The titan (ttn) mutants of Arabidopsis exhibit striking alterations in 
chromosome dynamics and cell division during seed development. Endosperm defects 
include aberrant mitoses and giant polyploid nuclei. Mutant embryos differ in 
cell size, morphology and viability, depending on the locus involved. Here we 
demonstrate that three TTN genes encode chromosome scaffold proteins of the 
condensin (SMC2) and cohesin (SMC1 and SMC3) classes. These proteins have been 
studied extensively in yeast and animal systems, where they modulate chromosome 
condensation, chromatid separation, and dosage compensation. Arabidopsis 
contains single copies of SMC1 and SMC3 cohesins. We used forward genetics to 
identify duplicate T-DNA insertions in each gene. These mutants (ttn7 and ttn8) 
have similar titan phenotypes: giant endosperm nuclei and arrested embryos with 
a few small cells. A single SMC2 knockout (ttn3) was identified and confirmed by 
molecular complementation. The weak embryo phenotype observed in this mutant may 
result from expression of a related gene (AtSMC2) with overlapping functions. 
Further analysis of titan mutants and the SMC gene family in Arabidopsis should 
provide clues to chromosome mechanics in plants and insights into the regulation 
of nuclear activity during endosperm development.