The cytokinin side chain commands shooting

Abstract

Functional distinction between structurally diverse cytokinins as essential plant hormones has remained enigmatic for decades. In this issue of Developmental Cell, Kiba et al. (2013) provide compelling evidence for the central role of CYP735A1/2 in synthesizing trans-hydroxylated cytokinins, which specify shoot growth, vital for energy and biomass production.

Figure 1. Trans-Zeatin Specifies Shoot Growth Regulation in Arabidopsis thaliana

Basal cytokinin levels are quantitatively essential for SAM (shoot apical meristem) and RAM (root apical meristem) activity and maintenance, and basic shoot and root development. High-level tZ is dispensable for root growth and development, but specifically promotes shoot growth. Bidirectional transport systems via xylem and phloem likely support the predominant shoot destination of tZ, tZR or tZRPs originated from either shoots or roots. (Inset) The cytokinin synthesis pathway is initiated by IPTs (adenosine phosphate-isopentenyltransferases) using DMAPP (dimethylallyl diphosphate) and ATP, ADP or AMP to generate iPRPs (iP riboside 5'-phosphates). CYP735A1/A2 convert iPRPs to tZRPs. LOGs (cytokinin nucleoside 5'monophosphate phosphoribohydrolase) release active cytokinins, N⁶-(Δ²-isopentenyl)adenine (iP) and trans-zeatin (tZ).