Biology Reference
In-Depth Information
groups of four, to investigate how pollen ontogeny may explain the rise and
establishment of this character. We followed pollen development using immu-
nolocalization and cytochemical characterization of changes occurring from
anther differentiation to pollen dehiscence.
Results
Our results show that, following tetrad formation, a delay in the dissolution
of the pollen mother cell wall and tapetal chamber is a key event that holds
the four microspores together in a confined tapetal chamber, allowing them to
rotate and then bind through the aperture sites through small pectin bridges,
followed by joint sporopollenin deposition.
Conclusion
Pollen grouping could be the result of relatively minor ontogenetic changes
beneficial for pollen transfer or/and protection from desiccation. Comparison
of these events with those recorded in the recent pollen developmental mutants
in Arabidopsis indicates that several failures during tetrad dissolution may
convert to a common recurring phenotype that has evolved independently sev-
eral times, whenever this grouping conferred advantages for pollen transfer.
Background
Pollen development is a well characterized and highly conserved process in
flowering plants [1-3]. Typically, following anther differentiation, a sporog-
enous tissue develops within the anthers producing microsporocytes or pollen
mother cells. Prior to meiosis, pollen mother cells become isolated by a wall
with the deposition of a callose layer. Each pollen mother cell, as the result
of the two meiotic divisions, generates four haploid cells forming a tetrad
and, for a short time, these four sibling microspores are held together in a
persistent pollen mother cell wall that is surrounded by callose. The tapetum
then produces an enzyme cocktail that dissolves the pollen mother cell wall
and the microspores are shed free and become independent [2]. The unicel-
lular microspores go through an asymmetric mitotic division (pollen mitosis
I) to produce a pollen grain with two cells, a larger vegetative cell that hosts a
smaller generative cell; the latter will divide once more to produce two sperm
cells (Pollen mitosis II). Pollen mitosis II can take place before or after pollen
release and, depending on when it occurs, the pollen will be bicellular or tri-
cellular at the time of anther dehiscence. Throughout the manuscript we will
use the term “pollen tetrads” for mature pollen to avoid confusion with the
tetrads of early developmental stages (“microspore tetrads”).
