Riboswitches are RNA elements that bind small molecules and in turn regulate gene expression. This mechanism allows the cell to sense the intracellular concentration of these small molecules. A particular riboswitch typically regulates its adjacent gene by altering the transcription, the translation or the splicing of this gene. Recently, a riboswitch that binds thiamin pyrophosphate (TPP) was characterized and found to regulate thiamin biosynthesis in plants and algae. Furthermore, it appears that this element is an essential regulator of primary metabolism in plants. Manipulation of endogenous riboswitch activity resulted in metabolic phenotypes that underlined the role of these elements and their ligands in preserving metabolic homeostasis. This situation supports the hypothesis that riboswitches could be remnants of the most ancient metabolic regulators. Here, we review the mode of action of the plant and algal TPP riboswitch and its relevance to the metabolic network. We also discuss the potential engineering of riboswitches as metabolite sensors in plants and platforms for gene control. Whether additional such RNA-based mechanisms exist in plants and in algae is still an open question, yet, the importance of these elements to metabolic regulation is beyond doubt.