A number of plant pathogenic fungi belonging to the genus Rhizopus are infamous for causing rice seedling blight. This plant disease is typically initiated by an abnormal swelling of the seedling roots without any sign of infection by the pathogen1,2,3,4. This characteristic symptom is in fact caused by the macrocyclic polyketide metabolite rhizoxin that has been isolated from cultures of Rhizopus sp.5,6. The phytotoxin exerts its destructive effect by binding to rice β-tubulin, which results in inhibition of mitosis and cell cycle arrest7,8. Owing to its remarkably strong antimitotic activity in most eukaryotic cells, including various human cancer cell lines, rhizoxin has attracted considerable interest as a potential antitumour drug9,10. Here we show that rhizoxin is not biosynthesized by the fungus itself, but by endosymbiotic, that is, intracellular living, bacteria of the genus Burkholderia. Our unexpected findings unveil a remarkably complex symbiotic-pathogenic relationship that extends the fungus–plant interaction to a third, bacterial, key-player, and opens new perspectives for pest control.
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We thank A. Perner for MS and HPLC-MS measurements, G.-M. Schwinger for strain cultivation, I. Löschmann for sequencing and A. Hartmann for practical help using the laser microscope. We are grateful to J. Piel for advice and discussions, and H. G. Floss and A. Brakhage for critically reading earlier versions of the manuscript. Financial support by the Leibniz Gemeinschaft is acknowledged.
The 16S rDNA nucleotide sequences of Rhizopus sp. symbionts have been deposited at the EMBL Nucleotide Sequence Database under the accession numbers AJ938141–AJ938144. Reprints and permissions information is available at npg.nature.com/reprintsandpermissionsu. The authors declare no competing financial interests.
MSn spectrum of rhizoxin (1,) produced by cultivated symbiont. Inset shows diagnostic fork — shape UV spectrum of the triene moiety of 1. (PDF 10489 kb)
Generation of symbiont-free Rhizopus strain and reinfection with cultivated symbiont. Agarose (2 %) gel electrophoresis of 16S rDNA PCR products amplified with universal primers. As template served fungal metagenomic DNA of symbiont-free R. microsporus ATCC 62417 (lane 1), and R. microsporus ATCC 62417 after reinfection (lane 2). Lanes 3 and 4 are positive and negative controls, respectively. (PDF 128 kb)
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Partida-Martinez, L., Hertweck, C. Pathogenic fungus harbours endosymbiotic bacteria for toxin production. Nature 437, 884–888 (2005). https://doi.org/10.1038/nature03997
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