The specialized chalazal endosperm in Arabidopsis thaliana and Lepidium virginicum (Brassicaceae)

Endosperm of the nuclear type initially develops into a large multinucleate syncytium that lines the central cell. This seemingly simple wall-less cyt oplasm can, however, be highly differentiated. In developing seeds of membe rs of the family Brassicaceae the curved postfertilization embryo sac compr ises three chambers or developmental domains. The syncytium fills the micro pylar chamber around the embryo, spreads as a thin peripheral layer surroun ding a large central vacuole in the central chamber, and is organized into individual nodules and a large multinucleate cyst in the chalazal tip. Late r in development, after the endosperm has cellularized in the micropylar an d central chambers, the chalazal endosperm cyst remains syncytial and shows considerable internal differentiation. The chalazal endosperm cyst consist s of a domelike apical region that is separated from the cellularized endos perm by a remnant of the central vacuole and a basal haustorial portion whi ch penetrates the chalazal proliferative tissue atop the vascular supply. I n the shallow chalazal depression of Arabidopsis thaliana, the cyst is mush room-shaped with short tentacle-like processes penetrating the maternal tis sues. The long narrow chalazal channel of Lepidium irginicum is filled by a n elongate stalklike portion of the cyst. In both, the dome contains a laby rinth of endoplasmic reticulum, dictyosomes with associated vesicles, nucle i, and plastids. The basal portions, which lack the larger organelles, exhi bit extensive wall ingrowths and contain parallel arrays of microtubules. T he highly specialized ultrastructure of the chalazal endosperm cyst and its intimate association with degrading chalazal proliferative cells suggest a n important role in loading of maternal resources into the developing seed.