Valorization of xylose mother liquor into salidroside by engineered Kluyveromyces marxianus

Large amounts of xylose mother liquor (XML) are produced as a byproduct of the xylose industry and are often treated as waste. Salidroside is a high-value glycoside with pharmaceutical and nutraceutical applications, but conventional extraction from plants cannot meet market demand and threatens the sustainability of <i>Rhodiola</i> resources. Here, XML was valorized for <i>de novo</i> salidroside production using engineered <i>Kluyveromyces marxianus</i>. The <i>KmPDC1</i> disruption promoted pyruvate accumulation, which in turn enhanced the accumulation of a shikimate pathway precursor (phosphoenolpyruvate). Introduction of the salidroside biosynthesis pathway enabled salidroside production in <i>K. marxianus</i>. Additionally, salidroside production was improved through three strategies: expressing feedback-resistant 3-deoxy-d-arabino-heptulosonate-7-phosphate (DAHP) synthase in the shikimate pathway, expressing phosphoketolase to boost erythrose-4-phosphate (another precursor of the shikimate pathway), and expressing chorismate mutase/prephenate dehydrogenase to facilitate tyrosine synthesis. The control of pH during fermentation further enhanced salidroside production. The final strain YSAL07 produced 10.17 ± 0.12 g/L salidroside in shake-flask cultivation, with a productivity of 84.75 mg/(L·h), which is higher than previously reported values. In the bioreactor, YSAL07 produced 13.18 ± 0.18 g/L salidroside within 96 h using glucose as the substrate, whereas fermentation with XML achieved a salidroside titer of 15.49 ± 0.69 g/L within 120 h. This study demonstrates the first gram-scale (≥10 g/L) salidroside production in <i>K. marxianus</i> directly from XML, providing a sustainable and economically viable route for the manufacture of high-value natural products.