The elusive cytokinin biosynthetic pathway

Although 40 years have passed since the first discovery of a naturally occurring cytokinin (CK) in plants, the biosynthetic pathway still remains unresolved. Numerous factors have contributed to the lack of success in this area, but most alarming has been the reliance upon unproven hypotheses and preconceived ideas. The greatest obstacle has been the identification and isolation of CK biosynthetic enzymes in plants, which have only recently been isolated and characterised in Arabidopsis. This has led to the classical isoprenoid CK biosynthetic pathway being partly challenged and the suggestion that multiple pathways might exist in plants. CK biosynthetic mutants, which overproduce CKs as a result of IPT overexpression, are also starting to be identified and this should help overcome the past difficulties experienced in determining the site(s) of CK biosynthesis in plants. On the other hand, aromatic CK biosynthesis has seen no progress in the last few years and remains strictly speculative. This review aims to evaluate the current status of knowledge with respect to CK biosynthesis, to identify gaps in the knowledge and to suggest ways by which to move forward.