Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Abstract Only
Publication Acceptance Date: 12/30/2009
Publication Date: 1/15/2010
Citation: Stipanovic, R.D., Puckhaber, L.S., Bell, A.A., Liu, J. 2010. Phytotoxicity of fusaric acid and analogues to cotton [abstract]. Proceedings of Beltwide Cotton Conferences, January 4-7, 2010, New Orleans, Louisiana. 2010 CDROM.
Interpretive Summary: A pathogen of the cotton plant Fusarium oxysporum produces a compound called fusaric acid that is toxic to plants including cotton. New virulent isolates of this pathogen produce large amounts of this plant toxin. To more fully understand how this toxin operates, we have studied how changes in its structure alter its toxicity. We found that the carboxylic acid group present in the molecule is essential to its toxicity, and alterations in this group can increase or decrease toxicity. These compounds may have applications as herbicides in organic farming.
Technical Abstract: Fusaric acid has been isolated from 11 Fusarium species including the cotton pathogen, Fusarium oxysporum f. sp. vasinfectum. Although fusaric acid shows low toxicity to animals, it is classified as a wilt-inducing toxin in many plants including cotton. The latter is particularly sensitive to this compound. We are interested in determining the phytotoxicity of fusaric acid analogues and as such have developed a cotton cotyledonary leaf assay. We used this assay to compare the toxicity of fusaric acid to various analogues. When the molecule fusaric acid is deconstructed, one finds it is composed of three structural components, the central pyridine ring, the butyl side chain and the carboxylic acid group. We now report the results of our preliminary study to determine the essential components responsible for its phytotoxicity. To this end, we eliminated, changed or rearranged the butyl and the carboxylic acid groups. Our results show that the carboxylic acid group is essential for toxicity. That is, 3-butylpyridine is not toxic. Conversion of the carboxylic acid group to the methyl ester provided a more toxic compound, while reduction of the carboxylic acid to the aldehyde provided a less toxic compound. Moving the carboxylic acid from the 2- to the 3-position (i.e., 5-butylnicotinic acid) or conversion of the fusaric acid side chain to a propyl or a pentyl group all reduced toxicity. Of the compounds tested, fusaric acid and its methyl ester were among the most phytotoxic compounds showing severe wilting at 8.0 mM and detectable necrosis at the point of piercing at 0.5 mM. Thus, it would appear that evolutionary pressures within Fusarium oxysporum have combined to produce an effective phytotoxin from the basic pyridine carboxylic acid building blocks.