The plant hormone auxin regulates growth and development by modulating the stability of auxin/indole acetic acid (Aux/IAA) proteins, which in turn repress auxin response factors (ARFs) transcriptional regulators. In transient assays performed in immature sunflower embryos, we observed that the Aux/IAA protein HaIAA27 represses transcriptional activation by HaHSFA9, a heat shock transcription factor (HSF). We also found that HaIAA27 is stabilized in immature sunflower embryos, where we could show bimolecular fluorescence complementation interaction between native forms of HaIAA27 and HaHSFA9. An auxin-resistant form of HaIAA27 was overexpressed in transgenic tobacco seeds, leading to effects consistent with down-regulation of the ortholog HSFA9 gene, effects not seen with the native HaIAA27 form. Repression of HSFs by HaIAA27 is thus likely alleviated by auxin in maturing seeds. We show that HSFs such as HaHSFA9 are targets of Aux/IAA protein repression. Because HaHSFA9 controls a genetic program involved in seed longevity and embryonic desiccation tolerance, our findings would suggest a mechanism by which these processes can be auxin regulated. Aux/IAA-mediated repression involves transcription factors distinct from ARFs. This finding widens interpretation of auxin responses. H aHSFA9 and the ortholog factors (HSFA9) are specialized heat shock transcription factors (HSFs) that are expressed only in seeds and perform functions during embryogenesis at normal growth temperature. The heat stress response in plants involves multiple HSFs, but HSFA9 does not have a role in the vegetative response to high temperature (1, 2). In Arabidopsis, transcription of the HSFA9 gene is activated by ABA-insensitive 3 (ABI3), a key regulator controlling late-seed development (2). Target genes of HSFA9 encode different heat shock proteins (HSP) (1-5). Gain of function (3, 4) and loss of function (5) approaches determined that in sunflower (Helianthus annuus L.) and tobacco (Nicotiana tabacum L.), HSFA9 activate transcription of specific small heat-shock protein (shsp) genes. Our previous studies (3-5) indicated that HSFA9 factors are involved in the control of a genetic program that regulates seed longevity and embryonic desiccation tolerance. This program includes genes that encode different HSP but not late embryogenesis abundant (LEA) proteins (3-5). To search for additional transcription factors (TFs) involved in the regulation of this process, we used a yeast two-hybrid system to identify embryo TFs that interact with HaHSFA9. Surprisingly, we found that the auxin/ indole acetic acid (Aux/IAA) protein HaIAA27 interacts with HaHSFA9.Aux/IAA are unstable proteins that are further destabilized in response to the major naturally occurring auxin, indole-3-acetic acid (IAA) (6). Aux/IAA proteins act as nuclear-localized transcriptional repressors of auxin response factors (ARFs) (7). In the current model of Aux/IAA function, auxin alleviates repression of ARFs by inducing Aux/IAA degradation in the 26S proteasome (i.e., refs. 8-10 an...