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The recent finding that proline-induced root elongation is mediated by reactive oxygen species (ROS) prompted us to re-evaluate other developmental processes modulated by proline, such as flowering time. By controlling the cellular redox status and the ROS distribution, proline could potentially affect the expression of transcriptional factors subjected to epigenetic regulation, such as <i>FLOWERING LOCUS C</i> (<i>FLC</i>). Accordingly, we investigated the effect of proline on flowering time in more detail by analyzing the relative expression of the main flowering time genes in <i>p5cs1 p5cs2/P5CS2</i> proline-deficient mutants and found a significant upregulation of <i>FLC</i> expression. Moreover, proline-deficient mutants exhibited an adult vegetative phase shorter than wild-type samples, with a trichome distribution reminiscent of plants with high <i>FLC</i> expression. In addition, the vernalization-induced downregulation of <i>FLC</i> abolished the flowering delay of <i>p5cs1 p5cs2/P5CS2</i>, and mutants homozygous for <i>p5cs1</i> and <i>flc-7</i> and heterozygous for <i>P5CS2</i> flowered as early as the <i>flc-7</i> parental mutant, indicating that FLC acts downstream of <i>P5CS1</i>/<i>P5CS2</i> and is necessary for proline-modulated flowering. The overall data indicate that the effects of proline on flowering time are mediated by FLC.