Biology Reference
In-Depth Information
Degeneria vitiensis [33] in the Magnoliales, Laurelia novae-zelandiae [34] and
Liriodendron tulipifera [35] in the Laurales or Asarum in the Piperales [30] as
well as in monocots as Sisyrinchium [36].
Starch accumulated prior to meiosis and the first pollen mitosis and vanished
with the onset of these two divisions; this also occurred 6 days before anther de-
hiscence, preceding the shedding of starchless pollen. The accumulation of starch
in PMC has also been reported in other primitive angiosperms, such as Anaxag-
orea brevipes [37] or Austrobaileya maculata [38], and in other evolutionarily
more recent angiosperms [39,40]. Vacuolization also follows a conserved pattern
[41,42]. The cytoplasm enlarges through a first vacuolization and, later on, fol-
lowing the first pollen mitosis, small vacuoles appear as starch builds up.
In most species a dehydration process takes place prior to pollen shedding and
starch is hydrolyzed to form sucrose that protects pollen against desiccation [43].
Starchless pollen is the most common pollen type in the angiosperms [44], being
more frequent in bicellular than in tricellular pollen [45,46]. In A. cherimola,
pollen is shed in a highly hydrated stage [47] and this lack of dehydration may
explain why the second mitotic division continues in free pollen after pollen shed-
ding, producing a mixed population of bi and tricellular pollen [48]. However,
both types of pollen are starchless at anther dehiscence.
Pollen Cohesion
Several reasons could account for the release of pollen in groups of four. In Arabi-
dopsis, failure of different enzymes during the dissolution of the pectic layer that
surrounds the PMC wall has been reported in two quartet mutants [10,49]. In
our work the immunolocalization of esterified and non-esterified pectins showed
that, although they were clearly present in the PMC wall, the pectins disappeared
following tetrad formation. A closer examination of the photographs reveals that
the PMC wall, which stains for cellulose, remained beyond the tetrad stage. In-
terestingly quartet mutants of Arabidopsis also show a defect in the degradation
of the PMC wall [10]. Cellulase has also been shown to be involved in the break-
down process of the PMC wall [50] and a delay in its action could lead to this
phenomenon. However, this failure does not seem permanent, since 25 days later
this wall is completely dissolved. Thus, the permanence of the PMC wall appears
as a key factor contributing to pollen grouping as pollen tetrads in A. cherimola,
similar to the observations in Arabidopsis mutants. A mixture of enzymes is re-
quire to break down the complex PMC wall [2], and a failure of one or more of
these enzymes could result in a similar final result.
Different events that take place during the retention of this wall may explain
pollen adherence once this wall disappears. The confining of pollen within the
