Ammonium-excreting Azospirillum sp. become intracellularly established in maize (Zea mays) para-nodules |
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Authors: | C. Christiansen-Weniger J. Vanderleyden |
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Affiliation: | (1) F. A. Janssens Laboratory of Genetics, Catholic University of Leuven, Willem de Croylaan 42, B-3001 Heverlee, Belgium |
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Abstract: | Maize seedlings develop nodule-like tumour knots (para-nodules) along primary roots when treated with the auxin 2,4-dichlor-phenoxy-acetic acid (2,4-D). Inoculated NH4+-excreting Azospirillum brasilense cells were shown to colonize these tumours, mostly intracellularly, promoting a high level of N2 fixation when microaerophilic conditions were imposed. The nitrogenase activity inside the para-nodules was less sensitive to free O2 than in non-para-nodulating roots. Both light and electron microscopy showed a dense bacterial population inside intact tumour cells, with the major part of the cell infection along a central tumour tissue. The bacteria colonized the cytoplasm with a close attachment to inner cell membranes. In an auxin-free growth medium, young 2,4-D-induced para-nodules grew further to become mature differentiated root organs in which introduced bacteria survived with a stable population. These results provide evidence that gramineous plants are potentially able to create a symbiosis with diazotrophic bacteria in which the NH4+-excreting symbiont will colonize para-nodule tissue intracellularly, thus becoming well protected. |
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Keywords: | Ammonium excretion Azospirillum brasilense Auxine 2,4-Dichlor-phenoxy-acetic acid Nitrogen fixation Paranodulation Maize Zea mays Electron microscopy |
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