No abstract

Filamentous fungi occupy a uniquely favorable position in the bioproduction of organic acids. Intracellular stress is the main stimulator in filamentous fungi to produce and accumulate organic acids with high flux. However, stress can affect the physiological activities of filamentous fungi, thereby deteriorating their fermentation performance. Herein, we report that peptide supplementation during Rhizopus oryzae fermentation significantly improved fumaric acid production. Specifically, fumaric acid productivity was elevated by approximately 100%, fermentation duration was shortened from 72 to 36 h, while maintaining the final titer. Furthermore, transcriptome profile analysis and biochemical assays indicated that the overall capabilities of the stress defense systems (enzymatic and nonenzymatic) were significantly improved in R. oryzae. Consequently, glycolytic metabolism was distinctly enhanced, which eventually resulted in improved fumaric acid production and reduced fermentation duration. We expect our findings and efforts to provide essential insights into the optimization of the fermentation performance of filamentous fungi in industrial biotechnology and fermentation engineering.

Rhizopus oryzae (R. oryzae) can effectively produce organic acids, and its pellet formation in seed cultures has been shown to significantly enhance subsequent fermentation processes. Despite advances in strain development, simple and effective methods for inducing pellet morphology and a basic understanding of the mechanisms controlling this process could facilitate substantial increases in efficiency and product output. Here, we report that 1.5% triethanolamine (TEOA) in seed culture medium can activate the growth of R. oryzae spores in compact and uniform pellets which is optimal for fermentation conditions. Analysis of fermentation kinetics showed that the production of fumaric and L-malic acid increases 293% and 177%, respectively. Transcriptomic analysis revealed that exposure of R. oryzae to 1.5% TEOA during the seed culture activated the phosphatidylinositol and mitogen-activated protein kinase signaling pathways. Theses pathways subsequently stimulated the downstream carbohydrate-active synthases and hydrolases that required for cell wall component synthesis and reconstruction. Our results thus provide insight into the regulatory pathways controlling pellet morphology germane to the viability of seed cultures, and provide valuable reference data for subsequent optimization of organic acid fermentation by R. oryzae.