Five unaddressed questions about cytokinin biosynthesis.

Cytokinins, a class of phytohormones, play crucial roles in regulating plant growth and stress responses through finely tuned feedback loops involving metabolic and signaling cascades. Over the past 25 years, studies have identified key genes involved in cytokinin biosynthesis and inactivation pathways. Nevertheless, several gaps remain in our understanding, particularly regarding the movement of intermediate metabolites between subcellular compartments and the discrepancy between the products of adenosine phosphate-isopentenyltransferase (IPT) and the substrate preferences of subsequent reactions. Recent gene discoveries related to lonely guy (LOG)-independent pathways suggest a spatial extension of cytokinin biosynthesis into the apoplast. Other intriguing issues remain to be addressed, such as elucidating the synthetic pathway for cis-zeatin and unraveling the molecular mechanisms governing selective substrate use by the cytokinin biosynthetic enzyme Tumor morphology root (Tmr) from the phytopathogen Agrobacterium tumefaciens. Further studies are needed to reveal a fully comprehensive picture of cytokinin metabolism.