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JXB Advance Access published online on June 9, 2006
Journal of Experimental Botany, doi:10.1093/jxb/erj214
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© The Author [2006]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received November 24, 2005
Accepted March 24, 2006

RESEARCH PAPER

Root exudation from Hordeum vulgare in response to localized nitrate supply

Eric Paterson 1 *, Allan Sim 1, Dominic Standing 2, Mairi Dorward 2, and A. James S. McDonald 2

1 Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
2 School of Biological Sciences, Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3UU, UK

* To whom correspondence should be addressed.
Eric Paterson, E-mail: eric.paterson{at}macaulay.ac.uk


  Abstract

Root proliferation as a response to exploit zones of nutrient enrichment in soil has been demonstrated for a wide range of plant species. However, the effectiveness of this as a strategy to acquire nutrients is also dependent on interactions with the soil microbial community. Specifically, C-flow from roots modifies microbial activity and probably the balance between nutrient mineralization and immobilization processes in the rhizosphere. In this study, near-natural abundance 13C-labelling and gene-reporter methods were applied to determine the effects of uneven nitrate supply to roots of Hordeum vulgare on assimilate partitioning and root exudation. Plants were initially grown in uniform nitrate supply in split-root, sand microcosms after which one treatment continued to receive uniform supply, and the other received nitrate to one root compartment only. At the time of imposing the treatments, the CO2 supplied to the plants was switched to a cylinder source, providing a distinct {delta}13C-signature and allowing the fate of new assimilate within the plants to be determined. The labelling approach allowed quantification of the expected preferential allocation of new C-assimilate to roots in enriched nitrate, prior to any measurable effect on whole biomass or root architecture. Biosensor (lux-marked Pseudomonas fluorescens 10586 pUCD607) bioluminescence, quantified spatially by CCD imaging, demonstrated that root exudation was significantly increased for roots in enriched nitrate. This response of root exudation, being primarily associated with root apices and concurrent with enhanced assimilate supply, strongly suggests that C-flow from roots is an integral component of the proliferation response to nitrate.

Keywords: Biosensor reporting; carbon partitioning; 13C-labelling; Hordeum vulgare; nutrient patches; rhizodeposition; root exudation; root proliferation.
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