Plant Cell Walls

Arabinogalactan proteins as molecular markers in Arabidopsis thaliana sexual reproduction

by Mario Costa

Sílvia Coimbra, João Almeida, Vítor Junqueira, Mário Costa and Luís Gustavo Pereira

Abstract
Some of the most important changes that occur in
plants during sexual reproduction involve the... more Abstract
Some of the most important changes that occur in
plants during sexual reproduction involve the transition
from a sporophytic to a gametophytic type of develop-
ment. In this paper, these changes were evaluated for
Arabidopsis thaliana. The results obtained clearly show
differences in the pattern of distribution of specific
arabinogalactan protein (AGP) sugar epitopes, during
anther and ovule development. AGPs are hydroxyproline-
rich glycoproteins that are massively glycosylated and
ubiquitous in plants. The molecular mechanism of
action of AGPs is still unknown, mainly due to the
difficulties posed by the complex saccharide chains.
However, the complex structure of the sugar fraction
of AGPs makes them a potential source of signalling
molecules. The selective labelling obtained with AGP
mAbs JIM8, JIM13, MAC207, and LM2, during Arabi-
dopsis pollen and pistil development, suggests that
some AGPs can work as markers for gametophytic cell
differentiation. Specific labelling of the first gameto-
phytic cells in the pistil, the strong labelling of the
secretory cells of the embryo sac, the synergid cells,
and the labelling of the integument micropylar cells,
apparently outlining the pollen tube pathway into its
final target, the embryo sac, have all been shown. In
the anthers, the specific labelling of gametophytic
cells, and of the male gametes that travel along the
pollen tube, may indicate AGP epitopes acting as
signals for the pollen tube to reach its final destiny.
The specific labelling of cells destined to go into
programmed cell death is also discussed.

Pollen grain development is compromised in Arabidopsis agp6 agp11 null mutants.

by Mario Costa

Sílvia Coimbra, Mário Costa, Brian Jones, Marta Adelina Mendes, and Luís Gustavo Pereira.

Abstract
Arabinogalactan proteins (AGPs) are structurally complex plasma membrane and cell wall proteoglycans... more Abstract
Arabinogalactan proteins (AGPs) are structurally complex plasma membrane and cell wall proteoglycans that are
implicated in diverse developmental processes, including plant sexual reproduction. Male gametogenesis (pollen
grain development) is fundamental to plant sexual reproduction. The role of two abundant, pollen-specific AGPs,
AGP6, and AGP11, have been investigated here. The pollen specificity of these proteoglycans suggested that they
are integral to pollen biogenesis and their strong sequence homology indicated a potential for overlapping function.
Indeed, single gene transposon insertion knockouts for both AGPs showed no discernible phenotype. However, in
plants homozygous for one of the insertions and heterozygous for the other, in homozygous double mutants, and in
RNAi and amiRNA transgenic plants that were down-regulated for both genes, many pollen grains failed to develop
normally, leading to their collapse. The microscopic observations of these aborted pollen grains showed
a condensed cytoplasm, membrane blebbing and the presence of small lytic vacuoles. Later in development, the
generative cells that arise from mitotic divisions were not seen to go into the second mitosis. Anther wall
development, the establishment of the endothecium thickenings, the opening of the stomium, and the deposition of
the pollen coat were all normal in the knockout and knockdown lines. Our data provide strong evidence that these
two proteoglycans have overlapping and important functions in gametophytic pollen grain development.

