Thermogenesis in plants involves significant increases in their cyanide-resistant mitochondrial alternative oxidase (AOX) capacity. Because AOX is a non-proton-motive ubiquinol oxidase, the dramatic drop in free energy between ubiquinol and oxygen is dissipated as heat. In the thermogenic skunk cabbage (Symplocarpus renifolius), SrAOX is specifically expressed in the florets. Although SrAOX harbours conserved cysteine residues, the details of the mechanisms underlying its redox regulation are poorly understood. In our present study, the two mitochondrial thioredoxin o cDNAs SrTrxo1 and SrTrxo2, were isolated from the thermogenic florets of S. renifolius. The deduced amino acid sequences of the protein products revealed that SrTrxo2 specifically lacks the region corresponding to the α3-helix in SrTrxo1. Expression analysis of thermogenic and non-thermogenic S. renifolius tissues indicated that the SrTrxo1 and SrAOX transcripts are predominantly expressed together in thermogenic florets, whereas SrTrxo2 transcripts are almost undetectable in any tissue. Finally, functional in vitro analysis of recombinant SrTrxo1 and mitochondrial membrane fractions of thermogenic florets indicated its reducing activity on SrAOX proteins. Taken together, these results indicate that SrTrxo1 is likely to play a role in the redox regulation of SrAOX in S. renifolius thermogenic florets.