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Mutant Studies on Root Hair Function |
Root hair cells are specialized
epidermal cells that greatly increase the surface area of roots. As
such, they are widely believed to play an important role in plant
nutrition by facilitating the absorption of water and nutrients. This
is a reasonable working hypothesis but one, like all hypotheses, that
needs to be tested and refined. Indeed, this proposed function of root
hairs raises a host of questions. For example, are the rates of
absorption of all nutrients raised in parallel by the increase in root
surface area resulting from the presence of root hairs? Do
root hairs function in processes other than absorption (e.g. in
anchorage)? If root hairs are such a valuable adaptation for
absorption, why doesn't every root epidermal cell differentiate into a
root hair cell, and why do some plants, such as many conifers, live
quite comfortably without them? This month's The Hot and the
Classic examines the role of root hair mutants, especially
those of Arabidopsis, in addressing fundamental questions concerning
root hair function.
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Root Hairs Are Dispensable under Some Conditions |
Wen and Schnable (1994)
isolated several maize (Zea
mays) mutants with abnormal root hair morphologies. One of
these mutants (rth3) initiated root hair primordia that
failed to elongate, but the plants grew vigorously nevertheless. This
finding suggests that under some environmental conditions, root hairs
are dispensable for plant growth.
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Root Hairs Confer a Competitive Advantage under Low P
Availability |
Arabidopsis root hairs grow longer and denser in response to
low P availability (Bates and Lynch, 1996; Ma et al., 2001b). Bates and
Lynch (2000a) tested the hypothesis that wild-type (WT) Arabidopsis
plants acquire more P under P-limiting conditions than do mutants
without root hairs. The growth and P acquisition of WT Arabidopsis were
compared with two root hair mutants (rhd6 and
rhd2) under different P treatments. At the lowest P
treatment, all plants were small and showed severe P stress
symptoms. Under medium P conditions, WT plants had greater shoot
biomass, absolute growth rate, total P, and specific P absorption than
did the two root hair mutants. At the highest P treatment, there was no
difference between genotypes in any of the parameters measured. The
authors concluded that the response of increased root hair growth under low P availability in Arabidopsis is important in increasing P acquisition under P-limiting conditions.
Bates and Lynch (2000b) compared the efficiency of root hairs in P
acquisition at high and low P availability. Root hair growth, root
growth, root respiration, plant P uptake, and the plant P content of WT
Arabidopsis were compared with two root hair mutants (rhd6 and rhd2) under high and low P
availability. A cost-benefit analysis was performed based on these
measurements. Under high P availability, root hairs did not have an
effect on any of the parameters measured. Under low-P availability,
however, WT Arabidopsis had greater total root surface area, shoot
biomass, P per root length, and specific P uptake. The cost-benefit
analysis shows that under low P conditions, WT roots acquire more P for
every unit of carbon respired than do the mutants. The authors conclude that the response of root hairs to low P availability is an efficient strategy for P acquisition.
More recently, Bates and Lynch (2001) used a root-hairless mutant of
Arabidopsis to assess the effect of root hairs on plant competition
under contrasting P regimes. WT plants were grown with hairless plants.
At high P availability, WT and mutant plants were equal in growth, P
acquisition, fecundity, and relative crowding coefficient (RCC). At low
P availability, however, hairless plants accumulated less biomass and
P, and they produced less seed when planted with WT plants. WT plants
were unaffected by the presence of hairless plants in mixed genotype
plantings. WT plants had RCC values greater than 1 while hairless
plants had RCC values less than 1. These results confirm that root
hairs increase the competitiveness of plants under low P availability
but do not reduce growth or competitiveness under high P availability.
The finding that root hairs confer a great advantage to plants growing
under low P conditions has been confirmed using a root-hair-deficient barley (Hordeum vulgare) mutant. The mutant bald
root barley (brb) absorbed only one-half as much
P than did WT in rhizosphere studies (Gahoonia et al., 2001). The acid
phosphatase activity near the roots of WT was higher and WT
mobilized more organic P in the rhizosphere than did the mutant. Hence,
root hairs are important for increasing plant uptake of inorganic P as
well as for increasing the mobilization of organic P in soils.
