Study of microtubules and F-actin during endosperm development revealed distinct developmental domains in the micropylar chamber (MC) containing the embryo, the large curved central chamber (CC), and the small chalazal chamber (ChC). As is characteristic of nuclear endosperm development, the common cytoplasm is organized into nuclear cytoplasmic domains defined by nuclear-based radial microtubule systems which determine placement of walls at the cellularization stage. Both microtubules and F-actin are reorganized in preparation of the syncytium for simultaneous cytokinesis. New findings are 1) the interrelationships between microtubules and F-actin during development in all three chambers, 2) an early stage of syncytial endosperm characterized by unusual fusiform to multangular nuclei sheathed by parallel arrays of microtubules, and 3) organization of a multinucleate cyst of cytoplasm in the ChC that never cellularizes. Comparison of Arabidopsis thaliana and Coronopus didymus shows endosperm development to be a conservative process in mustard seeds. Supported in part by NSF MCB-9726968 to R.C.B and B.E.L. and Louisiana Board of Regents Fellowship LEQSF-1998-03-GF-28 to H.N.

Key words: Arabidopsis, embryology, F-actin, microtubules