1. Leibniz Institute for Natural Products Research and Infection Biology, HKI, Beutenbergstr. 11a, 07745 Jena, Germany

Correspondence to: Christian Hertweck¹ Correspondence and requests for materials should be addressed to C.H. (Email: Christian.Hertweck@hki-jena.de). The 16S rDNA nucleotide sequences of Rhizopus sp. symbionts have been deposited at the EMBL Nucleotide Sequence Database under the accession numbers AJ938141–AJ938144.

Abstract

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 pathogen^{1, 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 -tubulin, which results in inhibition of mitosis and cell cycle arrest^{7, 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 drug^{9, 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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