Download Border cells versus border-like cells: are they alike?

Journal of Experimental Botany, Vol. 61, No. 14, pp. 3827–3831, 2010
doi:10.1093/jxb/erq216 Advance Access publication 19 July, 2010
OPINION PAPER
Border cells versus border-like cells: are they alike?
Azeddine Driouich*, Caroline Durand, Marc-Antoine Cannesan, Giuseppe Percoco and Maité Vicré-Gibouin
Laboratoire Glyco-MEV, IFRMP 23, Plate-forme de Recherche en Imagerie Cellulaire de Haute Normandie, Université de Rouen,
F-76821 Mont Saint Aignan, France
* To whom correspondence should be addressed: E-mail: [email protected]
Received 24 March 2010; Revised 14 June 2010; Accepted 16 June 2010
Abstract
Roots of many plants are known to produce large numbers of ‘border’ cells that play a central role in root protection
and the interaction of the root with the rhizosphere. Unlike border cells, border-like cells were described only
recently in the model plant Arabidopsis thaliana and other Brassicaceae species and very little is known about the
functional properties of border-like cells as compared with ‘classical’ border cells. To stimulate discussion and
future research on this topic, the function of border cells and the way border-like cells are organized, maintained,
and possibly involved in plant protection is discussed here.
Key words: Arabinogalactan protein, border cells, border-like cells, cell wall, homogalacturonan, mucilage, plant defence, plant
root cap, xylogalacturonan.
What are border cells?
The current definition of border cells as put by M. Hawes
and co-workers is as follows: ‘Border cells are those cells
that separate from the root tips of higher plants and
disperse individually into suspension immediately after their
contact with water’. (Hawes et al., 2000, 2003). The number
of border cells produced by a root in a given period of time
by a given family is conserved (from a few hundreds to
several thousands) and the cells remain viable for weeks
after their detachment (Fig. 1A).
Based on the above definition, it was assumed that
Arabidopsis thaliana, a well-studied species in plant biology,
does not produce border cells (i.e. dispersed cells when
placed in water). However, it has been possible to show that
root tips of A. thaliana seedlings produce sheets of attached
cells that remain associated together after their release (Vicré
et al., 2005). They never become dispersed individually as
single cells when put into water as happens, for example, to
pea border cells. A similar border-like cell phenotype was
found in other Brassicaceae, including rapeseed (Brassica
napus), Brussels sprout (Brassica oleraceae), and mustard
(Sinapis alba) (Driouich et al., 2007). These cells do not seem
to fit the definition of ‘border cells’ as cited above and,
therefore, to emphasize their unusual organization pattern,
they were named border-like cells (Fig. 1B, C).
Function of border cells
For many years, border cells were considered of little or no
interest but it has become clear that these cells play a major
role in the interaction of plant roots with the rhizosphere
(Hawes et al., 2000). A wide range of studies has demonstrated that border cells have an impact on plant health and
survival by protecting the root meristem from pathogenic
infection. For instance, border cells of pea, the most widely
used plant model for border cell studies, are capable of
inhibiting growth of the fungus Nectria haematococca in
vitro (Gunawardena and Hawes, 2002). They are also
capable of repulsing the fungus, thereby preventing infection of the root tip (Gunawardena et al., 2005). They
seem to do so by encasing the hyphae in a kind of mantle,
and once the mantle is removed the root tip remains free of
infection. Interestingly, such a parasite-expulsion strategy
has also been observed in intestinal mammalian cells, which
are under continuous renewal and were shown to repel the
invading nematode parasite, Trichuris trichuria from colonizing the gut (Cliffe et al., 2005).
Border cells can also repel pathogenic bacteria by means
of their secreted mucilage. In addition, it has been shown
that border cells of legumes export a large number of
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3828 | Driouich et al.
antimicrobial enzymes, including chitinase, peptidase, and
glucanase (Wen et al., 2007; De la Peña et al., 2008).
One of the most interesting recent findings related to
border cell function is that, in pea, they secrete extracellular
DNA that is involved in root tip resistance to fungal infection
(Wen et al., 2009). It is notable that extracellular DNA plays
an analogous role in several other systems. Human white
blood cells form an extracellular structure called the neutrophil extracellular trap (NET) that contains DNA along with
antimicrobial peptides and enzymes. The NET is formed by
activated neutrophils in response to microbial invasion and
traps and kills the pathogens (Wartha et al., 2007). Extracellular DNA is also known to be present in bacterial secretions
that form biofilms and to be essential for biofilm stability
(Arundhati and Paul, 2008). The role of the DNA in these
cellular fortifications remains to be elucidated.
Border cells were also shown to be involved in the
response to abiotic stress. For instance, exposure of border
cells to aluminium induces secretion of an important layer
of mucilage that chelates aluminium and prevents it from
penetrating the root tip (Miyasaka and Hawes, 2001).
