Editors' ChoicePlant biology

An “A tZ” for Shoots, Not Roots

Sci. Signal.  10 Dec 2013:
Vol. 6, Issue 305, pp. ec299
DOI: 10.1126/scisignal.2004977

Cytokinins are plant hormones that regulate the development of tissues from the shoot and root apical meristems. N6-(∆2-isopentenyl)adenine (iP)–type and trans-zeatin (tZ)–type cytokinins are the major cytokinins in Arabidopsis thaliana and differ structurally in their side chains. Kiba et al. examined the physiological role of the side-chain modification (trans-hydroxylation) that forms tZ from iP-type cytokinins (see highlight by Sheen). In double-mutant (cypDM) plants deficient in the cytochrome P450 monooxygenase CYP735A, the concentration of tZ-type cytokinins markedly decreased, whereas the concentration of iP-type cytokinins proportionally increased, confirming that CYP735A catalyzes the trans-hydroxylation of the side chain on iP to synthesize tZ, which has been previously reported from studies in vitro. The phenotype of cypDM plants, which had shorter growth of shoots, smaller and rounder leaves in circular arrangements (rosettes), and delayed bolting time (production of a flowering stem), indicated that shoot development and maturation was impaired by the loss of tZ-type cytokinins. The shoot biomass was substantially reduced in cypDM plants, but the biomass, cellular organization, and vasculature in the roots were unaffected. Additionally, the expression of cytokinin-inducible type A Arabidopsis response regulators (ARRs) was reduced in the shoots but not the roots of cypDM plants. Furthermore, complementation with CYP735A or application of exogenous tZ but not iP recovered the wild-type phenotype and restored the balance of tZ to iP cytokinins in cypDM plant shoots but did not affect the roots, together suggesting a shoot-specific function for tZ-type cytokinins. The findings show that the side-chain modification of cytokinins confers functional specificity in plant development.

T. Kiba, K. Takei, M. Kojima, H. Sakakibara, Side-chain modification of cytokinins controls shoot growth in Arabidopsis. Dev. Cell 27, 452–461 (2013). [PubMed]

J. Sheen, The cytokinin side chain commands shooting. Dev. Cell 27, 371–372 (2013). [PubMed]