Tomato (Solanum lycopersicum) HD-Zip I Transcription Factor, SlHZ24, Positively Regulates the Ascorbic Acid Biosynthetic Gene, GDP-D-Man Pyrophosphorylase

Ascorbic acid (AsA) is an antioxidant that can scavenge the reactive oxygen species (ROS) produced when plants encounter stressful conditions. Here, it was revealed by a yeast one-hybrid assay that a tomato (Solanum lycopersicum) HD-Zip I family transcription factor, SlHZ24, binds to the promoter of an AsA biosynthetic gene, SlGMP3. A transient expression system and an electrophoretic mobility shift assay (EMSA) showed that SlHZ24 binds to a regulatory cis-element in the SlGMP3 promoter, and overexpression of SlHZ24 in transgenic tomato lines resulted in increased AsA levels, while suppressing expression of the gene using RNA interference (RNAi) had the opposite effect. These data suggest that SlHZ24 can positively regulate AsA accumulation, and in support of this it was shown that SlGMP3 expression increased in the SlHZ24-overexpressing lines and declined in SlHZ24-RNAi lines. SlHZ24 also affected the expression of other genes in the D-mannose/L-galactose (D-Man/L-Gal) AsA biosynthesis pathway, such as SlGMP4, SlGME2, SlGGP and SlcAPX. The EMSA indicated that SlHZ24 bound to the promoters of SlGME2 and SlGGP, indicating a multiple-targeted regulation of AsA biosynthesis. Finally, SlHZ24-overexpressing plants showed less sensitivity to oxidative stress and so we conclude that SlHZ24 promotes AsA biosynthesis, which in turn enhances oxidative stress tolerance.