The Endoreduplication Cell Cycle: Regulation and Function 83
development progresses initially through distinct stages of acytokinetic mi-
tosis, cellularization, and mitosis coupled to cell division. From around
8–10 days after pollination (DAP), central endosperm cells gradually switch
to an endoreduplication cell cycle. By 20 DAP, ploidy levels reach and can ex-
ceed 96–192C, indicating up to six or more endoreduplication cycles (Larkins
et al. 2001; Sabelli et al. 2005b) (Fig. 1). Considerable attention has been paid
to the regulation of the endoreduplication phase of maize endosperm devel-
opment for several reasons. First, it provides an excellent system in which
to study how plant cells transition from a mitotic to an endoreduplication
cell cycle. Second, this phase of endosperm development is correlated with
dramatic enlargements in nuclear and cell size, the accumulation of storage
compoundssuchasstarchandstorageproteins,andtherapidgrowthofthe
caryopsis. Third, endoreduplication also occurs during endosperm develop-
mentinrice,wheat,andsorghum,andthusthestudyofendoreduplication
in maize endosperm is likely to provide a blueprint that can be applied to all
cereal crops.
Fig. 1 Endoreduplication during maize endosperm development. Schematic diagrams of
the mitotic and endoreduplication cell cycles are superimposed on micrographs of kernels
at 5 and 15 days after pollination (DAP), respectively. Nuclei are stained with propidium
iodide. Nuclear, cell, and endosperm sizes are clearly correlated with endoreduplication.
Factors that appear up- ()ordown-() regulated as cells switch from a mitotic to an
endoreduplication cell cycle are listed in boxes. Thebottom panelsshow nuclear ploidy
classes at different stages of endosperm development. Nu, nucellus; Em, embryo; En,
endosperm; Pe, pericarp; SAl, subaleurone layer; CSEn, central starchy endosperm; Pl,
placentochalaza; TC, transfer cells