Effect Of Furfural On Validamycin-A Fermentation Utilizing Xylose Mother Liquor

Validamycin-A(VAL-A),an agricultural antibiotic,which can efficiently treat rice sheath blight disease and many other crop diseases,is wildly used in China.In industry,VAL-A is produced by Streptomyces hygroscopicus using corn or rice-based carbon sources.However,the dependence on such grain-based media has limited the green and sustainable production of VAL-A.Previous research indicated that S.hygroscopicus var.jinggangensis 5008 could produce VAL-A using xylose mother liquor,a by-product of xylose extraction from corncob hydrolysate,as carbon source,which provided an experimental evidence of VAL-A fermentation utilizing non-food,cost-effective hemicellulose as raw material.However,it is well known that hemicellulose hydrolysates contain numerous inhibitors including furfural,which may exert negative effects on microbial fermentation.Whether furfural will affect VAL-A fermentation has not been reported.This study at first investigated the effects of different concentrations of furfural on the fermentation of VAL-A in mixed sugar(xylose,arabinose,and glucose)medium,which simulated the composition of xylose mother liquor.In the medium supplemented with furfural,cell growth was slightly lagged in the initial 12 hours,and then quickly reached or exceeded that of the control(without furfural addition).Furthermore,furfural addition at 1g/L and 3 g/L significantly increased the production titer of VAL-A,which was 12% and 15% higher than that of the control at the end of fermentation(day 5),respectively.Furfural was further added to 7%(v/v)xylose mother liquor at about 1 g/L and 1.5 g/L,which could enhance VAL-A production by up to 39% and 47% compared with the control(containing 0.07 g/L background furfural),respectively.To understand the physiological and metabolic effects of furfural addition on S.hygroscopicus 5008,the metabolism of furfural and various cell responses were analyzed in mixed sugar fermentation.The furfural in medium up to 1 g/L was converted to furfuryl alcohol and furoic acid by this strain.The direct conversion of furfural to furfuryl alcohol and furoic acid was confirmed by in vitro enzymatic reactions.During the conversion of furfural to furoic acid,the generation of hydrogen peroxide was accompanied.By measuring the intracellular hydrogen peroxide level under different furfural concentrations,it was found that the intracellular hydrogen peroxide level increased with a higher addition amount of furfural(up to 3 g/L).Furthermore,under furfural addition,transcriptional levels of structural genes(valABC,valKLMN,valG)in VAL-A biosynthesis and ValG enzyme activity were increased.The results combined with previous reports suggested furfural addition may promote VAL-A biosynthesis by inducing intracellular reactive oxygen species(ROS).In summary,furfural was found to have positive effects on VAL-A fermentation,and its action mechanism on the physiology and metabolism of S.hygroscopicus 5008 were preliminarily revealed from intracellular ROS level,gene transcription level and enzyme activity.This work was helpful to the understanding of VAL-A fermentation utilizing hemicellulose hydrolysates,and may contribute to green,sustainable and efficient production of VAL-A and other metabolites.