Another function of border cells and their secreted mucilage
is lubrication that helps root penetration into compact soil.
The total number of border cells released from the root tip
of maize was shown to increase significantly in compacted
sandy soil compared with loose sand (Ijima et al., 2004).
Also, the release of border cells is dependent on the water
status of the soil with fewer cells being produced in dry
sandy soil compared with wet soil (Ijima et al., 2003).
Although border-like cells differ from border cells in their
pattern of release, it is likely that they function similarly.
However, a direct involvement of border-like cells in
defence awaits further investigation. One of the approaches
to evaluate the role of border-like cells in defence is to
assess the resistance of mutants lacking such cells to
infection with soil-borne pathogens. One such mutant is fez
that produces fewer border-like cells than does the wild type
(Willemsen et al., 2008; C Durand, A Driouich, unpublished
results; Fig. 2A). FEZ is a NAC-domain transcription
factor that is required for root cap development. It is active
in root cap stem cells and allows the production of cap
tissues via the control of the orientation of cell division. The
activity of FEZ in the epidermal/lateral root cap cell initials
is able to promote the formation of root cap cells including
border-like cells (Willemsen et al., 2008). It is predicted that
fez roots will be readily succumbing to pathogen attack.
Fig. 1. Border cells (BC) are released by P. sativum root tips as
isolated cells (A). Morphological phenotypes of root tips showing
border-like cells (BLC) of the wild-type Arabidopsis (B, C). In (C)
BLC are stained with Calcofluor. Bars, 50 lm (A) or 20 lm (B, C).
proteins (the secretome) during their detachment from the
root cap (Wen et al., 2007). The secretome is a fundamental
protective component of the root cap that contains many
The organization pattern of border-like cells
is dependent on homogalacturonan
The unusual, adherent, habit of border-like cells when
compared with border cells raised the question of the
components responsible for cell adhesion. Using immunofluorescence imaging, it has been possible to show that
border-like cells of Arabidopsis secrete abundant arabinogalactan protein and homogalacturonan epitopes (Vicré et al.,
2005). However, they do not secrete as much mucilage as
Border cells versus border-like cells | 3829
Fig. 2. Morphological phenotypes of root tips showing border-like cells (BLC) of the mutants fez (A) and quasimodo1-1 (B). Note the
abundant mucilage (M) that encloses the BLC of quasimodo1-1. fez mutants produce almost no BLC. Bars, 10 lm (A) or 50 lm (B).
pea border cells do. By examining a series of selected
Arabidopsis mutants deficient in the biosynthesis of xyloglucan, cellulose, and pectin, Durand et al. (2009) showed
that homogalacturonan is a fundamental component of
border-like cell organization. The quasimodo1-1 (qua1-1)
mutant, which is deficient in homogalacturonan biosynthesis (Bouton et al., 2002), releases border-like cells that fully
disperse in the surrounding environment, a phenotype that
resembles that of border cells in pea. This observation
is consistent with the role of homogalacturonan in the
control of cell attachment and adhesion. Separation of the
border-like cells in qua1-1 is accompanied by concomitant
production and secretion of a mucilaginous matrix (Fig.
2B), not usually seen in the wild type. The mucilage is
abundant and contains mainly the LM8 xylogalacturonan
(XGA) epitope and arabinogalactan protein epitopes as
revealed by probing with specific antibodies (Durand et al.,
2009).
It is interesting to note that the sheaths of abundant
mucilage enclosing border cells resembles biofilm bacterial
secretions, which aid in cell-to-cell aggregation, protection
from desiccation, and resistance against harmful substances
(Davey and O’Toole, 2000). Such a border cell ‘biofilm’
formation in qua1-1 root is perhaps a key factor for their
survival, stability, and thus their defence activity.
Does qua1-1 make functional border cells?
As described above, the major difference between the
border cells of many plants and the border-like cells of the
Brassicaceae is the latter’s persistent adhesion. Therefore,
qua1-1 provides the ideal material to test the relative
performance of border cells when they are kept adherent or
allowed to separate. The mucilage produced by qua1-1
seems to retain the separated cells close to each other and
close to the root tip, thereby preventing them from moving
away in the presence of water. It is currently not known
whether the retention of cells in the vicinity of root tips via
the mucilage in qua1-1 mutant or as a block of attached
cells in the wild-type is important for their function. But
unity makes strength or ‘l’union fait la force’!
Mucilage has been implicated in the defensive function of
border cells in pea (Hawes et al., 2000). It is also possible
that the mucilage produced by qua1-1 has a protective
activity.
