Dead pipes can still regulate plants’ water

In a new twist on zombie botany, Harvard University physiologists have found that the pipes in a plant’s water plumbing can regulate the flow speed, despite the disability of being dead.

Round spots (lower left), which contain a pectin hydrogel, change porosity when ion concentrations vary in the water flowing through. Zwieniecki

The stack of dead cells, called xylem vessel members, respond to what’s in the water whipping through, report Maciej A. Zwieniecki and his colleagues. When the cells carry high concentrations of calcium, potassium, or other ions, the researchers say, membranes between cells become more porous, speeding the stream.

Yes, even a dead cell can manage such bottleneck control, the researchers argue in the Jan. 26 Sciencexpress, an online compilation of articles to appear later in Science. They describe experiments suggesting the regulation comes from the properties of absorbent goo called a hydrogel.

Xylem, the core of plant stems, comprises living and dead cells. Drawn by evaporation from leaves, water whooshes up through the dead cells. The water keeps the whole plant’s chemistry humming.

The old view recognized only two states for the flow through these pipes. The water either streams or, when blocked by a bubble, stalls.

After recently reading about a 1978 lab mishap, Zwieniecki began to wonder whether the system is more complex. A physiologist had reported that flow rate jumped in a stem when he ran out of deionized water and substituted tap water.

To see if that quirk could prove useful, Zwieniecki monitored sections of stem from laurel plants. The higher the concentration of potassium chloride in the water flowing though the sections, the faster its flow, he found. Tissue in an intact plant showed a similar reaction. Xylem tissue is “more dynamic than we thought,” Zwieniecki says.

He then looked for the structure that carries out this regulation. After about 50 attempts, each lasting an hour, Zwieniecki succeeded in threading a fine tube into a single cell in a cut stem. The cell connected to two channels, but only one of them was separated from the original cell by a membrane. When Zwieniecki pumped in test solutions, just the membrane-divided branch showed a change in flow rate. The membrane, he decided, was the regulator.

While doing these experiments, Zwieniecki came across a new publication on hydrogels. These tangles of interconnected polymers in water swell when the concentration of dissolved ions in the surrounding water is low. In further tests of the plant membranes with various solutions, his team observed an abrupt flow change typical of hydrogel membranes.

This raises the possibility of great sophistication in plant plumbing, Zwieniecki says. A sun-drenched branch might trigger a rise in ion concentration, which in turn might widen membrane pores and trigger faster inflow of water to a biochemically active area.

“That is our vision,” Zwieniecki says, cautioning that it’s still speculative.

Another student of water transport and the editor of the American Journal of Botany, Karl J. Niklas of Cornell University, calls the new study “technically a very good paper.” Now, he wants to see more about how such a system would work in a plant.

“Water transport is a contentious topic,” Niklas volunteers, “probably because it’s so important.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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