УДК 581.1
PLANT FIBERS: INITIATION, GROWTH, MODEL PLANTS, AND OPEN
QUESTIONS
© 2010 Simcha Lev-Yadun
Department of Science Education-Biology, Faculty of Science and Science Education,
University of Haifa-Oranim, Tivon, Israel
Received July 20, 2009

Fibers, which are used as a major raw material in the paper industry, as structural components
in timber for building, and in the manufacture of wooden items, are among the most important
renewable resources. Billions use wood as a major energy source, and fibers are an energy-rich
component of wood. They are used for various textiles and as raw material for composites. In
this review, I describe the basic characters of fibers, their structure, development, uses, and
some of the current major model plants for fiber formation. I discuss open developmental
questions and various aspects of further research. Most of the recent progress in the biology of
fiber formation, especially in their cell-wall chemistry, emerged from studies of several model
plants: Arabidopsis thaliana, Populus sp., Eucalyptus sp., flax, and hemp. I stress the critical
need to combine the use of modern methods of research with classical botany. Approaching the
issue of fiber formation only by molecular or only by classical methods will not only limit the
progress, but may result in critical mistakes. Considering the importance of fibers to humanity,
it is surprising how little we know about the biology of fiber formation and how little it is
studied as compared, for instance, to the effort to study the genetics and cell biology of flower
organ identity.

Key words: fibers – sclerenchyma – bast – differentiation – wood – intrusive growth –
secondary cell wall – hormonal regulation
——————————————
Corresponding author: Simcha Lev-Yadun. Department of Science Education-Biology,
Faculty of Science and Science Education, University of Haifa-Oranim, Tivon 36006, Israel;
E-mail: levyadun@research.haifa.ac.il
1 Материал был доложен на Международном симпозиуме “Plant Fibers: View of Fundamental Biology”,
Казань, 28–31 мая 2009 г.

INTRODUCTION
The term fiber describes several botanical entities: a mechanical cell type belonging to
the sclerenchyma (true fibers), long cells used to produce paper (true fibers and tracheids),
vascular bundles used for textiles or to produce ropes (manila and sisal), long trichomes used
for textiles (cotton), and the so-called dietary fibers. Some of these botanical entities, i.e.,
sclerenchyma fibers and fibers for paper production, partially overlap. In this review, I will
discuss only "true fibers", those belonging to the sclerenchyma.
The sclerenchyma is a simple mechanical plant tissue composed of either fibers or
sclereids, or both. In addition, both cell types can be one of the cell types that build various
complicated tissues such as the xylem, phloem, and cortex, along with several other cell types.
Alternatively, fibers and sclereids may differentiate as single cells among many other different
cells, a situation known as ideoblasts [1]. There are intermediates between fibers and sclereids
called fiber sclereids (e.g., [2]). Fibers serve mostly mechanical support and defense from
herbivory, being both hard and flexible, but when alive they can also serve in storage [1].
Fibers can be part of both the primary and secondary plant body. They can be
derivatives of the procambium (primary phloem fibers, fibers surrounding vascular bundles in
monocotyledons) [1, 3] or even differentiate from the ground meristem [4]. In the secondary
body, fibers are mostly cambial derivatives (fiber bundles in the secondary phloem and fibers
of the secondary xylem) [1, 3]. There are also regenerative fibers that differentiate from
parenchyma cells after wounding [5]. Fibers appear in several cellular types. They can be alive
or dead, and uninuclear or multinucleate [1, 3, 6]. Many fibers have septa that divide them into
several chambers [1, 3]. Fibers can be short, sometimes less than 1 mm long as in Arabidopsis
thaliana [2], or they may reach a length of 55 cm, as in Boehmeria nivea (ramie) [7].
There are many patterns of fiber arrangements at the tissue, organ, and whole plant
levels that may significantly influence the biomechanics of plants, but this aspect has not been
studied much because of the typical botanist's limitation in the ability to deal with theoretical
aspects of engineering (but see [8, 9]).

