Patterns of cytoskeletal organization reflect distinct developmental domains in endosperm of Coronopus didymus (Brassicaceae)

New data on the interrelationships of F-actin and microtubules during endosperm development reveal distinct domains in the micropylar chamber (MC) containing the embryo, the large curved central chamber (CC), and the small chalazal chamber (ChC). Reported for the first time in any endosperm are the following: (1) data on the interrelationships between microtubules and F-actin during development in all three chambers, and (2) an early stage of syncytial endosperm characterized by unusual fusiform to multangular nuclei sheathed by parallel arrays of microtubules. As is characteristic of nuclear endosperm development, the common cytoplasm is organized into nuclear cytoplasmic domains (NCDs) defined by 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. Cellularization directly follows formation of adventitious phragmoplasts in the MC but is delayed in the CC as polarized NCDs elongate and anticlinal walls grow centripetally, thus forming a layer of open-ended alveoli overtopped by the advancing front of syncytial cytoplasm containing the adventitious phragmoplasts. Periclinal divisions in the alveoli cut off the first layer of true cells and displace the alveoli inward. Repeated cycles of alveolation and periclinal division complete initial cellularization of endosperm in the CC. The ChC becomes occupied early on by a multinucleate cyst of cytoplasm that fails to organize NCDs and that never cellularizes or that cellularizes irregularly very late in seed development but remains distinct from the remainder of the endosperm. Comparison of Arabidopsis thaliana and Coronopus didymus shows endosperm development to be a highly conservative process in mustard seeds.