Heart cells beat together as the heart pumps. Single-celled amoebae
pulsate as they move. But pulsing plant cells? That's precisely what certain
plant cells do as they grow, says a study publishing online this week in the
Proceedings of the National Academy of Sciences.
Using sophisticated video imaging techniques, a team led by University
of Wisconsin-Madison botany professor Simon Gilroy captured the growth of
tiny extensions of individual root cells, called root hairs. Blanketing the
surfaces of roots by the millions, these long, skinny projections hugely
expand the surface area for water and mineral uptake from the soil, but how
they form has remained something of a mystery.

When the scientists trained their cameras on the hairs, rather than
witnessing a slow, steady lengthening, they were surprised to see the hairs
undergoing rhythmic pulses of growth every 20 seconds, orchestrated by
rapid-fire changes in pH and levels of reactive oxygen species (ROS).

The findings show that plants are hardly the slow or even static
beings we normally take them for, says Gilroy. Instead, they're responding
to cues and regulating their growth on timescales of seconds to
milliseconds - a realm usually associated only with animal responses.

"This is one of the first real-time views of plant growth," says
Gilroy, who recently came to UW-Madison from Penn State University. "And
what everyone notices is that plants are actually much more dynamic than
you'd think by looking at them just sitting there, soaking up the sun."

What makes the feat even more remarkable is that, unlike animal cells,
enlarging plant cells must contend with a cell boundary made of cellulose
that is, pound for pound, stronger than steel. Known as the cell wall, the
reinforcement is needed because the insides of plant cells are under
tremendous water pressure, or turgor, Gilroy explains.

In the case of root hairs - which grow from their tips - this means
the cell must somehow make the wall just flexible enough at the tip so that
it stretches from that internal force without bursting. At the same time,
the cell must quickly strengthen the wall directly behind the tip once
growth has taken place.

"The plant cell has to work out where to make the wall stiff, where to
make it loose, and control this so finely that the turgor pressure doesn't
absolutely shoot the end off the cell and kill it," says Gilroy. "It's
always been a conundrum: How on earth does a plant cell do this?"

Based on previous work, Gilroy's team hypothesized that a drop in pH
at the tip might trigger growth, since acidity is known to loosen the
cellulose matrix and let the cell wall stretch. Their first idea was that
the plant cell carefully doles out just the right number of protons - which
control pH - to create a gradient of acidity that results in steady
elongation.

What they saw instead was much more lively. When protons flow into the
wall, the wall stretches and the tip lengthens as expected. But rather than
letting this continue, the plant cell sucks the protons back out again
almost immediately, causing the pH to rise and the cellulose strands to lock
in place once more After a brief pause, another influx of protons loosens
the wall again and the tip extends a little more.

Why the stops and starts? Because the root hair is at such risk of
weakening the wall too much and bursting, Gilroy thinks it takes a quick
breather to check on things.

"There's a lot of regulation invested in slowing things down and
waiting to see if the cell is okay, before going on," says Gilroy. Because
the effects of pH are reversible, the wall is easily strengthened or
loosened again if a mistake is made.

That's only half the story, though. As growth proceeds, the portion of
the wall once located at the growing tip moves further back, and the cell
needs to make it permanently rigid to prevent further stretching. This is
where the highly reactive form of oxygen, ROS, comes in, says Gilroy. In
addition to the falling and rising of pH, his team has documented a burst of
ROS just behind the tip during each growth pulse. There, ROS cross-links the
wall, rendering it permanently stiff.

"Everything's coordinated. It's like a dance," says Gilroy. What's
more, the entire complicated ballet occurs up to three times a minute.

"If you pick up your potted plant, every 20 seconds probably 10
million of these events are going on in the root system, provided the plant
has a big one," says Gilroy.

The same goes for the roots that snake everywhere underground. It
gives you a whole new perspective on what's going on under your feet.

[www.wisconsin.edu]