Interestingly, the mucilage secreted in qua1-1 mutants
was found to be enriched in the XGA epitope detected by
LM8 antibodies (Durand et al., 2009). The same observation was made in qua2-1 (Fig. 3), a mutant deficent in
a putative methyl transferase (Mouille et al., 2007). XGA is
an a-(1/4)-linked D-galacturonic acid chain highly
substituted with b-D-xylose (Zandleven et al., 2005).
Although the precise function of XGA is not clearly
established, it was speculated that XGA could be involved
in the resistance to pathogen attack (Willats et al., 2004;
Jensen et al., 2008). This hypothesis was based on the fact
that substitution of galacturonans with xylose makes XGA
more resistant to digestion by endopolygalacturonases. As
pathogens invade plant tissues, they synthesize and secrete
a set of enzymes including endopolygalacturonase in order
to degrade the plant cell walls. It is thus possible that the
presence of XGA in the mucilage of the qua mutants would
restrain the progression of pathogens and provide root
protection against infection.
It will be interesting to analyse to what extent qua1-1
appears to have either gained or lost putative border cell
functionality. In our laboratory, the current focus is on
comparing the response of qua1-1 to the wild type when
challenged with various pathogens and abiotic stresses,
thereby improving our understanding of the unusual adherent border cell phenotype of A. thaliana and its relatives.
Arabinogalactan proteins might control
binding of micro-organisms to border cells
Unlike homogalacturonan, arabinogalactan proteins do not
seem to be involved in the attachment and organization of
3830 | Driouich et al.
Fig. 3. Border-like cells (BLC) characterization in qua2-1, a mutant deficient in a putative methyl transferase (Mouille et al., 2007).
Border-like cells are released individually from the root tip (RT) (A) and are embedded in a thick mucilage (M) as revealed by the
ruthenium red staining (B). Immunofluorescence labeling with the mAb LM8 revealed the presence of the XGA epitope both in the
mucilage and at the surface of border-like cells in the qua2-1 mutant (C). Bars, 20 lm (A),40 lm (B) or 8 lm (C).
border-like cells in the wild type, although these proteins
(especially fasciclin-like arabinogalactan protein) were suggested to be important for cell adhesion (Johnson et al.,
2003; Driouich and Baskin, 2008). A careful examination of
all fasciclin-like arabinogalactan protein mutants did not
reveal any alteration of border-like cells pattern (C Durand,
A Driouich, unpublished data). Separated cells were not
observed.
Arabinogalactan proteins may fulfil other functions at the
cell surface of border-like cells. The alteration of arabinogalactan protein biosynthesis or incorporation within the cell
wall induces a significant reduction in the binding of
rhizobacteria to border-like cells and the root cap in
Arabidopsis (Vicré et al., 2005). Consistent with the implication of cell wall arabinogalactan proteins in the attachment
of soil-borne bacteria is the observation that the rat1
AtAGP17 mutant, containing a T-DNA insertion in the
promoter region of a gene encoding an arabinogalactan
protein, is resistant to infection and transformation by
Agrobacterium tumefaciens. Resistance to transformation
was shown to be correlated to a deficiency of A. tumefaciens
in binding root cells (Nam et al., 1999; Gaspar et al., 2004).
Also, incubation of Arabidopsis roots with b-glucosyl Yariv,
an agent known to bind arabinogalactan proteins, inhibits
attachment and transformation by the same bacteria. Therefore, it appears that, while homogalacturonan produced by
border-like cells is involved in their attachment to each other,
arabinogalactan proteins seem to function in binding and,
possibly, in recognition of micro-organisms.
Conclusion and prospects
Border cells of the legume, pea, are clearly involved in
defence against fungal pathogens. But what about the
border-like cells of Arabidopsis and other Brassicaceae?
One of the most important questions with regard to the
function of border-like cells is related to their role in
defence. Are their formation and release stimulated in the
presence of specific pathogens? Do they produce specific
anti-microbial molecules in response to biotic stress, and
also in response to abiotic stress? How is their adherent
phenotype adaptive? Is the abundant mucilage secreted by
the cells of the mutant qua1-1 related to their role in
defence?
We should now move forward and search for the
components required for the function of border-like cells
of the Brassicaceae (some of which are agriculturally
important crops). The use of large-scale transciptomic or
metabolomic profiling, for instance, would be a powerful
approach for the identification of novel defensive molecules
produced by border-like cells in response to specific microbe
infections.
Acknowledgements
We are grateful to Professor T Baskin (University of
Massachusetts) for helpful comments and critical reading
of the manuscript. La région de Haute Normandie and the
Border cells versus border-like cells | 3831
‘Grand Réseau de Recherche–Végétal, Agronomie et Transformation des Agro-ressources–VATA’ are also acknowledged for their financial support to AD and MVG.
Ijima M, Barlow PW, Bengough G. 2003. Root cap structure and
cell production rates of maize (Zea mays) roots in compacted sand.
New Phytologist 160, 127–134.
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