Molecular basis for cytokinin biosynthesis.

Cytokinins (CKs) are a group of phytohormones that play a crucial role in the regulation of plant growth and development. Identification of the enzymes and the corresponding genes that are involved in CK metabolism allowed us to understand how plants synthesize CKs and adjust CK activity to optimal levels. A major accomplishment toward these goals was the identification of genes for the first enzyme in the CK biosynthetic pathway, adenosine phosphate-isopentenyltransferase (IPT). In Arabidopsis thaliana and Agrobacterium tumefaciens, detailed analyses of IPTs were conducted through not only enzymatic characterization but also molecular structural approaches. These studies revealed the molecular basis for the Agrobacterium-origin of IPT used for the efficient biosynthesis of trans-zeatin that promotes tumorigenesis in host plants. Another landmark in CK research was the identification of CYP735A as an enzyme that converts iP-nucleotide to tZ-nucleotide. Furthermore, the identification of a CK-activating enzyme, LOG, which catalyzes a novel activation pathway, is a remarkable recent achievement in CK research. Collectively, these advances have revealed the complexity of the entire metabolic scheme for CK biosynthesis.