|Crutcher, Frankie - Montana State University|
|Stipanovic, Robert - Retired ARS Employee|
|Bell, Alois - Al|
|Nichols, Robert - Cotton, Inc|
|Lawrence, Katheryn - Auburn University|
Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2017
Publication Date: 10/6/2017
Citation: Crutcher, F.K., Puckhaber, L.S., Stipanovic, R.D., Bell, A.A., Nichols, R.L., Lawrence, K.S., Liu, J. 2017. Microbial resistance mechanisms to the antibiotic and phytotoxic fusaric acid. Journal of Chemical Ecology. 43(10):996-1006.
Interpretive Summary: Fusarium wilt pathogens produce fusaric acid (FA) which is toxic to soil microorganisms and plants; this contributes to the pathogen’s virulence to plants and allow them to out-compete other microbes in the soil. Many microorganisms have also developed mechanisms to mitigate toxic effects of FA. Understanding these mechanisms will provide effective tools to enhance resistance against FA in plants to control Fusarium wilt and in biocontrol agents to increase their efficacy. Soil microbes were screened for FA resistance and their ability to convert FA into less toxic compounds. Bacteria were found unable to convert FA, but resisted FA by pumping out FA, with the Gram-negative bacteria among the most resistant ones. Resistance in Gram-negative bacteria was positively correlated with the number of these FA efflux pumps present in the genomes. Protein sequence analysis of these pumps revealed that pumps with sequences very similar to the previously characterized FA resistant pump FusC or Fdt are the main contributors to FA resistance. Most of the fungi tested, on the other hand, converted FA into less toxic compounds; five were detected with one remaining unidentified. Either oxidative products or reductive products were produced, rarely both. These products were produced by widely different groups of fungi, indicating that these FA detoxification mechanisms are highly conserved and new mechanisms may still await discovery. Fungi also utilize FA transporters different from those of Gram-negative efflux pumps to resist FA. Deployment of both efflux and conversion mechanisms thus may be a common feature of fungal resistance to FA.
Technical Abstract: Fusaric acid (FA) produced by Fusarium oxysporum plays an important role in disease development in plants, including cotton. This non-specific toxin also has deleterious effects on microorganisms, providing for a potential pool of unique detoxification methods. Bacteria and fungi from soils infested with Fusarium and from laboratory sources were evaluated for their ability to grow in the presence of FA and alter its structure into less toxic compounds. For bacteria, highly FA resistant strains were found only in Gram-negative bacteria, mainly in the genus of Pseudomonas. None of the bacterial strains were able to chemically modify FA. FA resistance of Gram-negative bacteria was positively correlated with the number of tetrapartite efflux pumps containing fusaric acid resistant proteins (FUSC) present in the genome. Phylogenetic analysis of FUSC proteins revealed that only FUSC proteins having the highest sequence identities with the functionally characterized FusC or Fdt/FuaA proteins from strains with demonstrated FA resistances clustered in their respective clade, indicating the proteins in these two clades are the major contributors of FA resistance. In contrast, most of the fungi demonstrated the ability to convert FA to less toxic compounds regardless of level of FA resistance they possess. Five derivatives were detected with one of them remains to be unidentified. The derivatization of FA involved either oxidative reactions on butyl side chain or reductive reactions on carboxylic acid group of FA, rarely both. The production of these derivatives including new metabolites from widely different phyla indicates that resistance to FA by altering its structure is highly conserved and there are still mechanisms to detoxify FA that have yet to be discovered. A few FA resistant saprophytic or biocontrol strains were incapable of altering FA, indicating a possible involvement of efflux transporters. In Fusarium species, both efflux by transporters and derivatization by oxidative enzymes were responsible for FA resistance. Deployment of both efflux and derivatization mechanisms may be a common feature of the FA resistance of fungal isolates.