Atmospheric CO2 concentration is predicted to reach 800 ppm by 2100 and will involve shifts in plant growth rates as well as influences on plant developmental timing. The direction and magnitude of flowering time shifts in response to rising CO2 are variable within species, ranging from large accelerations through major delays, with some genotypes showing no change at all.  In addition, it has been challenging to understand the genetic mechanisms driving shifts in flowering time in response to increasing CO2, and often times these are not predictable by simply extrapolating changes in plant growth rates to the onset of flowering (i.e., plant size at flowering may shift under elevated CO2 conditions). Furthermore, adaptive change over evolutionary time scales may further alter developmental timing in response to elevated CO2, since changes in fitness stemming from new developmental patterns may drive strong selective pressures in a given environment.       In our work, we found that an Arabidopsis thaliana genotype previously selected for high fitness at elevated CO2 (SG) showed delayed flowering and larger size at flowering when grown at elevated (700 ppm) versus current (380 ppm) CO2. This response was correlated with prolonged expression of FLOWERING LOCUS C (FLC), a vernalization-responsive floral repressor gene. To determine if FLC was directly driving the delays in flowering at elevated CO2 in SG, we used vernalization treatments (extended cold) to independently downregulate FLC expression. We hypothesized that vernalization would eliminate delayed flowering (defined as leaf number at flowering) at elevated CO2 through the direct reduction of FLC expression, eliminating differences in flowering time between current and elevated CO2.       As predicted, we found that with downregulation of FLC expression via vernalization, SG plants grown at elevated CO2 no longer delayed flowering relative to current CO2-grown plants. Thus, vernalization returned the normal flowering time phenotype, counteracting the delayed effects of elevated CO2 on flowering. This study indicates that elevated CO2 can delay flowering directly through FLC, and downregulation of FLC under elevated CO2 reverses this effect. Moreover, this study demonstrates that increasing CO2 may potentially drive major changes in development through FLC and allowed us to advance in our mechanistic understanding of the influence of rising CO2 on plant developmental pathways.