In other words, once CebR is neutralized, once it has been withdrawn from the chromosome, Streptomyces scabies starts to produce thaxtomin constitutively, that is to say, without needing to perceive the external triggering signal any longer (cellobiose or cellotriose). The result is visible to the naked eye. The cultures of the wild-type strain of S. scabies do not produce thaxtomin (yellow-pigmented), while the culture prepared with the strain of S. scabies where the cebR gene has been inactivated is completely yellow!

A new herbicide onto the market?

Thaxtomin has the particularity of attacking the enzyme which synthetizes cellulose in plants and, consequently, prevents them from growing. In other words, it is a powerful weed-killer. It has the added advantage of being biodegradable: soils already contain microorganisms that are capable of using them as a source of nutrition (5). Thaxtomin, as a biopesticide, offers an interesting alternative to chemically synthesized pesticides. Several companies hold patents for the exploitation of this molecule but marketing thaxtomin is not very profitable because the cost of producing it is very high. A detailed consultation with the suppliers of this molecule revealed that one thousandth of one gramme currently costs between 260 and 590 Euros.

The discovery made by Sébastien Rigali, Samuel Jourdan and their colleagues at the University of Florida is a game-changer because, thanks to this genetic manipulation involving removal of the famous closing key, the mutant bacteria massively produce thaxtomin, or ‘constitutively’, as it is described in the jargon. They are now able to produce grams of thaxtomin at cost-effective industrial scale.

Streptomyces scabies is therefore nothing like a black sheep…after one genetic modification! Their findings will do nothing to prevent the appearance of common scab: potatoes will remain covered with these scab-like lesions.  “Potatoes covered with scab lesions are eatable. It is up to people to change their consumption habits”!  It would undoubtedly be an easier task to stop buying with one’s eyes. Their research has just been published (6) in mBio, an online journal which is a reference in microbiology matters. Furthermore, their work has just been recommended in F1000Prime as being of special significance in the field of pathogenic microbiology. Although the two researchers from Liege have no intention of transforming themselves into thaxtomin dealers, a patent has been filed in collaboration with the University of Florida.

“That we made thaxtomin exploitation possible at an industrial scale is a great step forward for the herbicide to become commercially viable. Our findings come right on time as glyphosate, the most used chemical herbicide notably under the trade name Roundup, has recently been predicted to cause cancer. Investigations on the toxicity of thaxtomin still have to be performed. However, because thaxtomin is naturally produced in the soil, plants and animal species have learned to live with this biomolecule and, importantly, the microorganisms neighboring S. scabies had hundreds of millions of years to exactly know how to degrade this compound. We therefore have many reasons to believe that, when the use of an herbicidal agent will be necessary, this next-generation natural phytotoxin should alter the environment as little as possible”. 

(5) Doumbou CL, Akimov VV, Beaulieu C. 1998. Selection and characterization of microorganisms utilizing thaxtomin A, a phytotoxin produced by streptomyces scabies. Applied and environmental microbiology 64:4313-4316.
(6) The Cellobiose Sensor CebR is the Gatekeeper of Streptomyces scabies Pathogenicity, Francis and Jourdan et al. mBio, 2015.