DiSUMO-like DSUL is required for nuclei positioning, cell specification and viability during female gametophyte maturation in maize

Reversible post-translational modification of numerous proteins by small ubiquitin-related modifiers (SUMOs) represents a major regulatory process in various eukaryotic cellular and developmental processes. To study the role of sumoylation during female gametophyte (FG) development in maize, we identified Zea mays genes encoding SUMO (ZmSUMO1a and ZmSUMO1b) and a diSUMO-like protein called ZmDSUL that contains two head-to-tail SUMO-like domains. Whereas ZmSUMO1a and ZmSUMO1b are almost ubiquitously expressed, ZmDSUL transcripts were detected exclusively in the egg apparatus and zygote. ZmDSUL was selected for detailed studies. ZmDSUL is processed close to the C-terminus, generating a dimeric protein that is similar to animal FAT10 and ISG15, which contain two ubiquitin-like domains. Whereas GFP fused to the ZmDSUL N-terminus was located in the cytoplasm and predominately in the nucleoplasm of some transiently transformed maize suspension cells, C-terminal GFP fusions exclusively accumulated at the nuclear surface. GFP or ZmDSUL-GFP under control of the ZmDSUL promoter first displayed GFP signals in the micropylar-most position of the FG at stage 5/6, when migration of polar nuclei and cellularization occurs. Mature FGs displayed GFP signals exclusively in the egg cell, but the strongest signals were observed shortly after fertilization and disappeared during the first asymmetric zygotic division. RNAi silencing of ZmDSUL showed that it is required for FG viability. Moreover, nuclei segregation and positioning defects occurred at stage FG 5 after mitotic nuclear divisions were completed. In summary, we report a diSUMO-like protein that appears to be essential for nuclei segregation and positioning, the prerequisite for cell specification during FG maturation.