USES OF FIBERS
Fibers are among the most important renewable resources for humanity. Wood is a
major energy source for many in the world [10]. The energy content of wood significantly
increases when the wood is rich in highly lignified fibers. At the same time, the mechanical
properties of timber for construction and as raw material for various industries are greatly
influenced by their fiber content, chemistry, and structure. The importance of fibers in the
production of textiles is also well known. From both commercial and biotechnological points
of view, the most important fiber types are bast (phloem) fibers, mostly used for textiles, and
fibers of the secondary xylem used as the major raw material for paper production [11]. Their
length and chemistry are critical quality components in the paper industry [11, 12]. Elimination
of lignin from fibers during paper production is a major cost component in the paper industry
and an important environmental polluting process [13, 14].
Phloem (bast) fibers are a common source of commercial fibers originating in several
crops including Linum usitatissimum (flax), Cannabis sativa (hemp), Corchorus capsularis
(jute), Boehmeria nivea (ramie), and Hibiscus cannabinus (kenaf) [11, 15–17]. In addition to
the traditional uses of fibers mentioned above, in the last several decades they have been used
in the production of many types of composite materials, but this broad technological issue is
outside the scope of this essay.
To compensate for the ever-increasing demand for wood fibers for the paper industry
and to reduce the pressure on the dwindling natural forests, more wood of higher quality will
have to be produced on less land by planting high quality and highly productive trees.
Achieving this goal by traditional studies on tree genetics, physiology, and development will
take more time than the current global trend of de-forestation allows. The urgent global need to
improve tree growth and quality for the paper industry, to reduce forested habitat destruction,
and to produce biofuel and sequester carbon make the study of producing more and better
fibers a critical global issue. In this respect, transgenic technology goes hand in hand with
nature conservation, contrary to the belief of the general public and some extreme
environmental activists.

