Development of a bioprocess with Streptomyces setonensis to produce a novel herbicidal sugar as an alternative to glyphosate

The controversial herbicide glyphosate is widely applied to reduce weeds worldwide, to increase agricultural yields to feed the world's population. Emerging plant resistances and its negative impact on the environment and living beings urge for an effective as well as ecologically acceptable alternative. Streptomyces setonensis, a natural producer strain of such a novel herbicidal substance, seems a promising biocatalyst for the large-scale production of 7-deoxy-sedoheptulose (7dSh). However, this strain has been marginally investigated. Therefore, we developed a microbial production process for 7dSh, systematically optimizing it by investigating essential factors such as medium composition, phosphate and nitrogen limitation, as well as process parameters and modes. Starting on a small scale, the experiments were conducted in a high throughput microbioreactor system (BioLector), in shake flasks employing a Design of Experiments approach and complemented by real-time oxygen transfer measurements with RAMOS® (Respiration Activity MOnitoring System) to gain a deeper understanding of the bacterial metabolism. Elevated osmolarity emerged as a critical factor for 7dSh production. Surprisingly, increased phosphate concentrations enabled an altered metabolism after the growth phase and extended culture longevity, allowing for the application of a feeding strategy. The optimized process was successfully scaled up to a 15L stirred tank bioreactor. Ultimately, a 200-fold increase in 7dSh titer from 22mg/L to over 4g/L was achieved. This represents the highest reported concentration to date and provides a solid foundation for pilot-scale production. The obtained titers pave the way for downstream formulation studies to establish 7dSh as a sustainable herbicide in modern agriculture.