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 endos perm development reveal distinct domains in the micropylar chamber (MC) con taining the embryo, the large curved central chamber (CC), and the small ch alazal chamber (ChC). Reported for the first time in any endosperm are the following: (1) data on the interrelationships between microtubules and F-ac tin during development in all three chambers, and (2) an early stage of syn cytial endosperm characterized by unusual fusiform to multangular nuclei sh eathed by parallel arrays of microtubules. As is characteristic of nuclear endosperm development, the common cytoplasm is organized into nuclear cytop lasmic domains (NCDs) defined by radial microtubule systems, which determin e placement of walls at the cellularization stage. Both microtubules and F- actin are reorganized in preparation of the syncytium for simultaneous cyto kinesis. Cellularization directly follows formation of adventitious phragmo plasts in the MC but is delayed in the CC as polarized NCDs elongate and an ticlinal walls grow centripetally, thus forming a layer of open-ended alveo li 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 o r that cellularizes irregularly very late in seed development but remains d istinct from the remainder of the endosperm. Comparison of Arabidopsis thal iana and Coronopus didymus shows endosperm development to be a highly conse rvative process in mustard seeds.