INITIATION OF FIBERS
Fibers initiate regularly in certain parts of the plant body, e.g., primary phloem fibers,
and this regularity is useful in studying their development. Classic anatomical studies of fiber
development were done by Esau [18], for instance, and reviewed in depth in some of her books
[3, 16]. Parallel studies of the hormonal regulation of fiber formation indicated that various
GAs are involved in determination of fiber length and cell wall structure [19–21]. A synergistic
effect of a combination of auxin (IAA) and GA on secondary xylem (which includes many
fibers) differentiation was established during the same years [22]. I stress that the early studies
indicating the role of GA in fiber differentiation were almost forgotten and are cited very
infrequently. This gives many current scientists the wrong impression that this level of
understanding of the hormonal regulation of fiber formation was achieved only in the 1980s.
However, wound effects seemed to influence the experimental results when wounding was part
of the experiments with hormonal application, and these effects must be taken into
consideration (e.g., [23–25]).
In spite of the wound effects in some of the studies, these classic studies and others
produced the basic understanding of tissue-level and cell-level processes of fiber
differentiation, and a general picture of the hormonal regulation of fiber differentiation
emerged. However, the plethora of genes involved, the delicate developmental details at the
cellular level, and the networks of regulatory cellular and tissue level signaling of these
complicated processes were impossible to study in great detail with the classical methods of
organismic biology. Now we have various new techniques that allow us to address these
delicate questions with precision undreamt of not so long ago.
The major unsolved problem concerning fiber initiation is that when we see fiber cells,
even at early stages of differentiation, the cellular factors that determine which cells will
acquire a fiber fate may have already ceased to operate. In some studies there was confusion
between fiber initiation (not demonstrated in spite of the claims) and fiber lignification, which
was the actual measured factor. Such mistakes should be avoided. Theoretically, the factors
that control later phases of fiber differentiation (when we can already identify them as
developing fibers) may also operate at the first stage of fiber cell fate determination, but these
factors have not been identified yet. Identifying these elusive factors may eventually have a
great impact on the economic aspects of fiber production and also have important contributions
in theoretical aspects of differentiation. >From the wonderful studies in flower organ identity
regulation or from similar studies on cell fate determination in embryos or in the shoot or root
apex, we know that very delicate signaling networks operate there. I assume that similar
regulatory mechanisms are involved in the determination of fiber initiation in the primary
phloem, the primary wavy fiber band in inflorescence stems of A. thaliana, and in the initiation
of secondary phloem and xylem fibers. The problem is that because of the importance for
industry and trends in funding, current studies are focused mostly on the regulation of the
chemistry of fiber cell walls and mechanisms influencing fiber length. For vessel members, we
have the very good Zinnia system that allows using cell cultures to study many cellular aspects
of differentiation (e.g., [26, 27]). However, we still have no such system for fibers (or
sclereids). Because there is no such system, many genomic, proteomic, and metabolomic
studies of fiber development result in data representing averaged developmental stages and cell
types that were pooled together. Some of these problems were overcome by extracting fibers of
specific developmental stages, but this cannot be done in all cases.
It is already clear that fiber differentiation is a very complicated issue. The current
studies on fiber development are usually devoted either to the specific role of a regulator
(hormone or gene), the monomers or polymers involved in cell wall formation, the genes that
produce the studied components and determine cell wall structures, compilation of large
numbers of genes involved via genomics, or to identifying the proteins involved in the process,
indirectly via genomics or directly via proteomics. There is a great need, only partly met, to
combine these approaches. The lists of genes and proteins resulting from studies of genomics
or proteomics give us only a crude understanding of their potential role in fiber differentiation.
There is a need to focus on the cellular and sub-cellular levels of actions of each of the many
identified genes, their proteins, the products of structural proteins, and biochemical function of
nonstructural proteins. Currently, the methods of RNA or protein harvesting used in many
cases to study fibers are not cell type and developmental stage specific, resulting in studying a
mixture of cell types and stages of cell differentiation that may differ significantly in gene
expression and protein function. More specific approaches were used in initiating cotton
"fibers" (e.g., [28]) and should be used for bast fibers and fibers of the secondary xylem. Large
scale and detailed studies of the relevant tissues at different developmental stages using in situ
hybridization of each relevant gene and immunogold or immunofluorescence of each of the
proteins are needed. I call this approach cytonomics, similar to genomics and proteomics. Such
tedious but systematic studies, which may be at least partially mechanized, may expose
potential specific roles of the genes even before mutant or transgene analyses are done.
Moreover, some of the genes, which operate at the same time (concerning specific cell age or
developmental stage) and place, have a good chance of being associated biochemically by the
formation of protein complexes like many such complexes known in cell biology. Identifying
their associations may shed light on both their function and mode of operation. Such large-
scale studies probably demand an international consortium similar to those that allowed whole
genome sequencing. However, this may be the best chance to advance in the understanding of
fiber formation in large steps.

FIBER GROWTH
Fibers usually attain their length in two stages, in the beginning by symplastic growth
and later by intrusive growth [1, 3, 29, 30]. In certain trees, fibers grow in length even in the
second year after the beginning of their differentiation [16]. Fibers several millimeter long are
common in many plant species [1, 3, 12, 16], but fibers in Boehmeria nivea (ramie) may reach
a length of up to 55 cm. These long fibers start as initials of approximately 20 µm, and over a
number of months grow 27 500 times longer [7]. During this growth, the fiber tips of ramie,
like those of other species, must pass through the middle lamella between up to tens of
thousands of cells, disrupting a very large number of plasmodesmata [31–33]. However, there
are usually no wound responses following the intrusive growth of fibers [34].
Many questions have been raised concerning the intrusive growth of fibers [34], but
there has only been modest progress in solving them (e.g., [35–37]).

