Plants have evolved to cope with changing environmental conditions. One way 
plants achieve this is through post-translational modification of target 
proteins by ubiquitination and SUMOylation. These post-translational modifiers 
(PMs) can alter stability, protein-protein interactions, and the overall fate of 
the protein. Both of these systems have remarkable similarities in terms of the 
process leading to attachment of the PM to its substrate : having to undertake 
activation, conjugation, and finally ligation to the target. In the ubiquitin 
system, there are a vast number of ubiquitin ligase enzymes (E3s) that provide 
specificity for the attachment of ubiquitin. With the SUMO system, only a small 
number of SUMO E3 ligases have so far been identified in the fully sequenced 
plant genomes. In Arabidopsis thaliana, there are only two SUMO E3s, compared to 
over 1400 ubiquitin E3s, a trend also observed in crop species such as Oryza 
sativa and Zea mays Recent research indicates that removing SUMO from its 
substrate by the enzymatically active SUMO proteases is a vital part of this 
system. A class of SUMO proteases called ubiquitin-like proteases (ULPs) are 
widespread in all eukaryotes; within plants, both monocot and dicot kingdoms 
have conserved and divergent ULPs and ULP-like proteases. This paper examines 
the roles ULPs have in stress responses and highlights the 'fine-tuning' of SUMO 
attachment/removal in balancing growth versus stress.