Plant Physiology Preview
Published on February 17, 2006; 10.1104/pp.105.075879
Received December 20, 2005
Returned for revision January 19, 2006
Accepted February 14, 2006
Defective Long Distance Auxin Transport Regulation in the Medicago truncatula sunn Mutant
Giel E. van Noorden , John J. Ross , James B. Reid , Barry G. Rolfe , and Ulrike Mathesius *
Australian Research Council Centre of Excellence for Integrative Legume Research; Genomic Interactions Group, Research School of Biological Sciences, The Australian National University, Canberra ACT 0200, Australia
School of Plant Science, University of Tasmania, Hobart, Tasmania 7001, Australia
Australian Research Council Centre of Excellence for Integrative Legume Research; School of Biochemistry and Molecular Biology, The Australian National University, Canberra ACT 0200, Australia
* Corresponding author; email: Ulrike.Mathesius{at}anu.edu.au.
Long distance auxin transport was examined in Medicago truncatula and in its supernodulating mutant sunn 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.
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