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Published on July 14, 2006; 10.1104/pp.106.080093

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Received March 6, 2006
Accepted June 30, 2006

The Ethylene Insensitive sickle Mutant of Medicago truncatula Shows Altered Auxin Transport Regulation during Nodulation

Joko Prayitno , 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, Australian Capital Territory 0200, Australia
Australian Research Council Centre of Excellence for Integrative Legume Research, School of Biochemistry and Molecular Biology, The Australian National University, Canberra, Australian Capital Territory 0200, Australia

* Corresponding author; email: ulrike.mathesius{at}anu.edu.au.

We studied the ethylene-insensitive, hypernodulating mutant, sickle (skl), to investigate the interaction of ethylene with auxin transport during root nodulation in Medicago truncatula. Grafting experiments demonstrated that hypernodulation in skl is root-controlled. Long distance transport of auxin from shoot to root was reduced by rhizobia after 24 h in wild type but not in skl. Similarly, the ethylene precursor ACC (1-amino cyclopropane-1-carboxylic acid) inhibited auxin transport in wild type but not in skl. Auxin transport at the nodule initiation zone was significantly reduced by rhizobia after 4 h in both wild type and skl. After 24h, auxin transport significantly increased at the nodule initiation zone in skl compared to wild type, accompanied by an increase in the expression of the MtPIN1 and MtPIN2 (pin-formed) auxin efflux transporters. Response assays to different auxins did not show any phenotype that would suggest a defect of auxin uptake in skl. The auxin transport inhibitor NPA (N-1-naphthylphtalamic acid) inhibited nodulation in wild type but not skl, even though NPA still inhibited auxin transport in skl. Our results suggest that ethylene signaling modulates auxin transport regulation at certain stages of nodule development, partially through PIN gene expression, and that an increase in auxin transport relative to the wild type is correlated with higher nodule numbers. We also discuss the regulation of auxin transport in skl in comparison to previously published data on the autoregulation mutant, sunn (super numerary nodules, van Noorden et al., 2006).




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