells."

Fang is a professor of chemistry at the Key Laboratory of Molecular Nanostructure and Nanotechnology of the Chinese Academy of Sciences in Beijing. She points out that polystyrene nanoparticles and quantum dots are the very few nanomaterials which have been reported to be internalized into plant cells.

"No delivery was attempted with these nanoparticles" says Fang. "Polystyrene nanoparticles were used for protoplasts, where the cell wall is enzymatically removed together with certain cell surface proteins. Quantum dot uptake was achieved only after starving the cells for 24 hours. Moreover, quantum dots are generally not used as delivery vehicles due to their toxicity. Recently silica nanoparticles have been used successfully for molecule delivery into both protoplasts and walled cells (see: "Mesoporous silica nanoparticles deliver DNA and chemicals into pla...). However, when testing with intact plant cells, the silica nanoparticles need to be conjugated with gold nanoparticles and be shot by gene gun into the cells."

In their work, Fang and her colleagues present the first evidence that nanomaterials can be internalized into intact walled cells without external assistant and pretreatment. In addition, they demonstrate that SWCNTs conjugated with either small dye molecules or DNA can been transported into cells, showing the potential of SWCNT as a new nanotransporter for plant cells. What is more, the team showed that it even is possible to deliver different cargo into different compartments in a plant cell via the SWCNTs.
The team reported their findings in a recent issue of Nano Letters ("Carbon Nanotubes as Molecular Transporters for Walled Plant Cells").

Compared to existing delivery methods for walled plant cells – such as gene gun, electroporation and microinjection – a nanoparticle based delivery strategy is advantageous due to its easy operation and wide applicability.

The Chinese team's study on the transportation of molecules into the walled plant cells by SWCNTs opens a new delivery approach for plant cells, for instance, DNA/RNA molecules may be delivered for genetic transformation or manipulation of plant cells. Delivery of intracellular imaging agents or other regulators may allow the real-time imaging or study of cellular processes which will lead to a better understanding of plant cell biology.

Fang says that this has been only a preliminary work on the application of SWCNT to plant cells. "Next, we want to know that the molecules delivered by SWCNTs are active and functional inside the cells," she says. "We are also interested in investigating the detailed mechanism of how SWCNTs were uptaken by the plant cells, and why they had different distributions for the two SWCNT conjugates that we used".

She points out that one of the challenges to this research is that, in contrast to animal cells, scientists' knowledge of the endocytosis mechanism for plant cells is very limited. "For many years people have been skeptical about the plant cell endocytosis in the presence of turgor pressure (the main pressure of the plant cell contents against the cell wall). Although general accepted now, and receiving increased research interest, the mechanism is far from understood as compared to that in mammalian cells."

By Michael Berger. Copyright 2009 Nanowerk LLC