A Seed-specific Heat-shock Transcription Factor Involved in Developmental Regulation during Embryogenesis in Sunflower

Abstract

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...

Role of Heat Stress Transcription Factor HsfA5 as Specific Repressor of HsfA4

“…Although our knowledge is still very limited, functional diversification seems to be the main reason for the coexistence of more than 20 Hsfs in plants. Remarkable cases of specialization by selective expression were reported for HsfA2 as a strongly hs-induced protein (10,11) and for HsfA9 with exclusive expression during seed maturation (20,37). 3 On the other hand, the well studied examples of three tomato Hsfs, A1, A2, and B1, impressively illustrated the extent of functional diversification and cooperation of Hsfs.…”

“…They represent recognition sites for heat stress transcription factors (Hsf), which are the key regulators of the hs response (3)(4)(5)(6)(7)(8). In contrast to other organisms, plant genomes encode extraordinarily complex Hsf families both in terms of the total number of genes (usually more than 20) as well as in terms of the structural and functional diversification of the Hsfs. There is good evidence that the Hsf mixture in cells strongly changes in a tissue-specific manner and in response to stress treatments (Refs.…”

“…Detailed investigations on the structural and functional diversification so far are restricted to Hsfs A1, A2, A3, A9, and B1 (10,11,(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). 3 An interesting new aspect of functional diversification of Hsfs was reported when a mutant of rice (spl7) with spontaneous necrotic lesions in mature leaves was identified as a point mutation with a Trp Ͼ Cys substitution in the N-terminal part of the DNA binding domain of HsfA4d.…”

Repression by an auxin/indole acetic acid protein connects auxin signaling with heat shock factor-mediated seed longevity

“…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).…”

“…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)(2)(3)(4)(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.…”