PATTERNS OF FIBER FORMATION IN SECONDARY TISSUES
The current understanding of the regulation of tissue patterning in the secondary xylem
and phloem of plants is inadequate. In many dicotyledons, the axial system of the secondary
xylem is composed of vessel members, tracheids, fibers, and parenchyma, and the axial system
of the secondary phloem is composed of sieve elements, companion cells, parenchyma, and
bands of fibers or sclereids (e.g., [1, 38]). Tissue patterns in the secondary xylem and phloem
have mostly been studied only from a descriptive point of view (e.g., [16, 38, 39]) and much
less in terms of the regulation of pattern formation (e.g., [40]). Patterns of alternating wood
areas rich in vessels and paratracheal parenchyma with wood areas rich in fibers are common
in thousands of woody species (e.g., [38, 41, 42]), indicating the generality of such phenomena.
However, there is considerable variability in the relations between vessels, parenchyma, and
fibers. For instance, in Acacia albida many vessels are found within broad bands of
parenchyma, but others are found within fiber bands [43]. The fiber-surrounded vessels in
Stemonurus luzoniensis, Rubinia pseudoacacia, Homalium foetidum, and other examples [41]
further indicate the variability in cellular patterns of fibers, for which we have no
understanding whatsoever of their regulation. The various patterns of fibers in secondary
tissues indicate migrating zones of signals that induce either fibers or other cell types in the
cambial region. The fact that there are zones with and without signals for fiber formation is an
opportunity to identify the genes involved in fiber initiation.

GELATINOUS FIBERS OF TENSION WOOD
The cell wall of fully differentiated fibers appears in two gross variations. In the first,
normal fibers are significantly lignified. In the second type, gelatinous fibers of tension wood
(classic reaction wood of dicotyledons) are not lignified [44]. While the lignin is critical for the
mechanical functions of normal fibers in living plants, when plants are considered as raw
material, the lignin may be either an advantage or burden depending on intended use. For
timber and firewood, production of lignin adds to the value of the product, but for the paper
industry, lignin must be eliminated from the product. Therefore, the study of fibers is greatly
influenced by the specific needs of various funding agencies. The low lignin content of
gelatinous fibers is a natural laboratory for reducing lignin in fibers, and as such should be
fully exploited in research (e.g., [45]) but has not been studied enough yet.
In general, the current hypothesis is that lower levels of polar auxin transport (or auxin
level) in the cambial region induce differentiation of gelatinous fibers (e.g., [44, 46]).
Gibberellin is also involved in tension wood formation [47, 48], and probably also ethylene
[49]. In spite of some advances in characterizing the genes involved in tension wood
production (e.g., [50, 51]), the whole issue of the hormonal regulation of tension wood
production and gelatinous fiber differentiation is not well understood (e.g., [52]). Since reduced
lignin levels in fibers are very important for the paper industry, differentiation of tension wood
fibers deserves much more attention than it has received so far.

USING POLARIZED LIGHT TO STUDY FIBERS
A very simple method to identify fibers at early (but not the earliest) stages of
differentiation is to examine the tissues under polarized light. As discussed in Lev-Yadun and
Flaishman [4], and in Lev-Yadun et al. [53], it is common to misidentify nonlignified fibers or
fibers produced among other lignified cell types (e.g., [54]). Secondary cell walls usually
contain both cellulose and lignin. However, identification of special cell types with secondary
cell walls found among other types in unstained material or by means of common tissue
staining is often difficult, and the use of polarized light can significantly help in this task. The
microfibrils of cell walls contain crystalline cellulose, which is birefringent (it has two
refractive indices). The refractive index to light waves parallel to the long axis of the cellulose
chains and microfibrils is larger than to light passing at right angles. Light polarized by passage
through a polarizing filter will not pass through a second polarizer oriented at 90° to the first.
When a piece of birefringent material (in this case a secondary cell wall) is placed between
such crossed polarizers, so that its axis of major or minor refraction is at 45° to the planes of
polarization of the polarizers, then the light passing through it becomes polarized and is able to
pass through the second polarizer. The birefringent material between the polarizers appears
bright to the viewer [55]. I propose routinely using this simple procedure before a decision is
made that a certain tissue produces no fibers (e.g., [53]).