Early germination of Arabidopsis pollen in a double null mutant for the arabinogalactan protein genes AGP6 and AGP11

by Mario Costa

Sílvia Coimbra • Mário Costa • Marta Adelina Mendes •
Ana Marta Pereira • João Pinto • Luís Gustavo Pereira

The pollen specificity of the Arabidopsis
arabinogalactan protein (AGP) genes AGP6 and AGP11
suggests that... more The pollen specificity of the Arabidopsis
arabinogalactan protein (AGP) genes AGP6 and AGP11
suggests that they are integral to pollen biogenesis, and
their high percent of sequence similarity may indicate a
potential for overlapping function. Arabidopsis agp6 agp11
double null mutants have been studied in our laboratory,
and in the present work, we characterize the germination
and growth of its pollen. When compared to wild type,
mutant agp6 agp11 pollen displayed reduced germination
and elongation, both in vivo and in vitro, and precocious
germination inside the anthers, provided that sufficient
moisture was available. This characteristic was not
observed in wild type plants, even in water content con-
ditions which for the mutant were sufficient for pollen
germination. Therefore, an additional distinctive pheno-
typic trait of arabinogalactan proteins AGP6 and AGP11
may be to avert untimely germination of pollen. Such
AGPs may control germination through water uptake,
suggesting an important biological function of this gene
family in pollen.

Is the recovery of microtubule orientation in pea roots dependent on the cell wall?

by Rosemary White

Holdaway NJ, White RG, Overall RL  (1995)  Is the recovery of microtubule orientation in pea roots dependent on the cell wall?  Cell Biology International 19: 913-919

This study tested several aspects of a model proposed by Williamson (1990, 1991) in which stresses in plant cell... more This study tested several aspects of a model proposed by Williamson (1990, 1991) in which stresses in plant cell walls, detected by stress-receptive portions of inelastic cellulose microfibrils, orient microtubules via interactions with cell wall-linked transmembrane proteins. Young expanding cells of pea root tips have highly ordered transverse arrays of microtubules oriented perpendicular to the direction of cell expansion. The recovery of these ordered MT arrays after depolymerisation with oryzalin was assessed. It was shown that treating roots with disruptors of microfibril synthesis (2,6-dichlorobenzonitrile and calcofluor white) or the disruption of Arg-Gly-Asp (RGD)-mediated wall-membrane links did not affect the orientation of recovering microtubule arrays. Furthermore, cell wall stresses themselves appeared unnecessary for regeneration of transverse arrays. The relevance of these findings to Williamson's hypothesis is discussed.

Ultrastructural specializations of the cell-wall sleeve around plasmodesmata

by Rosemary White

Badelt K, White RG, Overall RL, Vesk M  (1994)  Ultrastructural specializations of the cell-wall sleeve around plasmodesmata.  American Journal of Botany  81: 1422-1427

Specialized structures were observed in the sleeve of wall surrounding plasmodesmata in the rhizome tips of... more Specialized structures were observed in the sleeve of wall surrounding plasmodesmata in the rhizome tips of Nephrolepis exaltata and root tips of Spirodela oligorrhiza, Azolla pinnata, and Hordeum vulgare. Material was prepared either by fixation in glutaraldehyde/paraformaldehyde/tannic acid followed by cell wall digestion, or by rapid freezing and freeze substitution, prior to resin embedding. Two structures were identified: rings of electron-opaque material encircling the neck region of plasmodesmata and electron-opaque helices spiraling around the length of plasmodesmata. In some instances, particle-like subunits were observed in the electron-opaque rings. The possible role of these structures as external sphincters involved in the control of permeability of plasmodesmata is discussed.

Dynamics of aerenchyma distribution in the cortex of sulfate-deprived adventitious roots of maize

by Vassilis Protonotarios

Localization of Reactive Oxygen Species and Lignification in Leaves of Young Sulphate-Deprived Maize Plants

by Vassilis Protonotarios

Walls as technologies of government: the double construction of geographies of peace and conflict in Israeli politics, 2002–present

by Samer Alatout

Since 2002, consecutive Israeli governing coalitions have been building a separation wall in the West Bank for the... more Since 2002, consecutive Israeli governing coalitions have been building a separation wall in the West Bank for the declared purposes of security and separation from the Palestinian population. Building on earlier phases of control, which relied on military orders, cantonment, roadblocks, and checkpoints, the wall functions as a regime of government that colonizes Palestinian life by regulating every nexus of body and space and population and territory. Rather than establishing peace, the wall’s regime of government uses separation as a double
construction of peace and conflict that isolates peace on the Israeli side and conflict on the Palestinian side.