We report the cloning and functional characterization of the first heat-shock transcription factor that is specifically expressed during embryogenesis in the absence of environmental stress. In sunflower embryos this factor, HaHSFA9, trans-activated promoters with poor consensus heat-shock cis-elements, including that of the seed-specific Hahsp17.6G1 gene. Mutations that improved the heat-shock cis-element consensus at the Hahsp17.7G4 promoter impaired transient activation by HaHSFA9 in sunflower embryos. The same mutations did not affect heat-shock-induced gene expression of this promoter in transgenic tobacco plants but reduced the developmental activation by endogenous heat-shock transcription factors (HSFs) in seeds. Sunflower, and perhaps other plants such as tobacco, differs from the vertebrate animal systems in having at least one specialized HSF with expression and (or) activation patterns strictly restricted to embryos. Our results strongly indicate that HaHSFA9 is a transcription factor critically involved in the developmental activation of Hahsp17.6G1 and in that of similar target genes as Hahsp17.7G4.In eukaryotes, the heat-shock response and some developmental processes are under the control of a family of conserved DNA-binding proteins known as the heat-shock transcription factors (HSFs). 1 Although in some systems, as in Drosophila melanogaster, this regulation involves a single HSF (1), multigenic families of HSFs participate in vertebrate and in plant systems. These families have different sizes, which, together with particular gene expression and activation patterns for the HSFs, might have consequences in the degree of overlapping of regulatory functions mediated by these factors. The specific role of the different HSFs is mostly unknown, particularly for the plant HSFs, and for involvement in developmental processes, as the regulation of gene expression during embryogenesis (See for example, Ref. 2 and the reviews in Refs. 3 and 4).In vertebrate systems, three different HSFs (HSF1, HSF2, and HSF3) have ubiquitous expression patterns (for example, Refs. 5 and 6 and the review in Ref.3). A fourth HSF found in humans displays tissue-specific expression patterns, which suggested specialized functions but not related to embryogenesis (HSF4, Ref. 7). Plants contain the highest number of HSF genes in eukaryotes. This is inferred from in silico analyses from the fully sequenced Arabidopsis thaliana model and from functional analyses of different cloned HSFs in tomato, Arabidopsis, and other plants (reviewed in Refs. 4 and 8 and references therein). Plant HSFs share unique structural and phylogenetic relationships compared with the vertebrate HSFs (9). Fifteen of the 21 putative HSFs from A. thaliana thus contain an insertion of 21 amino acid residues in the oligomerization domain (characteristic of the plant class A HSFs), whereas class B HSFs have no such insertion. Gene expression studies for plant HSFs are very scarce, with only fragmentary data at the mRNA level and even scarcer reports for prote...