AbstractVitamin C deficiency in the Arabidopsis mutant vtc1 causes slow growth and late flowering. This is not attributable to changes in photosynthesis or increased oxidative stress. We have used the vtc1 mutant to provide a molecular signature for vitamin C deficiency in plants. Using statistical analysis, we show that 171 genes are expressed differentially in vtc1 compared with the wild type. Many defense genes are activated, particularly those that encode pathogenesis-related proteins. Furthermore, transcript changes indicate that growth and development are constrained in vtc1 by the modulation of abscisic acid signaling. Abscisic acid contents are significantly higher in vtc1 than in the wild type. Key features of the molecular signature of ascorbate deficiency can be reversed by incubating vtc1 leaf discs in ascorbate. This finding provides evidence that many of the observed effects on transcript abundance in vtc1 result from ascorbate deficiency. Hence, through modifying gene expression, vitamin C contents not only act to regulate defense and survival but also act via phytohormones to modulate plant growth under optimal conditions.