Plant Physiology Preview
Published on February 2, 2007; 10.1104/pp.106.093708
OPEN ACCESS ARTICLE
Received November 27, 2006
Accepted January 22, 2007
Feedback Regulation of Xylem Cytokinin Content Is Conserved in Pea and Arabidopsis
Eloise Foo , Suzanne E Morris , Kathy Parmenter , Naomi Young , Huiting Wang , Alun Jones , Catherine Rameau , Colin G.N. Turnbull , and Christine A. Beveridge *
Australian Research Council Centre of Excellence for Integrative Legume Research and School of Integrative Biology, The University of Queensland, St Lucia, QLD, 4072, Australia; INRA, Station de Génétique, Route de St Cyr, 78026 Versailles Cedex, France; Division of Biology, Imperial College London, Wye Campus, Wye, Kent TN25 5AH UK; Institute for Molecular Bioscience, The University of Queensland, St Lucia, QLD, 4072, Australia
* Corresponding author; email: c.beveridge{at}uq.edu.au.
Increased branching mutants of Pisum sativum (garden pea; ramosus, rms) and Arabidopsis thaliana (more axillary branches, max) 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 signalling mechanism appears independent of the number of growing axillary shoots because a suppressed axillary meristem (sax) 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.
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