Cytokinin Signalling and Mechanism of Action of Meta-Topolin and Its Derivatives

Abstract

The discovery of ortho-topolin, meta-topolin and its conjugates in the early 1990s prompted further research on the biological activity of these aromatic cytokinins. Structure-activity relationship studies (SAR) revealed that hydroxylation at the meta-position on the benzyl side chain of BAP and its riboside significantly improved the compound performance in standardized cytokinin bioassays compared to BAP. Elucidation of the molecular mechanism of the cytokinin signal transduction and identification of membrane-bound receptor histidine kinase (HK) led to deeper understanding of the function of topolins in planta. While aromatic cytokinins generally show weak interaction with Arabidopsis and other cytokinin-sensitive HKs, meta-topolin is able to provoke a strong response comparable to that observed with the most active isoprenoid free bases. In contrast, ortho-topolin and para-topolin are much less active, and it is possible that hydroxylation at these positions serves as a deactivation step. In contrast, derivatives of meta-topolin, although not directly interacting with cytokinin-sensitive HKs, have proven to be very useful in plant propagation techniques, and slow gradual release of the free active base allows for the potentiation of cytokinin effects. This chapter summarizes basic facts about cytokinin signalling and adds to our knowledge of signalling aspects and metabolism of aromatic cytokinins in various plant models.