Endosperm development

Endosperm stores proteins and carbohydrates for use during seed germination. It provides a nutritive role in seed development but may get mostly absorbed by the embryo before desiccation.  Endosperm is a source of hormones and signals that partly direct embryogenesis, and acts as mechanical support for the developing embryo.

There are four general phases of endosperm development: syncytial, cellularization, differentiation, and death. Endosperm development is initiated by successive divisions of the triploid nucleus. During initial stages of development, the endosperm is a syncytium (a multinucleated cell). Later, each nucleus becomes isolated by the development of cell walls (cellularization). Cellularization is followed by a differentiation of functional tissues. Eventually, most endosperm cells degenerate during seed maturation.

Interaction between embryo and endosperm.

In maize, it is possible to obtain endosperm by in vitro fusion of sperm and central cells; the endosperm follows a typical pattern of development suggesting that embryo development or the presence of maternal tissues is not necessary for endosperm development.

However in Arabidopsis cdc2 mutant (Nowack et al., 2006), the symmetric division of the generative cell is blocked such that the mature pollen grain has only one instead of two generative cells.  The one sperm cell of cdc2 pollen always fertilizes the egg, not the polar nuclei [(k) shows GUS expression only in embryo from a marker gene introduced in the cdc2 pollen; (j) shows expression in both embryo and endosperm of same marker introduced in wild-type pollen).  Fertilization of egg cell is sufficient to initiate proliferation of endosperm [stars in (k) mark endosperm nuclei].  So the endosperm has to be sensitive to the state of the fertilization state of the egg cell and one explanation is that the zygote sends a positive signal to the endosperm to begin proliferating. The endosperm stopped proliferating (seed abortion), indicating that the signal wasn't sufficient to maintain the proliferation.