Mutagenesis Breeding and Liquid Surface Culture Strategy for R-2-(4-Hydroxyphenoxy) Propionic Acid-Producing Strains

R-2-(4-hydroxyphenoxy)propionic acid (R-HPPA) is a globally sought-after intermediate for aromatic phenoxy propionic acid herbicides. In this study, a mutant strain of Beauveria bassiana, designated as ZJB23323 (CCTCC NO: M 2023584), was obtained through UV, atmospheric, and room temperature plasma, and N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis, exhibiting 7.24-fold improved production of R-HPPA. The concentrations of glucose, yeast extract, and R-2-phenoxypropionic acid (R-PPA) in the initial medium were optimized for R-HPPA production by using response surface methodology with Box-Behnken design. The maximum R-HPPA yield of 25.8 g/L was obtained at optimal concentrations of 64.7 g/L for glucose, 21.9 g/L for yeast extract, and 31.7 g/L for R-PPA, which increased by 48.7% compared to the original medium. Finally, the scaled-up biosynthesis of R-HPPA was successfully carried out in a 6-L plastic tray bioreactor, and the kinetic model and model parameters of the R-HPPA batch fermentation process were described. The metabolic characterization of strain ZJB23323 for the fermentative production of R-HPPA under three modes of batch culture, constant rate feeding, and exponential feeding was further investigated. The results showed that 100 g/L of R-PPA could be completely converted to R-HPPA within 13 days under the exponential feeding culture strategy.