Annals of Botany 80 : 125–129, 1997
Fibres and Fibre-sclereids in Wild-type Arabidopsis thaliana
S I M C H A L E V-Y A D U N*
Department of Plant Genetics, The Weizmann Institute of Science, RehoŠot 76100, Israel
Received : 13 August 1996
Accepted : 6 February 1997
Examination of the fibre system in roots and shoots of Arabidopsis thaliana (L.) Heynh. revealed three types of fibres,
distinguished according to their site of differentiation : (1) short fibre-sclereids in the secondary phloem of roots and
shoots at the rosette level ; (2) long fibres in the secondary xylem of the main root ; and (3) very long fibres in the xylem
of the inflorescence stems. These three types are in addition to the small number of primary phloem fibres that are
formed even in small A. thaliana individuals. These findings provide a basis for the use of Arabidopsis thaliana as a
model system to study the processes leading to fibre differentiation in dicotyledons.
# 1997 Annals of Botany Company
Key words : Arabidopsis thaliana, differentiation, fibre-sclereids, inflorescences, phloem fibres, xylem.
INTRODUCTION
Sclerenchyma is a mechanical tissue composed of two cell
types : fibres and sclereids (Fahn, 1990). The fibres give
plants strength and elasticity while hard sclereids protect
soft tissues from herbivores or mechanical damage. The
mechanical properties of the fibre system are well demonstrated in reeds, bamboo stems and wheat or rice culms,
which are strong and flexible because of their fibre bands.
The best known examples of the protective nature of the
sclereids are nutshells, which are mostly composed of thick
layers of sclereids. Fibres and sclereids either form a
sclerenchyma tissue or participate in the formation of
complex tissues with other cell types. Fibre cells are long
and sclereids are short. However, this definition is not
sufficient as very long sclereids exist and relatively short
fibres can be found (Fahn, 1990). The most important
characteristic of sclerenchyma cells is their thick, secondary
and usually lignified cell wall. Both fibres and sclereids have
several sub-types differing in their nuclear number, shape,
structure, viability and origin (Esau, 1965 ; Fahn, 1990).
Some of the fibres that differentiate in the phloem may have
characteristics of both fibres and sclereids, and were
therefore named fibre-sclereids (Esau, 1969). Fibres may
form a network in the cortex, a phenomenon known from
woody plants (Roth, 1981). Fibres may reach a length of
more than 50 cm by symplastic growth that lasts for several
weeks or even months (Fahn, 1990). In dicotyledons,
formation of both primary and secondary phloem fibres is
a normal aspect of ontogeny (Esau, 1965 ; Fahn, 1990).
These fibres have two origins : (1) primary phloem fibres are
formed from the procambium ; and (2) secondary phloem
fibres from the cambium (Esau, 1969). Many authors have
misclassified primary phloem fibres of the shoot as orig* Present address : The Zinman Institute of Archaeology, University of Haifa, Haifa 31905, Israel.
0305-7364}97}070125­05 $25.00}0
inating from the pericycle, and this has led to confusion
concerning the origin and nature of the primary phloem
fibres (Esau, 1969). In many dicotyledons fibres are also
formed from the cambium as part of the secondary xylem
(Fahn, 1990). Fibres also differentiate from ground meristem
(Fahn, 1990). Phloem fibres are a common source of
commercial fibres and can be produced from several plant
species including Linum usitatissimum (flax), Cannabis satiŠa
(hemp), Corchorus capsularis (jute), Boehmeria niŠea (ramie)
and Hibiscus cannabinus (kenaf) (Hayward, 1938 ; Hill,
1952 ; Esau, 1969).
Differentiation of fibres and sclereids follows several
steps, some of which may not occur : cell divisions, nuclear
divisions, elongation, intrusive growth, formation of septa,
lignification and cell death. The first schedule for differentiation is when fibres and sclereids differentiate from a
meristem such as the procambium or the cambium. A
different schedule occurs when parenchyma cells change
their fate following wounding, aging or external hormonal
application and redifferentiate to fibres or sclereids. Redifferentiation of pith parenchyma cells into sclereids in A.
thaliana was shown (Lev-Yadun, 1994). Fibre cells may be
uni- or multi-nucleate. Fibres that are multinucleate have
probably performed karyogenesis after the determination of
their fate to fibres. Many fibres are dead when mature, but
in certain cases both phloem and xylem fibres are alive for
many years (Fahn, 1990).
