SummaryTransient expression analyses in sunflower embryos demonstrated that ABI3, a seed‐specific transcription factor from Arabidopsis, activated chimaeric genes with the Ha hsp17.7 G4 promoter. Nucleotide substitutions at crucial positions of heat shock cis‐elements established that they are required for the transcriptional activation involving ABI3. Trans‐activation with Lp‐HSFA1, a heat shock factor from tomato, reproduced the activation patterns of wild‐type and mutant promoters observed with ABI3. In addition, ABI3 and Lp‐HSFA1 syner‐gistically activated the Ha hsp17.7 G4 promoter, but only when it contained the intact proximal and distal heat shock cis‐elements. The activation domain of Lp‐HSFA1 was necessary for promoter activation. An amino terminal deletion of ABI3 had dominant negative effects on activation by Lp‐HSFA1. We failed to detect a substantial transcriptional activation by ABI3 in the absence of either functional heat shock factors or heat shock elements (HSEs). Furthermore, the wild‐type, but not the mutant HSEs (from − 136 to − 49 in Ha hsp17.7 G4) were sufficient, in the context of a − 46 CaMV 35S promoter, to support activation by Lp‐HSFA1, or Lp‐HSFA1 and ABI3. These results demonstrate, for the first time, transcriptional activation of a heat shock protein promoter by ABI3. We also suggest that ABI3 functions as a transcriptional co‐activator through heat shock factors.