Agriculture Reference
In-Depth Information
Microsporogenesis and Pollen Differentiation
The anther hosts the microsporogenesis and male gametogenesis (McCormick
2004
). The spore producing tissue is organized in four microsporangia evolving in
four pollen sacs. Each of the microsporangia is surrounded by the tapetum, the tis-
sue playing important functions in controlling pollen development and in releasing
information essential for proper pollen/stigma interactions. In each microsporan-
gium, the microspore mother cell differentiation occurs. Before meiosis this cells
undergoes a cell wall callose deposition. Tetrads are formed after the meiosis and
the spores will be released from the tetrads upon enzymatic digestion of the callose
deposited on the external wall of the tetraspore. Each free microspore experiences
an asymmetric mitosis forming the male gametophyte made up by two cells, one
large and the other one small, named vegetative and generative cell, respectively.
The generative cell is progressively surrounded by the vegetative cell and later on
undergoes normal division forming the two sperm cells. This can happen at anther
level or after pollination. Referring to the timing of this event, pollen of different
species are usually classified as bi- or tri-nucleated, although at the end of dif-
ferentiation the pollen of all angiosperms are tri-nucleated. The pollen cell wall
is formed by an inner and outer layers, named intine and exine, respectively. The
latter undergoes deep modifications throughout deposition, giving rise to pollen
sculptures which are genotype specific and used as a taxonomic trait to recognize
different species and varieties. The majority of the genes expressed in the pollen are
shared by the sporophytic tissue. It has to be underlined that when the pollen leaves
anthers, the complete transcriptome necessary for pollen germination and pollen
tube development is already present in the vegetative cell.
Macrosporogenesis and Ovule Formation
Female macrosporogenesis occurs in the ovule within the pistils (Yadegari and
Drews
2004
). The ovule originates from the carpellar leaf, in marginal or central
position, from the placental tissue. It is made up by the nucellus surrounded by the
inner and outer integuments, that develop toward the apical part of the nucellus
defining the micropilar region. In opposite position to the micropyle, at the base of
the nucellus, is located the chalaza. Within the nucellus, the megaspore mother cell
after differentiation undergoes meiosis generating four megaspores. Three of these,
located at the micropylar site, undergo degeneration, while the basal one, after three
subsequent mitotic divisions, originates the embryosac (the female gametophyte).
At the beginning, the embryosac is an 8 cell structure composed, moving from the
micropylar to the chalazal region, as follow: in the upper part are the synergids and
between them the egg cell. Below, in the central part are the upper and lower polar
cells whose nuclei fuse shortly after division, and at the chalazal site, the three
antipodal cells. The female gametophyte differentiation is controlled by specific
genes, among which those involved in ovule and integuments development are the
most characterized.
