Development of a bioprocess for the production of a herbicidal sugar as sustainable alternative to glyphosate

Weeds pose severe threats to agricultural yields and areas used for settlement and transport. Until now, glyphosate is widely applied to reduce undesired plants worldwide [1]. Due to its negative impact on the environment, insects and humans [2-5] and likely prohibition in the EU until the end of 2023 [6], an effective as well as ecologically acceptable alternative is urgently needed. 7-deoxy-sedoheptulose (7dSh) is a novel herbicidal candidate. This sugar was isolated from culture supernatants of Synechococcus elongatus and has recently been reported to act as an inhibitor of a key enzyme of the shikimate pathway [7] analogous to glyphosate [1]. Yet the cyanobacterium S. elongatus produces very low amounts of 7dSh merely in the stationary phase of cultivation [7] and is therefore not a suitable candidate for an industrial production. Besides, chemoenzymatic synthesis of 7dSh has a low yield (20%) [7] and is not economically feasible due to high substrate costs such as 5-deoxy-D-ribose. Therefore, we aim to develop a microbial process with Streptomyces setonensis as a natural producer strain [7] for the large-scale production of 7dSh as an alternative herbicide. To evolve a scalable, well-characterized bioprocess for an unknown microbial system, firstly, parameters as media composition, phosphate and nitrogen limitation were examined using a high-throughput microbioreactor system (BioLector) which allows online-monitoring of growth. Moreover, a design of experiments (DOE) approach in shake flasks was conducted to investigate the influence of cultivation factors on each other. Results from the DOE approach suggest that elevated osmolarity through NaCl addition enables higher product titers whereas nitrogen starvation or addition has no influence. For other Streptomyces strains it is reported that the synthesis of antibiotics and other secondary metabolites is coupled to phosphate limitation [8]. Surprisingly, in contrast, 7dSh synthesis is enhanced by adding 150% more of both phosphate components than in the basic minimal medium [9] and decreased if phosphate starvation occurs. Therefore, the identification of the trigger for the 7dSh production is still ongoing, although the product titer was already increased by factor 100 from mg/L [7] to g/L. The identified beneficial culture conditions lay the foundation for the process scale-up to a stirred-tank reactor and to further improve product concentration by applying process control and defining the optimal operation regime. Moreover, organic residuals should be employed as substrates for a sustainable and cost-efficient process. REFERENCES [1] https://doi.org/10.1002/ps.1518 [2] https://doi.org/10.1038/s42003-021-02057-6 [3] https://doi.org/10.1371/journal.pbio.3001182 [4] DOI:10.3390/insects10100354 [5] DOI:10.1016/j.envpol.2020.114372 [6] https://www.bmuv.de/themen/wasser-ressourcen-abfall/boden-und-altlasten/bodenschutz-und-altlasten-worum-geht-es/faq-plan-zum-glyphosat-ausstieg [7] https://doi.org/10.1038/s41467-019-08476-8 [8] DOI: 10.1128/JB.186.16.5197-5201.2004 [9] DOI: 10.3389/fmicb.2018.02680