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PLANTS INTERACTING WITH OTHER ORGANISMS

Defective Long-Distance Auxin Transport Regulation in the Medicago truncatula super numeric nodules Mutant^1,[W]

Australian Research Council Centre of Excellence for Integrative Legume Research (G.E.v.N., B.G.R., U.M.), Genomic Interactions Group, Research School of Biological Sciences (G.E.v.N., B.G.R.), and School of Biochemistry and Molecular Biology (U.M.), the Australian National University, Canberra, Australian Capitol Territory 0200, Australia; and School of Plant Science, University of Tasmania, Hobart, Tasmania 7001, Australia (J.J.R., J.B.R.)

Long-distance auxin transport was examined in Medicago truncatula and in its supernodulating mutant sunn (super numeric nodules) to investigate the regulation of auxin transport during autoregulation of nodulation (AON). A method was developed to monitor the transport of auxin from the shoot to the root in whole seedlings. Subsequently, the transport was monitored after inoculation of roots with the nodulating symbiont Sinorhizobium meliloti. The sunn mutant showed an increased amount of auxin transported from the shoot to the root compared to the wild type. The auxin transport capacity of excised root segments was similar in wild type and sunn, suggesting that the difference in long-distance auxin transfer between them is due to loading in the shoot. After inoculation, wild-type seedlings showed decreased auxin loading from the shoot to the root; however, the sunn mutant failed to reduce the amount of auxin loaded. The time of reduced auxin loading correlated with the onset of AON. Quantification of endogenous auxin levels at the site of nodule initiation showed that sunn contained three times more auxin than wild type. Inoculation of sunn failed to reduce the level of auxin within 24 h, as was observed in the wild type. We propose a model for the role of auxin during AON of indeterminate legumes: 1) high levels of endogenous auxin are correlated with increased numbers of nodules, 2) inoculation of roots reduces auxin loading from the shoot to the root, and 3) subsequent reduction of auxin levels in the root inhibits further nodule initiation.

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: Ulrike Mathesius (ulrike.mathesius{at}anu.edu.au).

^[W] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.075879.

^* Corresponding author; e-mail ulrike.mathesius{at}anu.edu.au; fax 61–2–6125–0313.

Received December 20, 2005; returned for revision February 6, 2006; accepted February 14, 2006.

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