MODEL PLANTS FOR FIBER DEVELOPMENT
General genetic comment
The most important model plants for fiber formation are either crop plants (flax and
hemp), which passed a very strong genetic bottleneck and strong selection of most of their
characters, eliminating most of their variability, or forest trees (Populus and Eucalyptus) that
may in many cases even be clonal. A similar strong selection exists in the common laboratory
genotypes of A. thaliana. In polygenic characters, such as fiber formation, the action of many
of the involved genes depends on the specific genetic background. The strong selection that
these model plants passed may result in specific genetic situations, which may differ in specific
molecular, biochemical, and developmental characters from other genotypes of these species or
from other taxa.
Fibers in A. thaliana
The emergence of A. thaliana as an established model plant 20 years ago [56] was a
critical moment for plant biology. This modest, small annual plant became the plant
Drosophila, allowing an explosion of genetic, physiological, developmental, reproductive, and
evolutionary researches. The completion and publication of the full sequence of its genome
[57] was probably the single most influential study in plant biology ever. While the critical role
Arabidopsis had in advancing plant biology in general is common knowledge, its role in
studying fiber biology was much smaller.
Secondary wood rich in fibers is essential for quicker progress in studying the genetic,
biochemical, developmental, and physiological aspects of fiber production for the paper
industry. The use of A. thaliana for studying fiber development opens enormous opportunities
to study the role of all plant hormones and their combinations, as well as many other
physiological and environmental factors (e.g., photoperiod, temperature, water regime,
minerals, vibrations, and gravity) on fiber formation. Moreover, A. thaliana allows studying
the functions of various genes cloned from other species, which are much less convenient to
study.
The basics of some wood and fiber production in A. thaliana were described in Russian
before it became a model plant [58]. This study escaped the attention of the scientific
community both because of the language barrier and because of its publication several years
before Arabidopsis became the most important model plant. Dolan et al. [59] described the
production of a small amount of secondary wood in the roots of A. thaliana, but again it caused
no shift in the general understanding. The wrong paradigm was that A. thaliana has no
secondary tissues and so unfortunately it could not be used as a genetic model for wood and
fiber production.
The first intentional attempts to use A. thaliana as a model for fiber production were
made in plants grown individually in large pots to avoid competition and self-thinning, and in
which the inflorescence stems were cut every day for two to three months to avoid monocarpic
senescence [2, 60]. This simple method induced much more secondary wood, fiber, and
sclereid differentiation, especially in roots but also in stems by means of physiological rather
than genetic changes, indicating that A. thaliana can be an excellent genetic model for fiber
formation [2, 60]. The basic description of the fiber system in roots and shoots of A. thaliana is
given by Lev-Yadun [2]. A. thaliana has three major fiber systems distinguished according to
their site of differentiation, some of which have sub-systems: (1) bast fibers, produced more in
the main root than in the inflorescence stem or rosette level stem, a few of which are primary
phloem fibers, while others are short fiber-sclereids produced in the secondary phloem of roots
and shoots at the rosette level; (2) long fibers in the secondary xylem of the main root; and (3)
even longer primary and secondary fibers in the wavy band of fibers of the inflorescence stems
[2, 4, 60].
Concerning the wavy ring of lignified fibers in inflorescence stems, the most studied
fibers of A. thaliana, I give a more detailed description. The early studies done by Lev-Yadun
[2, 60] were conducted on mature plants. From these results and from the confusing captions to
plates in various other studies (e.g., [61–63]), it became clear that a developmental study of
inflorescence stems, beginning from the first day of their emergence until they become fully
mature, was needed. Otherwise, as will be discussed below, there is no way to understand both
their structure and various anatomical descriptions found in the literature. This essential
developmental study was therefore done [4] and indeed illuminated several complicated
developmental aspects of inflorescence stem anatomy. The inflorescence stem of A. thaliana is
characterized by a wavy ring of lignified fibers inward to the cortex. In the young stem,
primary fibers develop from the ground meristem, forming the outer part of the pith as the
wavy ring of fibers. In many cases where A. thaliana is grown on agar or under low light in
growth chambers, only this primary component is formed. In the mature stem of plants
growing under good growth conditions (individual plants in large pots and high illumination),
the fiber system is comprised of both these primary fibers and secondary ones that differentiate
around the primary ones, being derivatives of the cambium. The cambium formed in the
inflorescence stem is not always continuous. Therefore, the secondary xylem and the secondary
parts of the wavy band of fibers may sometimes form only in sectors at the circumference [4].
However, the progress in understanding the genetics and cell biology of fiber
production by using A. thaliana was slower than expected. Some physiological studies showed
that it is possible to manipulate fiber formation in A. thaliana by changing growth conditions
[4] and by external hormonal applications [64]. However, as with many studies of this model
plant, there were expectations for genetic breakthroughs. One promising mutant that induces
alterations in fiber differentiation is the A. thaliana mutant of the gene REVOLUTA (REV)
[61], which was studied and cloned in parallel as a different gene INTERFASCICULAR
FIBERLESS1 (IFL1), a name implying that the interfascicular fibers do not form [62, 63, 65–
68]. These two putative genes were found to be identical [66]. The gene encodes a class III
homeodomain-leucine zipper protein (HD-ZIP) and is involved in the regulation of
interfascicular fiber differentiation in inflorescence stems of A. thaliana [63, 69]. The
REV/IFL1 gene was found to be one of the putative target sequences of microRNA 165 that
cleaves the wild-type REV/IFL1 mRNA [68, 70]. However, the nature of the Rev/IFL1 gene
turned out to be different than considered for about a decade. Careful examination of the
published histological figures of the mutant [61–63, 65, 67] raised the possibility that fibers are
actually formed in the inflorescence stems and that the mutation mainly alters the secondary
cell-wall composition rather than causing fiber absence. To clear this issue, Lev-Yadun et al.
[53] grew plants of the mutant and characterized the interfascicular fiber band of their
inflorescence stems. They found that the mutant produced the typical primary wavy fiber
system of the inflorescence stems. A simple, but critical aspect in this study was the
examination of the microscopic sections under polarized light, which clearly demonstrated that
fibers were formed by the mutant. The impression of a total lack of the wavy band of fibers
was found to be a result of poorly lignified secondary walls in many of the specimens. This
specific gene that reduces lignification in fibers is of great significance for biotechnological
developments for the paper industry and thus for the global economy and ecology because of
the need to get rid of the lignin in the process of paper making. An important conclusion from
the studies of the fiber system of Arabidopsis is that a reliable and detailed knowledge of the
developmental aspects of the studied plant must be gained before mutants are characterized.
While there was little progress from the use of Arabidopsis concerning the genes
involved in fiber initiation, significant progress was made in the regulation of the structure and
chemistry of the secondary cell wall. Several transcription factors are involved in the regulation
of secondary cell walls in the fibers [63, 71–73]. This and progress in understanding other
factors involved in fiber development are essential in order to control fiber chemistry at the
genetic level. This understanding can allow reducing fiber cost, and reduce deforestation and
environmental pollution by the paper industry. The irrational public fear of transgenic plants
should also be reduced by education to allow the progress.
Populus and Eucalyptus
Growing plants of A. thaliana of various genotypes that produce enough secondary
growth for studies on fiber development need daily clipping of the inflorescence stems and
very strong illumination [60], and it was expected that a genetic solution would eventually
emerge, as it partly emerged only recently [74]. However, in addition to the use of model
herbaceous plants like A. thaliana that produce some secondary tissues, real trees are needed to
study some of the questions of fiber formation.
The current progress in making poplar a genetic model for wood and fiber formation
(e.g., [75–81]) and the completion of its genome sequencing [82] are very important for
studying fiber formation, in spite of the difficulty of genetic analysis in a perennial dioecious
species. The recent progress in xylem and phloem transcript profiling in Eucalyptus (e.g., [50,
83–85]) opens further opportunities to understand the regulation of fiber formation. Both
species produce fibers both in their bark and secondary xylem. Moreover, as they are important
trees for the paper industry, there are significant resources for their study. The ability to use
Arabidopsis to examine various genes (or their products) from Populus and Eucalyptus helps to
partly overcome the limitations of trees as model plants (e.g., [86]). The use of transgenic
Populus trees further supported the understanding of the role of GA in fiber growth [87, 88].
Flax, Hemp, Rice, Tomato, and Tobacco
Currently, flax seems to be the most studied annual commercial fiber plant (e.g., [45,
89–93]). Hemp (e.g., [94, 95]) is also important. Rice, a monocotyledonous cereal, for which
we now have excellent genetic knowledge, produces large quantities of fibers in its stems and
leaves. Unfortunately, rice is not used as a classic model plant for fiber production. The critical
importance of rice for human nutrition diverts the research effort from its fibers. However, this
can and should be changed. Since we expect that very soon the whole genome of several other
cereals rich in fibers, such as sorghum, wheat, and maize, will also be sequenced, cereals in
general may also become good models for fiber production.
There are no developmental or other biological reasons not to use other model plant
species, such as tomato and tobacco, for which we have very good genetic data, to study fiber
formation and thus to focus mostly on plants used for commercial fiber production. Any model
plant that will permit a quicker, cheaper, and deeper understanding of the biology of fiber
formation should be used. The practical application of the knowledge to actual crops will be
easier when we have more biological facts.

