Regulation of tocopherol biosynthesis through transcriptional control of tocopherol cyclase during cold hardening in Eucalyptus gunnii

A clone very similar to the maize sucrose export defective (SXD1) gene encoding a tocopherol cyclase (TC) was isolated from a Eucalyptus cold-acclimation downregulated cDNA library and was designated EgSXD1 (Eucalyptus gunnii sucrose export defective 1). As expected, the synthesis of tocopherols (vitamin E) from 2,3-dimethyl-5-phytyl-1,4-benzoquinol by the recombinant protein demonstrated that the EgSXD1 gene product is a TC. Northern hybridizations with RNA isolated from cell suspension cultures clearly showed a fast EgSXD1 downregulation by cold. This strong repression also evidenced by real-time reverse transcription polymerase chain reaction analysis on plantlets was associated with both an increased cold tolerance and a slight but significant decrease in leaf tocopherol content. These results, contrary to the commonly reported tocopherol accumulation after drought or salt stress, raise the question of the role of this gene regulation in cold hardening, which is clearly different from a direct antioxidant protection.