Root
Hairs
Root hairs are projections from surface cells of the root. They have
been proposed to play critical roles in water and nutrient uptake
and anchoring the plant in the soil.
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Root hair development exhibits two fascinating aspects of the regulation
of positional information during plant cell growth. First on initiation
the surface cell that will give rise to the root hair must break symmetry
and form a bulge in its side wall. Once the bulge is formed the subsequent
growth of the hair is confined to the tip leading to a long thin cell.
Quicktime
Time-lapse movie of root hairs growing under the microscope.
The
final length of each hair is approximately 0.1 mm. The actual elapsed
time is about 20 minutes. Click on the image to download the movie.
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to view this move.
The high degree of spatial resolution with which this growth gradient
is maintained has suggested that there is a gradient of some factor
or signal present at the tip that orients the growth process. We are
currently investigating what this signal may be. Our and others work
is revealing gradients in cytoplasmic calcium levels as an important
element in this tip signal.
We have
used Arabidopsis seedlings loaded with a fluorescent Ca2+-sensitive
indicator to directly image Ca2+ in developing, living, functioning
root hairs. With the increased quantitative and spatial resolution
offered by uv-confocal ratio-imaging we
have been able to observe a gradient in calcium in the tips of growing
root hairs that is absent in non-growing root hairs or root hairs
of mutant plants which are blocked in hair elongation.
Root hairs
are also extremely sensitive monitors of environmental stress. Here
is shown a root hair that bursts at the tip a few minutes after the
plant is stressed. The upper panels show normal microscope pictures
of the bursting (before and after) the lower panels show the dramatic
increase in cytoplasmic calcium associated with the root hair death.
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