CLASSIC PHYSIOLOGICAL EXPERIMENTS SHOULD BE REPEATED BUT WITH
GENOMIC AND PROTEOMIC APPROACHES
While molecular genetics has enabled unbelievable progress in understanding plant
biology in the last 25 years, even these sharp and powerful tools have limitations. When a
biological character involves many genes, it may currently be impossible to use mutants or
transgenic plants to study it. QTL analysis may partly bridge the gap between monogenic and
very complicated polygenic characters (e.g., [96]), but even QTL analysis has its limitations.
For instance, the whole cascade of genes involved in wood and fiber production in Arabidopsis
is not known yet in spite of significant efforts to study them. Fifteen or twenty years ago,
studying such complicated biological attributes using mutants in a single gene or similar
transgenes was not practical. Genomics, proteomics, and sequences of entire genomes were not
yet available. Therefore, using physiology, even if a studied physiological change was actually
a black box, allowed progress in the study of complicated processes. As already discussed
above, attempts were made to use A. thaliana as a model for wood and fiber production in trees
by using plants, in which the inflorescence stems were repeatedly cut to avoid monocarpic
senescence [60]. This simple method induced much more secondary wood, and fiber and
sclereid differentiation, especially in the main root, but also in stems, by means of
physiological rather than genetic changes. It took about 15 additional years to produce a
transgenic Arabidopsis that parallels it in producing secondary tissues [74]. Similar, if not
longer delays, in producing the right genetic combinations are expected for other complicated
physiological or developmental functions.
The polygenic character of fiber differentiation was also shown while studying the
genetic regulation of fiber formation in wheat (Triticum aestivum) using the GA-insensitivity
locus Rht1 [97]. One of these polygenes, a calmodulin gene, was found in a parallel study
conducted for other purposes [98]. The whole cascade of genes involved in fiber development
in wheat is not yet known.
The ability to fully or partly alter fiber differentiation was demonstrated in several
systems. In pea (Pisum sativum), differentiation of primary fibers was found to be dependent
on stimuli originating in young leaf primordia, but external application of auxin alone was not
a sufficient signal to replace the primordial and induce fibers [99]. In A. thaliana, submergence
eliminated fiber differentiation in inflorescence stems [4]. In various tree species, wounding
was found to arrest fiber production in the secondary xylem for weeks (e.g., [100]). However,
enhanced fiber production was also shown. It is possible to increase fiber length (e.g., [19–21,
87, 101] and change its lignin composition [102] by manipulation of auxin and gibberellins.
All these systems and similar ones, in which fiber development is altered, open the door to the
use of genomic and proteomic approaches (e.g., [92–94, 103–107]) to identify the cascade of
genes involved in fiber initiation and development, even when critical mutants and transgenes
do not exist.
A good example of the use of classic experiments are those of Wareing et al. [22],
which were recently repeated in Populus, but with the current ability to study the transcriptome
[78] and give new insights that were impossible to achieve almost half a century ago.

