A Belgian tale

It was Christmas Eve 2012, when suddenly, bingo! Just as preparations were being made for the annual festivities, PREDetector detected DNA motifs (locks) recognized by the transcriptional repressor (the key) associated with the thaxtomin-synthesizing genes in Streptomyces scabies. It is this molecule that allows S. scabies to attack the roots and tubers of plants in order to extract their nutritional reserves and which is behind these horrible brown-tinted craters on the potatoes.

On returning to the laboratory, Samuel Jourdan was informed about the PREDetector predictions and few weeks later, the latter confirmed that the key which fits the lock is indeed CebR, the transcription repressor for the use of cellulose. The researchers from Liege decided to share their discovery with Prof. Rosemary Loria of the University of Florida in Gainesville (USA), a pioneer in this area of research for about 20 years. In Gainesville, another Belgian, Dr. Isolde Francis, was awarded a post-doctoral grant for the specific study of the induction mechanisms involved in the production of thaxtomin in S. scabies.

“Revealing results that are unpublished is an unusual practice in the realm of science. In general, individuals prefer to keep their discoveries to themselves, partly out of fear of being cheated but also because any public disclosure eliminates the possibility of filing a patent.  Nevertheless, we have always contacted the main interested parties in order to comment upon and share our work and this applies to all our research subjects. Not to contact Rosemary Loria would have been a professional mistake! She is the most important specialist in this subject; we were delighted to work with her”.
 
There is therefore a perfect understanding between the Belgian researchers in Liege and those in Gainesville, and the work is equitably shared: the in vitro experiments are conducted in Liege and the in vivo approaches in Florida. They quickly discovered the way in which S. scabies triggers its pathogenic behavior and generates a series of mutants that produce high quantities of thaxtomin, or that, conversely, do not produce thaxtomin at all.

A double locking system

How does it work? “S. scabies has to detect the presence of its host in the environment. Without perception, there is no production of thaxtomin and therefore no attack on the plants”, observes Samuel Jourdan. “With our colleagues from the University of Florida we discovered how this signal is perceived by Streptomyces scabies and how this triggers the production of the phytotoxin”.

But what is this signal? Rosemary Loria and her colleagues had already found the environmental elicitors for thaxtomin in 2007, those which act as the ‘starting pistol’ for the production of the toxin (3). The molecules which give the ‘green light’, are cellobiose and cellotriose (dimers and trimers of glucose respectively), that is to say, disaccharide and trisaccharide components of cellulose that are found in the walls of plant cells (4).

“In response to cellobiose or cellotriose emanating from the walls of the plant cells, CebR no longer represses the expression of the genes that code not only for the production of thaxtomin, but also for a gene called txtR, which codes for an activator for the expression of these same synthesis genes. We are therefore in the presence of a locking system with two keys: one key which closes called CebR, which controls a key that opens called TxtR. When the cellobiose or cellotriose fixes on CebR, the latter can no longer prevent the expression of the key that opens, TxtR. The inactivation of the cebR gene results therefore in a constant expression of TxtR, and S. scabies presents a hypervirulent behavior because it excessively produces phytotoxin”, explains Sébastien Rigali (opposite).

(3) Wach MJ, Krasnoff SB, Loria R, Gibson DM. 2007. Effect of carbohydrates on the production of thaxtomin A by Streptomyces acidiscabies. Archives of microbiology 188:81-88.
(4) Johnson EG, Joshi MV, Gibson DM, Loria R. 2007. Cello-oligosaccharides released from host plants induce pathogenicity in scab-causing Streptomyces species. Physiological and Molecular Plant Pathology 71:18-25.