Symbiotic Cooperation in the Biosynthesis of a Phytotoxin

Abstract

Natural products play a key role in symbiotic interactions between microorganisms and higher organisms, covering all kingdoms of life. The function of these secondary metabolites may range from signaling compounds in mutualism to virulence factors and antibiotics in parasitic relationships. In many cases the interactions involve multiple partners and thus the biogenetic basis of chemical mediators can be quite complex. This complexity is well exemplified by the unparalleled tripartite relationship among the rice-seedling-blight fungus Rhizopus microsporus, its host plant Oryza sativa and endosymbiotic bacteria that reside in the fungal cytosol. The bacterial symbionts (Burkholderia species) produce a phytotoxin complex to assist the phytopathogenic fungus in colonizing rice seedlings. In turn the bacteria profit from a safe niche and access to nutrients released from the decaying plant. Initially, the macrolide rhizoxin (1, Figure 1) and various congeners such as WF-1360F (2) were isolated from cultures of R. microsporus van Tieghem var. chinensis and identified as the causative agent of rice seedling blight. Rhizoxin efficiently inhibits eukaryotic cell proliferation by binding to b-tubulin and thus blocking the formation of the mitotic spindle. Notably, the pure compound alone evokes the typical symptoms of seedling root swelling. Only recently, through detection, isolation, and cultivation of the endosymbionts we could unequivocally prove that actually associated bacteria are the true producers of the toxin complex. The importance of this metabolic capability has been underlined by the finding that fungal reproduction depends entirely on the presence of the bacterial symbionts. Survival of the toxinogenic symbiosis is warranted by the strict sporulation control and exclusive dispersal of spores harboring endosymbionts. Moreover, the unusual mutualism has been fine-tuned through symbiosis factors such as a type-III secretion system and a novel lipopolysaccharide O-antigen that sets the symbionts into a “stealth mode” by decorating the outer membrane of the endosymbionts. The host, on the other hand, acquired resistance towards rhizoxin by mutation of the b-tubulin. Because of its ecological and medicinal relevance as an antimitotic agent, the biosynthesis of rhizoxin has been studied. Cloning, sequencing, and molecular analyses of the rhizoxin (rhi) biosynthetic gene cluster in the genome of Burkholderia rhizoxinica revealed the molecular basis for a complex polyketide assembly line required for the biosynthesis of the virulence factor. Whereas the biosynthesis of the macrolide backbone has been decoded by mutational analyses, polyketide tailoring mechanisms and the biological role of the bis(epoxidation) have remained elusive. Herein we elucidate the dual epoxidation of rhizoxin and its impact on rice seedling blight and report an unprecedented case for symbiotic cooperation in the biosynthesis of an ecologically relevant natural product. In a broader survey on rhizoxin-positive Rhizopus species we discovered that the unusual bacterial–fungal association is not restricted to a single isolate but has spread worldwide. We have identified eight related Burkholderia–Rhizopus associations from geographically highly different regions on five continents; these findings underlign the ecological imporFigure 1. A) Structures of rhizoxin and congeners. B) Phylogenetic relationship of Rhizopus microsporus strains; structures of the corresponding metabolites indicate which strains can produce bisepoxides. The numbers on top of the branches indicate the clade probability values; the scale on the left site relates the length of a branch to the distance (number of changes that have taken place along a branch).