OPEN QUESTIONS
Almost a decade ago [34], I reviewed various questions concerning fiber
differentiation. Here I sum them up briefly, add new questions, and evaluate the progress since.
(1) What are the signals that induce fiber initiation? (no progress). (2) Is the determination of
fiber cell fate reversible, and untill what stage? (no progress). (3) What are the signals that
initiate cell elongation? (no progress). (4) What is the mechanism/force enabling intrusive
growth? (little progress). (5) How does the cell tip soften the middle lamella during intrusive
growth? (little progress). (6) Is intrusive growth the result of tip growth alone, or do other cell
parts take place in the process? (little progress) (7) How does a cell determine the direction of
growth? (no progress). (8) What brings intrusive growth of fibers to an end? (no progress). (9)
When intrusive growth occurs, how does the plant sense that the penetration into the tissue is
not foreign or hazardous (growth of fungal hyphae or mouth parts of an insect)? (no progress).
(10) What are the mechanisms that coordinate cellular functions in very long cells such as
fibers? (no progress). (11) What determines fiber cell wall composition? (moderate progress).
(12) Since many fiber cells are multinuclear, how is their multinuclearity regulated? (no
progress). (13) Many fibers have septa, how is septum formation regulated? (no progress). (14)
Many fibers die at the end of differentiation, how and why? (little progress).

