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DEVELOPMENT AND HORMONE ACTION

Feedback Regulation of Xylem Cytokinin Content Is Conserved in Pea and Arabidopsis^1,[C],[OA]

Australian Research Council Centre of Excellence for Integrative Legume Research and School of Integrative Biology (E.F., S.E.M., K.P., C.A.B.), and Institute for Molecular Bioscience (A.J.), The University of Queensland, St Lucia, Queensland 4072, Australia; Institut National de la Recherche Agronomique, Station de Génétique, 78026 Versailles cedex, France (K.P., C.R.); and Division of Biology, Imperial College London, Wye Campus, Wye, Kent TN25 5AH, United Kingdom (N.Y., H.W., C.G.N.T.)

Increased-branching mutants of garden pea (Pisum sativum; ramosus [rms]) and Arabidopsis (Arabidopsis thaliana; more axillary branches) were used to investigate control of cytokinin export from roots in relation to shoot branching. In particular, we tested the hypothesis that regulation of xylem sap cytokinin is dependent on a long-distance feedback signal moving from shoot to root. With the exception of rms2, branching mutants from both species had greatly reduced amounts of the major cytokinins zeatin riboside, zeatin, and isopentenyl adenosine in xylem sap compared with wild-type plants. Reciprocally grafted mutant and wild-type Arabidopsis plants gave similar results to those observed previously in pea, with xylem sap cytokinin down-regulated in all graft combinations possessing branched shoots, regardless of root genotype. This long-distance feedback mechanism thus appears to be conserved between pea and Arabidopsis. Experiments with grafted pea plants bearing two shoots of the same or different genotype revealed that regulation of root cytokinin export is probably mediated by an inhibitory signal. Moreover, the signaling mechanism appears independent of the number of growing axillary shoots because a suppressed axillary meristem mutation that prevents axillary meristem development at most nodes did not abolish long-distance regulation of root cytokinin export in rms4 plants. Based on double mutant and grafting experiments, we conclude that RMS2 is essential for long-distance feedback regulation of cytokinin export from roots. Finally, the startling disconnection between cytokinin content of xylem sap and shoot tissues of various rms mutants indicates that shoots possess powerful homeostatic mechanisms for regulation of cytokinin levels.

² Present address: School of Plant Science, University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Christine A. Beveridge (c.beveridge{at}uq.edu.au).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.093708

^* Corresponding author; e-mail c.beveridge{at}uq.edu.au; fax 61–(0)–7–3365–1699.

Received November 27, 2006; accepted January 22, 2007; published February 2, 2007.

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