THE TITAN MUTANTS OF ARABIDOPSIS ARE DISRUPTED IN MITOSIS AND CELL-CYCLE CONTROL DURING SEED DEVELOPMENT

We describe in this report a novel class of mutants that should facili tate the identification of genes required for progression through the mitotic cell cycle during seed development in angiosperms. Three non-a llelic titan (ttn) mutants with related but distinct phenotypes are ch aracterized. The common feature among these mutants is that endosperm nuclei become greatly enlarged and highly polyploid. The mutant embryo is composed of a few giant cells in ttn1, several small cells in ttn2 ,and produces a normal plant in ttn3. Condensed chromosomes arrested a t prophase of mitosis are found in the free nuclear endosperm of ttn1 and ttn2 seeds. Large mitotic figures with excessive numbers of chromo somes are visible in ttn3 endosperm. The ttn1 mutation appears to disr upt cytoskeletal organization because endosperm nuclei fail to migrate to the chalazal end of the seed. How double fertilization leads to th e establishment of distinct patterns of mitosis and cytokinesis in the embryo and endosperm is a central question in plant reproductive biol ogy. Molecular isolation of TITAN genes should help to answer this que stion, as well as related issues concerning cell cycle regulation, chr omosome movement and endosperm identity in angiosperms.