Reversible post-translational modification by small ubiquitin-related modifiers (SUMOs) regulates the activity and degradation of numerous proteins during various eukaryotic developmental processes. Now, Thomas Dresselhaus and colleagues describe the important role that a diSUMO-like protein called ZmDSUL plays in female gametophyte (embryo sac) development in maize (see p. ). The researchers identify ZmDSUL by searching maize egg-cell-specific expressed sequence tag data for SUMO-related proteins. They show that DSUL, which has a unique structure consisting of two head-to-tail SUMO-like domains, has a specific expression pattern during maize embryo sac development and early embryogenesis. Finally, using RNAi silencing, they show that DSUL is required for female gametophyte viability and for the correct segregation and positioning of the nuclei that are produced by mitotic nuclear division in the developing embryo. Thus, they conclude, DSUL is essential for female gametophyte maturation in maize because it ensures the correct positioning of the nuclei, a prerequisite for cell specification (see also the review on female gametophyte development by Sundaresan and Alandete-Saez on p. ).
