Small ubiquitin-like modifier (SUMO) is a small (∼12kDa) protein that occurs in 
all eukaryotes and participates in the reversible posttranslational modification 
of target cellular proteins. The three-dimensional structure of SUMO and 
ubiquitin (Ub) are superimposable although there is very little similarity in 
their primary amino acid sequences. In all organisms, conjugation and 
deconjugation of Ub and SUMO proceed by the same reactions while using 
pathway-specific enzymes. SUMO conjugation in plants is a part of the controls 
governing important biological processes such as growth, development, flowering, 
environmental (abiotic) stress responses, and response to pathogen infection. 
Most of the evidence for this comes from genetic analyses. Recent efforts to 
dissect the function of sumoylation have focused on uncovering targets of SUMO 
conjugation by using either a yeast two-hybrid screen employing components of 
the SUMO cycle as bait or by using affinity purification of SUMO-conjugated 
proteins followed by identification of these proteins by mass spectrometry. This 
chapter reviews the current knowledge regarding sumoylation in plants, with 
special focus on the model plant Arabidopsis thaliana.