Riboswitches are natural RNA sensors that affect post-transcriptional processes via their capacity to bind small molecules. To date, these mRNA structures have been shown to regulate the biosynthesis of essential metabolites, including vitamins and amino acids. Although bacterial riboswitches are widespread and characterized, only a single eukaryotic, thiamin-pyrophosphate-binding riboswitch has recently been discovered to direct gene expression by regulating mRNA splicing in fungi, green algae and land plants. It is unclear how widespread riboswitches are and what additional roles they have in eukaryotes. When engineered in plants, riboswitches can function autonomously to modulate gene expression. These discoveries not only trigger novel findings regarding RNA switches in plants, but also spur the exploitation of riboswitches for monitoring metabolite concentrations in planta.