A cytoskeletal basis for wood formation in angiosperm trees: the involvement of cortical microtubules |
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Authors: | Nigel Chaffey John Barnett Peter Barlow |
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Institution: | (1) Department of Botany, Plant Science Laboratories, The University of Reading, Whiteknights PO Box 221, Reading RG6 6AS, UK, GB;(2) IACR – Long Ashton Research Station, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol BS41 9AF, UK, GB |
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Abstract: | Rearrangements of cortical microtubules (CMTs) during the differentiation of axial secondary xylem elements within taproots
and shoots of Aesculus hippocastanum L. (horse-chestnut) are described. A correlative approach was employed using indirect immunofluorescence microscopy of α-tubulin
in 6- to 10-μm sections and transmission electron microscopy of ultrathin sections. All cell types – fibres, vessel elements
and axial parenchyma – derive from fusiform cambial cells which contain randomly oriented CMTs. At the early stages of development,
fibres and axial parenchyma cells possess helically arranged CMTs, which increase in number as secondary wall thickening proceeds
and simple pits develop. In contrast, incipient vessel elements are distinguished by the marking out of sites of bordered
pits; these sites first appear as microtubule-free regions within the reticulum of randomly oriented CMTs that characterises
their precursor fusiform cambial cells. Subsequently, the ring of CMTs which develops at the periphery of the microtubule-free
region decreases in diameter as the over-arching pit border is formed. Like bordered pits, large-diameter, non-bordered pits
(contact pits) which develop between vessel elements and adjacent contact ray cells originate as microtubule-free regions
and are also associated with development of a ring of CMTs at the periphery. In the case of contact pits, however, there is
no reduction in the diameter of the CMT ring during pit development. Tertiary cell wall thickenings are also a feature of
vessel elements and appear to form at sites where bands of laterally associated, transversely oriented CMTs, separated from
each other by microtubule-free zones, are found. Later, these bands of CMTs become narrower, and separate into pairs of microtubule
bundles located on each side of the developing wall thickening. Development of perforations between vessel elements is also
associated with the presence of a ring of CMTs at their periphery.
Received: 13 July 1998 / Accepted: 30 November 1998 |
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Keywords: | :Aesculus (wood formation) Cytodifferentiation Microtubule Secondary vascular tissue Xylem |
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