Very little is known about the sclerenchymatous tissues of
A. thaliana. Sclereids were induced in the pith of large
A. thaliana plants by repeated removal of inflorescences
(Lev-Yadun, 1994). The existence outside the phloem of a
layer of thickened fibre cells derived from the pericycle of
the unthickened root was briefly described by Dolan et al.
(1993). However, in spite of the substantial documentation
of the development of A. thaliana (Bowman, 1994 ;
Meyerowitz and Somerville, 1994), no further information
on fibres in A. thaliana has been provided.
bo970419
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126
LeŠ-Yadun—Fibre Types in Arabidopsis thaliana
F. 1. Longitudinal view of a cleared, peeled cortex of the main root. Dark bands of fibre-sclereids (arrows) and light parenchyma cells are seen.
Bar ¯ 200 µm.
F. 2. Longitudinal view of a cleared, peeled cortex of the main root under polarized light. Light bands of fibre-sclereids (asterisks) with blunt
ends are seen. Bar ¯ 100 µm.
F. 3. Longitudinal view of a cleared peeled cortex of the main root. Dark bands (large arrows) and individual pointed fibre-sclereids (small
arrows) are seen. Bar ¯ 200 µm.
F. 4. Magnification of Fig. 3. Dark bands (large arrows) and individual pointed fibre-sclereids (small arrows) are seen. Bar ¯ 100 µm.
LeŠ-Yadun—Fibre Types in Arabidopsis thaliana
127
The object of the present study was to characterize the
occurrence of fibres in the secondary xylem and phloem of
roots and stems of A. thaliana.
MATERIALS AND METHODS
Plant material
Seeds of Arabidopsis thaliana ecotypes C24 and Columbia
were germinated in a glasshouse. One-week-old rosettes
were transferred to 3-l pots. The development of larger
rosettes of A. thaliana was induced by repeated removal of
inflorescences (Lev-Yadun, 1994). The main root, the stem
at the rosette level and the largest inflorescence stems of five
Columbia ecotype plants were fixed in freshly prepared 3 %
paraformaldehyde and 2 % glutaraldehyde overnight at
room temperature, washed three times for 15 min in PBS
pH 7±2, dehydrated in a series of ethanol solutions (25, 50,
75, 96 and 100 %) and embedded in Spurr’s resin. Cross
sections (2 µm) were obtained using a Nova LKB ultratome
equipped with glass knives, and were stained with safranin
and fast-green. The main root and the stems at the rosette
level of ten mature plants of the C24 ecotype, as well as ten
inflorescence stems, were smashed using a metal cylinder.
The soft bark was then separated from the xylem of the
roots and the xylem was separated from the bark of the
inflorescence stems and cleared by overnight incubation in
95 % ethanol at room temperature. It was then rinsed in
water, boiled briefly in lactic acid, and kept immersed in the
acid for 16 h. The cleared tissues were immersed in water for
1 d before being stained with safranin and fast-green and
mounted with Entellan new (Merck). Thick cross and
longitudinal sections were prepared from the main root and
the stems at the rosette level of 15 mature plants of the C24
ecotype, as well as from 15 inflorescence stems, using a
sharp razor blade or a sliding microtome (Reichart). The
thick sections were cleared, stained and mounted by the
same procedure as that used for the separated bark.
Microscopy
Slides were examined under brightfield and polarized
light using a Leitz Dialux 20 microscope equipped with a
Nikon F3 camera at magnifications of ¬63 to ¬400.
F. 5. Cross section of the main root under polarized light, showing
the secondary xylem (SX) and many light bands of fibre-sclereids
(asterisks) in the secondary phloem. The outermost groups are the
primary phloem fibres. Bar ¯ 300 µm.
F. 6. A magnified section from Fig. 5. Secondary xylem (SX) and
bands of fibre-sclereids in the secondary phloem (asterisks) are seen.
Bar ¯ 100 µm.
longer) differentiate in the xylem of the inflorescence stems,
where they form a wavy hard band with a softer, nonlignified matrix on both the inner and outer sides (Figs 10
and 11). Longitudinal sections of the xylem in the
inflorescence stems show the compact bands of very long
fibres that compose this structure (Fig. 12).