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No Role in Anchorage |
Bailey et al. (2002) examined the roles played by lateral
roots and root hairs in promoting plant anchorage, specifically, resistance to vertical uprooting forces. Two species were studied, onion (Allium cepa) that has a particularly simple root
system and two mutants of Arabidopsis, one without root hairs
(rhd 2-1) and another with reduced lateral root
branching (axr 4-2). In uprooting tests on onion
seedlings, resistance to uprooting could be resolved into a series of
events associated with the breakage of individual roots. Peak pulling
resistance was explained in a regression model by a combination of a
measure of plant size and the extent to which the uprooting resistance
of individual roots was additive. This additive effect is termed root
co-operation. In similar uprooting tests on Arabidopsis, the mutant
axr 4-2, with very restricted lateral development,
showed a 14% reduction in peak pulling resistance when compared with
the WT plants of similar shoot dry weight. The uprooting force trace of
axr 4-2 was different from that of the WT, and the main
axis was a more significant contributor to anchorage than in the WT. By
contrast, the root-hair-deficient mutant rhd 2-1 showed
no difference in peak pulling resistance compared with the WT,
suggesting that root hairs, unlike lateral roots, do not normally play
a role in resisting uprooting.
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Root Hairs Offer Resistance to Oxygen Transfer |
Shiao and Doran (2000) examined the effect of root hairiness
on fluid flow and O2 transfer in hairy root cultures using
WT, transgenic, and root hair mutants of Arabidopsis. The root hair morphologies of the Arabidopsis lines were hairless, short hairs, moderately hairy (WT), and excessively hairy. Filtration experiments were used to determine the permeability of packed beds of roots; permeability declined significantly with increasing root hairiness. The
moderately hairy roots of WT Arabidopsis grew fastest with a doubling
time of 6.9 d, but the hairless roots exhibited the highest
specific O2 uptake rate. In experiments using a
gradientless packed bed reactor with medium recirculation, the liquid
velocity required to eliminate external mass transfer boundary layer
effects increased with increasing root hairiness, reflecting the
greater tendency toward liquid stagnation near the surface of roots
covered with hairs. External critical O2 tensions also
increased with increasing root hairiness, ranging from 50% air
saturation for hairless roots to circa 150% air saturation for roots
with excessive root hairs. These results are consistent with root hairs
providing a significant additional resistance to O2
transfer to the roots, indicating that very hairy roots are more likely
than hairless roots to become O2 limited in culture.
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No Role in Fe Uptake |
In some plant species including Arabidopsis, Fe deficiency
induces alterations in root physiology and morphology. Moog et al.
(1995) grew Arabidopsis WT and a root hair-less mutant
(rm57) on Fe-containing and Fe-deficient nutrient solutions.
In both genotypes, ferric chelate reductase (FCR) of intact roots was induced upon Fe deficiency. The approximately 4-fold stimulation of FCR
activity was independent of the formation of root hairs (see also
Schmidt et al., 2000).
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No Role in Si Uptake |
Unlike Arabidopsis, many cultivars of rice (Oryza
sativa) are Si accumulators. Little is known about the
mechanism responsible for the high uptake of Si by rice roots. Ma et
al. (2001a) investigated the role of root hairs and lateral roots in
the Si uptake using two mutants of rice, one defective in the formation
of root hairs (rh2) and the other defective in the formation
of lateral roots (rm109). Both short-term (up to 12 h)
and relatively long-term (26 d) uptake experiments showed that there
was no significant difference in Si uptake between rh2 and
the WT, whereas the Si uptake of rm109 was much less than
that of WT. Si uptake in the mature zone (1-4 cm from root tip) was
significantly lower in rm109 than in WT, whereas no
difference was found in Si uptake between WT and rh2. These
results indicate that lateral roots contribute to the Si uptake in rice
plant, whereas root hairs do not.
TOP
Mutant Studies on Root...
Root Hairs Are Dispensable...
the turbo iron reductase does not depend on the formation of root hairs and transfer cells.
Planta
195: 505-513