The rice SUMO conjugating enzymes OsSCE1 and OsSCE3 have opposing effects on drought stress.

Posttranslational modification of proteins by the small ubiquitin-related modifier (SUMO) protein is involved in diverse cellular processes. In sumoylation, SUMO-conjugating enzyme (SCE) conjugates SUMO to substrate proteins. Similarly to yeast and animals, Arabidopsis encodes a single SCE gene, but other plants encode at least two SCE genes. In this study, we report the molecular characterization of three Oryza sativa SCE genes. Their levels of expression are commonly upregulated by drought stress but are differentially regulated by hormones and sugars. Only the OsSCE1 gene showed photoperiod- and light-dependent diurnal oscillations in the leaves. Yeast two-hybrid assays showed that OsSCEs do not show SUMO isoform specificity. Three rice OsSCE proteins localize primarily to the nucleus. Interestingly, OsSCE1 is distributed in specific parts of the nucleus and shows sumoylation activities in the absence of a SUMO ligase in E. coli. In addition, overexpression of the OsSCE1 gene alters the biomass and grain yield parameters in transgenic rice plants. Overexpression of the OsSCE3 gene in transgenic rice plants enhances drought stress tolerance. In contrast, OsSCE1-OX transgenic rice plants are hypersensitive to drought stress. Our results suggest that these genes may be involved in different cellular processes.