Rooting out answers to growth

The work of an Irish scientist reveals how growing plant roots are encouraged to find their way around obstacles and spread into empty spaces.

AN IRISH SCIENTIST based in Norwich has helped explain how growing plant roots find their way around obstacles. A feedback system drives the process, encouraging fine roots to grow into empty space while ceasing growth after encountering something impenetrable.

Prof Liam Dolan is a graduate of University College Dublin and the University of Pennsylvania, now based at the John Innes Centre (JIC) in Norwich. The JIC is Britain's leading plant research labs and one of the top centres of its kind in Europe.

The work focuses on the fine, hair-like roots sent out by a plant as it grows. The root hairs are important because they extract phosphate from the soil, an essential substance for plant growth.

But how does the plant know where to send its roots, what is the mechanism that controls this and helps roots grow away from obstacles? Prof Dolan was lead author on a paper looking at these questions, published at the end of February in the journal, Science, which explained work done in collaboration with Japanese scientists.

"The cells grow like a fungus grows, they spread out into the soil. We discovered a positive feedback mechanism that explains how root hairs grow around obstacles. All of the action takes place at the tip of the cells," explains Dolan.

The process is dependent on a protein called RHD2. Dolan's group discovered it some years ago by studying a mutant plant that lacked the protein and couldn't grow these very fine roots. They cloned the gene to identify the protein.

"Once we identified the protein we tagged it with green fluorescent protein so we could follow it around the cell," Dolan says. This made it possible to see where in these fine roots the protein was active.

The team also discovered what the protein was doing, finding it had a similar role in both the plant and in the human version. It produces free radicals, reactive oxygen that in humans helps the immune system kill off bacteria. In plants, the reactive oxygen helps it to capture calcium from the soil.

This is the key to controlling the direction of growth, explains Dolan. The calcium stimulates RHD2 expression, which in turn increases calcium uptake, initiating a positive feedback so long as calcium is available.

"Growth will initiate stochastically at the tip and the feedback mechanism helps the hair cells to grow in the right direction."

If the hair cells come up against something solid, calcium will quickly be depleted and a lack of calcium inhibits the action of RHD2.

Growth stops at that point but not everywhere, says Dolan. Active hair cells encountering calcium will send out the stimulus for more growth in that direction and the process continues, at least for those parts of the root system.

The fluorescent images clearly show the RHD2 congregating at the very tip of the fine roots, regardless of where the protein is actually manufactured within the plant.

The findings are significant because of the way plants draw nutrients from the soil. "Of the main nutrients plants need, nitrogen and potassium can be sucked up with water. Phosphate is different because it sticks to the soil particles," Dolan explains.

Studies have shown that plants with long hair cells are much better at drawing phosphate from the soil than those with short ones. Dolan likens the process to drawing energy from the ground as oil, as opposed to coal.

"That is exactly what the root hairs have to do - they mine for phosphates," he says.

Understanding the mechanism may make it possible to develop plants that have longer root hairs. This would deliver two benefits. They may be able to survive and grow better in soils deficient in phosphate.

Equally important, it could allow reductions in the use of phosphate fertilizer, something which, when over-used, has caused eutrophication of water courses.

"Phosphate is mined and is a non-renewable resource and is becoming more and more expensive," says Dolan.