Sl-ERF2, a Tomato Ethylene Response Factor Involved in Ethylene Response and Seed Germination

Ethylene response factors (ERFs) are plant transcriptional regulators mediating ethylene-dependent gene expression via binding to the GCC motif found in the promoter region of ethylene-regulated genes. We report here on the structural and functional characterization of the tomato Sl-ERF2 gene that belongs to a distinct class of the large ERF gene family. Both spliced and unspliced versions of Sl-ERF2 transcripts were amplified from RNA samples and the search in the public tomato expressed sequence tag (EST) database confirmed the existence of the two transcript species in a number of cDNA libraries. The unspliced transcript contains two open reading frames yielding two hypothetical proteins, a small highly truncated version lacking the APETALA2 domain and a bigger protein lacking the N-terminal MCGGAAI(I)/(L) consensus peptide specific to ERF members from subfamily IV. Nevertheless, functional Sl-ERF2 protein may only derive from spliced transcripts since, depending on the tissue, the level of the spliced transcript is much higher than that of the unspliced transcript. Sl-ERF2 is expressed in all plant tissues tested, though its transcript accumulates preferentially in germinating seeds and ripening fruit. Overexpression of the Sl-ERF2 gene in transgenic tomato lines results in premature seed germination and enhanced hook formation of dark-grown seedlings, which is indicative of increased ethylene sensitivity. The expression of the mannanase2 gene is upregulated in Sl-ERF2-overexpressing seeds, suggesting that Sl-ERF2 stimulates seed germination through the induction of the mannanase2 gene. It is noteworthy that the exaggerated hook phenotype is abolished when ethylene perception is blocked, strongly suggesting that Sl-ERF2 requires other ethylene-dependent components to impact the hook formation process.