A passive repression mechanism that hinders synergic transcriptional activation by heat shock factors involved in sunflower seed longevity.

Different genetic programs that are active during late stages of zygotic embryogenesis are involved in seed phenotypes as longevity, basal thermotolerance, and desiccation tolerance. In sunflower, Heat Shock Factor A9 (HaHSFA9) controls one of such programs, the HaHSFA9 program. Overexpression of HaHSFA9 in seeds of transgenic tobacco showed the involvement of this transcription factor in basal thermotolerance and longevity; HaHSFA9 activated a genetic program that includes specific small Heat Shock Protein (sHSP) genes, which are expressed mainly (or exclusively) during zygotic embryogenesis in seeds (Prieto-Dapena et al., 2006). We also used an active-repressor dominant-negative (DN) form of HaHSFA9, and thus we obtained a substantial reduction of both the accumulation of specific sHSPs and of seed longevity (Tejedor-Cano et al., 2010). In contrast, transcription-inactive DN forms were inefficient. We therefore inferred the participation of additional HSF(s) in controlling the genetic program activated by HaHSFA9, rather than involvement of a single master control HSF (i.e., HaHSFA9 only). These additional HSF(s) would belong to class A, as HaHSFA9 (Tejedor-Cano et al., 2010). Furthermore, we obtained a similar reduction of seed longevity upon the overexpression of stabilized forms of HaIAA27, an Aux/IAA protein that represses HaHSFA9 in sunflower embryos. That observation connected plant seed longevity with auxin hormone action and lead us to predicting that the additional HSF(s) involved in the activation of the HaHSFA9 program should be also repressed by HaIAA27 (Carranco et al., 2010). Here, we identify HaHSFA4a as one of these additional HSFs. We demonstrate a transcriptional synergism between HaHSFA4a and HaHSFA9. Furthermore, we provide evidence for a, novel, passive repression mechanism of this synergism by HaIAA27.