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Identification of Symbiotically Defective Mutants of Lotus japonicus Affected in Infection Thread Growth

December 2006 , Volume 19 , Number  12
Pages  1,444 - 1,450

Fabien Lombardo , 1 Anne B. Heckmann , 1 Hiroki Miwa , 1 Jillian A. Perry , 1 Koji Yano , 2 Makoto Hayashi , 2 Martin Parniske , 3 Trevor L. Wang , 1 and J. Allan Downie 1

1John Innes Centre, Colney Lane, NR4 7UH, Norwich, U.K.; 2Department of Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan; 3The Sainsbury Laboratory, Colney Lane, NR4 7UH, Norwich, U.K.

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Accepted 14 July 2006.

During the symbiotic interaction between legumes and rhizobia, the host cell plasma membrane and associated plant cell wall invaginate to form a tunnel-like infection thread, a structure in which bacteria divide to reach the plant root cortex. We isolated four Lotus japonicus mutants that make infection pockets in root hairs but form very few infection threads after inoculation with Mesorhizobium loti. The few infection threads that did initiate in the mutants usually did not progress further than the root hair cell. These infection-thread deficient (itd) mutants were unaffected for early symbiotic responses such as calcium spiking, root hair deformation, and curling, as well as for the induction of cortical cell division and the arbuscular mycorrhizal symbiosis. Complementation tests and genetic mapping indicate that itd2 is allelic to Ljsym7, whereas the itd1, itd3, and itd4 mutations identified novel loci. Bacterial release into host cells did occur occasionally in the itd1, itd2, and itd3 mutants suggesting that some infections may succeed after a long period and that infection of nodule cells could occur normally if the few abnormal infection threads that were formed reached the appropriate nodule cells.

Additional keyword: Medicago , nitrogen-fixing nodules .

© 2006 The American Phytopathological Society