DISCUSSION
RESULTS
Short fibre-sclereids differentiated in the secondary phloem
of roots and of shoots at the rosette level. In the secondary
phloem of the main root they formed a dense network of
fibre-sclereid bands (Figs 1–4). The length of these fibresclereids was usually 40–100 µm, and their ends were blunt
(Fig. 2) or pointed (Figs 3 and 4). In thick main roots,
several fibre-sclereid bands were formed in the secondary
phloem (Figs 5 and 6). These fibre-sclereid bands can be
clearly seen in thick longitudinal sections of the rosette-level
stems (Fig. 7). The number of fibres in the fibre bands of the
secondary phloem varies, and when mature they have a
thick lignified secondary cell wall (Figs 8 and 9). Longer
fibres (120–200 µm) differentiate in the secondary xylem of
the main root (not shown). Very long fibres (300 µm and
This study describes the fibre system of the roots and stems
of Arabidopsis thaliana, which has only been partially
studied to date.
A. thaliana has a short life cycle and the plants are usually
too small to have very long fibres. There is a substantial
difference between the fibres formed in the phloem of the
main root and those of the stems at the rosette level, which
are very short and should be considered as fibre-sclereids.
Those of the xylem of the main root or of the inflorescence
stems are typical long fibres.
From a functional point of view, the shorter fibres of the
main root and rosette level stems as compared with those of
the inflorescence stems, reflect adaptations to the different
functions of these organs. Because the main root and rosette
level stems are short, they are not exposed to regular
128
LeŠ-Yadun—Fibre Types in Arabidopsis thaliana
F. 7. A thick, cleared longitudinal section of a stem at the rosette
level, showing vessel members and vessels (arrowheads) and dark
bands of fibre-sclereids in the cortex (arrows). Bar ¯ 200 µm.
F. 8. Cross section of the stem at the rosette level under polarized
light, showing a band of phloem fibres in the cortex with a thick
lignified secondary cell wall and a dark lumen. Bar ¯ 50 µm.
F. 9. Details as for Fig. 8 but seen under normal light, showing the
thick, dark, lignified secondary cell wall of the fibres (arrows) and a
light lumen. Large thin-walled cortex cells are also seen. Bar ¯ 50 µm.
mechanical loads. The inflorescence stems, however, are
exposed both to the load of their own weight and to wind
action. Their longer fibres and the arrangement of the
mechanical tissues in a hollow wavy band apparently
provide the inflorescence stems with the required mechanical
support (see Wainwright et al., 1976 ; Niklas, 1992).
The development of fibres in any plant raises questions
concerning their structure, function, regulation and development. Fibre differentiation can be divided into a
number of main stages : (1) determination of cell fate to fibre
following the action of a certain stimulus ; (2) elongation of
F. 10. Cross section of a section of the inflorescence stem showing the
pith parenchyma (P), primary xylem (arrows), phloem (asterisks),
cortex (C), epidermis (arrowheads) and a dark wavy fibre band of the
secondary xylem. Bar ¯ 100 µm.
F. 11. Cross section of an inflorescence stem under polarized light,
showing the dark pith parenchyma and a light wavy fibre band of the
secondary xylem (asterisks). Bar ¯ 300 µm.
F. 12. Longitudinal section of the xylem of an inflorescence stem
under polarized light, showing very long fibres composing the wavy
fibre band of the secondary xylem. Bar ¯ 100 µm.
the cell ; (3) deposition of a secondary lignified cell wall ; (4)
nuclear divisions and formation of coenocytes ; (5) intrusive
growth between other cells without elicitation of wound
responses ; and (6) programmed cell death.
The existence of several types of fibres in Arabidopsis
thaliana, as discussed here, suggests that this plant has great
potential as a model system for the study of different aspects
of fibre differentiation.
LeŠ-Yadun—Fibre Types in Arabidopsis thaliana
A C K N O W L E D G E M E N TS
The author was a recipient of a Sir Charles Clore PostDoctoral Fellowship. I thank Dvora Dolev for her assistance
and Hillel Fromm, Shahal Abbo, Gad Galili, Gideon Grafi
and an anonymous reviewer for their comments on the
manuscript. This paper is dedicated to Professor Abraham
Fahn on his 80th birthday.
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