The 5′ untranslated region of the FAD3 mRNA is required for its translational enhancement at low temperature in Arabidopsis roots

The FAD3 protein is the main ω-3 desaturase for maintaining the C18:3 content of membrane fatty acids in Arabidopsis roots. Previous studies have shown that the expression of the FAD3 gene can be upregulated mainly at the translational level upon temperature decreases. In an attempt to unveil the mechanism underlying the low temperature-induced translational increase (LTTI), we investigated the role of the untranslated regions (UTRs) in the regulation of the LTTI. The 5′ UTR and the 3′ UTR derived from the ArabidopsisFAD3 mRNA were ligated alone or in combination to the GUS reporter sequences of pBI121 to generate different transgenic Arabidopsis lines. Despite of a decrease in the GUS mRNA level, the GUS enzyme activity in the lines containing the 5′ UTR displayed a 1.6–2.0-fold increase upon low temperature induction (from 22 °C to 10 °C). No significant translational difference was observed in the lines containing the 3′ UTR alone. Furthermore, a series of 5′ UTR deletion constructs were created to identify the necessary sequences required for the high level of translation of the FAD3 mRNA. We found that the nucleotide region from −80 to −6 of the FAD3 5′ UTR was required for the LTTI. These results suggested that the low temperature-induced translational regulation was mediated by the 5′ UTR of the FAD3 mRNA.