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of water loss (Johnson, 1975). Trichomes in different plants vary dramatically in
form, size and density, and may be unicellular or multicellular, secretory glandular
or non-glandular hairs (Uphof, 1962; Esau, 1977; Theobald et al. , 1979). The mech-
anisms underlying their development are best understood in the genetic model plant
Arabidopsis and most of the data discussed in this review will focus on trichomes
in Arabidopsis .
9.3.1 Trichome differentiation
Leaf trichomes in Arabidopsis are unicellular and branched cells that cover the
surface of rosette leaves. Ecological studies have shown that they play a role in
the defence against insect herbivores (Mauricio & Rausher, 1997). Arabidopsis tri-
chomes develop from single protodermal cells. When they become recognizable as
trichomes, they are separated from each other by an average of about three proto-
dermal cells. On a given leaf the most mature trichomes are found at the tip, and
concomitant with further leaf growth new trichomes are initiated at the leaf base
(Larkin et al. , 1996). The first morphological change of an incipient trichome cell
is an increased nuclear size, which indicates that trichomes switch from mitotic cell
divisions to endoreduplication cycles (Hulskamp et al. , 1994). The surrounding epi-
dermal pavement cells continue to divide, thereby increasing the distance between
the already formed trichomes. The trichome cell undergoes on average four en-
doreduplication cycles, resulting in a mature trichome with a DNA content of about
32C (Melaragno et al. , 1993; Hulskamp et al. , 1994). The increase in nuclear size
is accompanied by an enormous growth. Initially the cell expands out of the plane
of the epidermis. Then two successive branching events occur with a stereotypical
arrangement of the branches (Hulskamp et al. , 1994; Folkers et al. , 1997). The ma-
ture trichome reaches a size of up to 0.5 mm and is separated from the neighbouring
trichomes by about 0.5-1 mm.
9.3.2
Why is a mechanism postulated to explain the trichome spacing
pattern and what kind of underlying principles are operating?
In principle, the distribution of trichomes could simply be random and, in this
case, no mechanism would be necessary to control the spatial arrangement. This,
however, is not the case. If the observed distribution of trichomes were random, one
would expect that trichomes would be initiated next to each other with a certain
probability that depends on their density. A careful statistical analysis of trichome
distribution revealed that clustered trichomes are much less frequently observed
than expected for a random distribution (Larkin et al. , 1996). Thus, some kind of
patterning mechanism must exist that controls trichome spacing. As no correlation
between the arrangement of underlying cells and trichome pattern was found, the
pattern is expected to be created de novo .Inprinciple, two types of mechanisms
could account for the observed distribution pattern:
1. A cell lineage mechanism by which a stereotypical series of divisions
segregates different cell fates;
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