282 J.M. Seguí-Simarro et al.
by fusing with the remaining cell plate assembly sites in the division plane
until the four new callosic cell walls are completed (Fig. 11E). Interestingly,
no dynamin rings have been identified in the wide tubular networks of the
cellularizing microsporocytes.
Transformation of the syncytial-type wide tubular networks into fenes-
trated sheets also seems to involve exploitation of the spreading forces as-
sociated with callose deposition along the cell plate membranes as in the
somatic-type cytokinesis systems (Otegui and Staehelin 2000b). However,
unlike in somatic-type cytokinesis, the callose deposits produced in the syn-
cytial walls persist after completion of the cell walls. There are several reasons
why endosperm callose-rich walls might be advantageous for these cell types.
For example, in contrast to cellulose, callose can be readily broken down into
its glucose building blocks by specific cell wall enzymes (Verma and Hong
2001). This property makes the callose-rich endosperm cell walls suitable for
use as carbohydrate reserves for developing embryos, as is the case inAra-
bidopsis, where most of the endosperm tissue is consumed by the embryo
during seed development. In addition, callose has gel-like properties that can
confer high plasticity to the endosperm cell walls, allowing for the spatial
remodeling of the endosperm around the expanding embryo. In the case of
the post-meiotic tetrad, the callose walls that break the symplastic connec-
tions and isolate the microspores from each other seems to be essential for
the expression of the gametophytic genome without interference either from
the parental sporophyte or from neighboring microspores (Shivanna et al.
1997). In addition, callose walls can be locally degraded to make room for the
primexine layer, the blueprint of the exine, which is deposited on the outer
surface of the microspores (Shivanna et al. 1997). Massive callose degradation
by callose-specific hydrolytic enzymes (callases) secreted from the surround-
ing tapetal cells is the mechanism that permits the release of the microspores
from the tetrad during maturation.
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