The ion pump is shown in use on the root system. Credit: Thor Balkhed

Drug delivery ion pumps, created from organic electronic components and traditionally limited to use in mammalian systems, have now been used to regulate plant physiology.

The idea first took root during a brainstorming session in 2008, when researchers were given the challenge to come up with the “craziest combination of technologies and applications,” Daniel Simon, Ph.D., associate professor from the Laboratory of Organic Electronics at Linköping University, told LabOutlook

Several years later, the team was lucky enough to get in touch with Ove Nilsson, director of the Umeå Plant Science Centre at Umeå University in Sweden, who connected Simon with Professor Markus Grebe. Together the teams submitted and were awarded a grant from the Knut and Alice Wallenberg Foundation to see if they could use organic electronic ion pumps (OEIPs) to investigate plant growth.

How OEIPs work

An OEIP, is essentially the same as a salt-bridge in classic electrochemistry, Simon explained —an electric field pushes charged molecules (ions). The difference is that unlike standard salt-bridges, or standard gel electrophoresis equipment, the ion pump is loaded with a charged gel polymer.

In the new work, described in the Proceedings of the National Academy of Sciences, the gel polymer “transport channel” was composed of positively charged materials.  This blocks the transport of positive ions, but allows negative ions to pass through, so the ionic current through the channel is only in one direction, Simon explained.

“We can therefore draw a direct relationship between this ionic current (dosage rate) and the electronic current we use to drive the device,” he said.

The transport with these devices is solely charged molecules or ions, not liquid or water. So, when the ions reach the end of the transport channel, or the delivery point, they diffuse into the target system in a very similar way to how cells release substances into their environment, be it neurotransmitter release, or plant hormone signaling.

“We can thereby deliver very precise amounts of charged molecules without disrupting fragile liquid microenvironments,” Simon said.

An unmet technological need

Studies investing the effects of hormones in plants have traditionally been conducted via exogenous application, the authors noted.

Some commonly used methods include spraying or soaking the plant, and applying gels, paraffin or polymer beads that have been soaked in known concentrations of compound or have been allowed to absorb compounds from the plants themselves.  Nanoscale functional systems have also been used for directed introduction of materials and molecules within plant cells and tissues.  However, these methods suffer from poor dynamic control or can introduce undesirable stresses on cells and tissues, the authors wrote.

Due to this there is an unmet technological need, as many long-standing questions in plant biology remain unanswered because of a lack of technology that precisely delivers plant hormones to regulate plant physiology.

The new study hopes to provide a starting point for more localized control of plant physiology.

David Poxson is working with the ion pump and the electronic roses, the Laboratory of Organic Electronics, Linköping University. Credit: Thor Balkhed

Observing auxin

The plant hormone auxin controls cell shape and growth, and is arguably the most important of all plant hormones, Simon said.  Specifically it controls cell elongation and the formation of root hairs.

“Deciphering auxin’s molecular and cellular modes of action is of fundamental importance for the elucidation of plant biology,” the authors wrote.

So the team, including David Poxson, assistant professor at the Laboratory of Organic Electronics, modified the ion pump to make it capable of transporting and delivering signaling compounds such as auxin.  Simon said that only minor modifications in the geometry of the delivery tip, so that it fit the specific plant setup, was needed.  The major difference was that the plant hormone required a new tailor-made ion transport material in the channel.  So the team fabricated OEIP devices based on a synthesized dendritic polyelectrolyte that enabled electrophoretic transport of the substances.

The scientists then worked closely to investigate highly-resolved delivery of auxin to the roots of a small flowering plant, the living thale cress, Arabidopsis thaliana, which is a major model organism for plant scientists.  The main question posed by Simon’s colleagues at the Umeå Plant Science Centre was how auxin gradients control and modulate this root hair development.

The response to auxin was observed in three ways, Simon said: (i) delivery caused the roots to significantly slow their growth (cease cell elongation); (ii) auxin uptake was visualized by a transgenic fluorescent-reporting plant; (iii) auxin gradients across the root (from delivery side to the other side) was visualized by another transgenic fluorescent-reporting plant.

So the fluorescent reporter proteins changed their intensity in the presence of auxin which helped researchers observe the response.

The results were promising, as the team showed electronically-controlled gradients of the plant hormone were taken up by the roots.

“We have accomplished a ground-breaking step for plant research by our multidisciplinary effort,” Grebe said in a statement. “The pump will likely allow us to locally apply not only auxin, but also a variety of other hormones to plants in an electronically controlled manner. This will help us to study the impact of these hormones on plant growth and development at tissue and cellular resolutions.”

A starting point

The researchers hope their recent work paves the way for further discoveries in plant science. 

“These new Dendrolyte materials also paves the way for future ion pump capabilities in a variety of areas, for example delivery of larger aromatic compounds like plant hormones or even certain pharmaceuticals,” Simon said.

Up next Simon said they plan to use OEIP technology to modulate a variety of plants’ physiological functions.  Some of the questions he’s investigating include: What do millisecond-scale dynamics do to plants? Can we tailor cell development with customized hormone gradients?

The recent advance is important not only because now scientists know they can use the ion pump in plants, but also that they can regulate their physiology and growth.

 “Our results provide a starting point for technologies enabling direct, rapid, and dynamic electronic interaction with the biochemical regulation systems of plants,” the authors wrote.

Simon noted that the without the willingness of Nilsson, Grebe and the Wallenberg Foundation to “work with ‘crazy engineers’, none” of the project would have been possible.