Abstract Small ubiquitin-like modifier (SUMO) conjugation/deconjugation to heat shock transcription factors regulates DNA binding of the peptides and activation of heat shock protein gene expression that modulates thermal adaptation in metazoans. SIZ1 is a SUMO E3 ligase that facilitates SUMO conjugation to substrate target proteins (sumoylation) in Arabidopsis (Arabidopsis thaliana). siz1 T-DNA insertional mutations (siz1-2 and siz1-3; Miura et al., 2005) cause basal, but not acquired, thermosensitivity that occurs in conjunction with hyperaccumulation of salicylic acid (SA). NahG encodes a salicylate hydroxylase, and expression in siz1-2 seedlings reduces endogenous SA accumulation to that of wild-type levels and further increases thermosensitivity. High temperature induces SUMO1/2 conjugation to peptides in wild type but to a substantially lesser degree in siz1 mutants. However, heat shock-induced expression of genes, including heat shock proteins, ascorbate peroxidase 1 and 2, is similar in siz1 and wild-type seedlings. Together, these results indicate that SIZ1 and, by inference, sumoylation facilitate basal thermotolerance through processes that are SA independent.