Protein sumoylation plays an important role in plant development, flowering-time regulation, and abiotic stress response. However, the molecular role of sumoylation in these pathways is largely unknown. It was shown previously that in mutants of the inner nuclear basket nucleoporin NUA a large increase in the abundance of high-molecular weight SUMO conjugated proteins correlated with nuclear retention of bulk mRNA. Here, the connection between sumoylation and mRNA export in plants was further investigated. Both SUMO-conjugate accumulation and mRNA retention were also found in a second nucleoporin mutant that does not affect NUA, and SUMO conjugates accumulated predominantly in the nucleus. Similarly, after heat and ethanol treatment, two abiotic stress treatments known to lead to the accumulation of sumoylated proteins, nuclear mRNA was retained. To establish a causal relationship between sumoylation and mRNA export, mutations in two enzymes in the SUMO pathway were tested. Mutating either SUMO E3 ligase or SUMO isopeptidase lead to nuclear mRNA retention, indicating that both an increase and a decrease in the pool of sumoylated nuclear proteins blocks mRNA export. Together, these data show that sumoylation acts upstream of mRNA export in plants, likely through the transient sumoylation status of one or more factors involved in mRNA trafficking.