CONCLUSIONS
Fiber differentiation can be divided into a number of main stages (determination of cell
fate to fiber and tissue patterning, elongation, deposition of a secondary, usually lignified cell
wall, intrusive growth between other cells without elicitation of wound responses, nuclear
divisions and formation of coenocytes, programmed cell death). All these complicated but
coordinated processes are induced by a combination of hormonal signals, mostly GA and
auxin, a fact known for half a century, but with complicated molecular interactions that are
very far from being understood. As can be seen, in spite of a certain progress in understanding
the genes involved in fiber formation and cell wall structure and chemistry, the general biology
of fiber differentiation is almost as enigmatic as it was when the basic hormonal regulation of
fiber formation was revealed decades ago.
An important aspect of studying fiber differentiation is the academic programs that
produce future scientists. We can see a global decline in research and the study of plant
development from the broad organismic and anatomical points of view. The sharp drop in
funding of such studies because they are not always "technically advanced" has reached the
point where, with the retirement of the currently small number of experts, there will be no new
good scientists who understand the relevant developmental aspects to replace them. This will
certainly damage future research and progress. This trend should be reversed in time – now.

ACKNOWLEDGMENTS
I thank the late Tsvi Sachs and Roni Aloni for their illuminating discussions on the
hormonal regulation of fiber formation, Shahal Abbo for the collaboration on the genetics and
physiology of fiber formation in wheat, Moshe Flaishman for our long cooperation on
Arabidopsis issues and his important comments on the manuscript, Sarah Wyatt for the
collaboration on Arabidopsis, and Ron Sederoff for many discussions on lignification and
forest biotechnology.


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