The Yang cycle in plants: a journey of methionine recycling with fascinating metabolites and enzymes

Methionine is a sulfur-containing amino acid that plays an essential role in plant growth and development. In contrast to its low abundance, methionine is highly demanded in various physiological processes, such as ethylene biosynthesis during fruit ripening. To sustain methionine levels, plants trade-off adenosine triphosphate to recycle the methylthio group through a metabolic pathway commonly known as the Yang cycle. Over the years, significant progress has been made in identifying the intermediate metabolites and enzymes involved in this cycle. While our understanding of the biological functions of certain metabolites and enzymes in the Yang cycle has expanded, there are still many important questions left unanswered. Notably, the aminotransferase responsible for the final step of the cycle has not yet been identified. This review provides a comprehensive overview of the metabolic roles of these metabolites and the biological significance of individual enzymes in the Yang cycle. We also discuss the regulatory influence of this cycle on ethylene production in plants.