Metatranscriptomics reveals sequentially expression of genes involved in the production of melanogenesis inhibitors by the defined microbial species in fermented unpolished black rice

Fermented rice products require metabolic enzymes from microbial community for desired final products. Although the role of these microorganisms in fermented products were investigated using metatranscriptomics approach for flavor improvement, no study has been reported so far on the function of microorganisms on producing compounds with biological activity. Our previous study, the unpolished black rice (UBR) was fermented using the E11 starter, containing Saccharomyces, Saccharomycopsis, Rhizopus, and Pediococcus species, to produce fermented UBR (FUBR) showed potent melanogenesis inhibition activity. The goal of this study was to investigate the function of these defined microbial species in producing the melanogenesis inhibitors in the FUBR using metatranscriptomics approach. The melanogenesis inhibition activity of the FUBR was increased as fermentation time-dependent manner. Genes related with melanogenesis inhibitors synthesis such as carbohydrate metabolism, amino acids/peptides synthesis, fatty acids/unsaturated fatty acids synthesis and carbohydrate transporter were analyzed. Most genes from Rhizopus and Pediococcus species were upregulated in the Early stage of the fermentation process, while those of Saccharomyces and Saccharomycopsis species were upregulated in the Late stage. Moreover, melanogenesis inhibition activity of fermented UBR from different combination of the four microbial species showed that all four species were required to produce the highest activity. The fermented UBR at least contains Rhizopus and/or Pediococcus species exhibited a certain level of melanogenesis inhibition activity, whereas the samples derived from Saccharomyces and/or Saccharomycopsis species did not reveal the activity. These findings were in agreement with the metatranscriptomics results. Overall results suggested that all the four species in the E11 starter sequentially and/or coordinately synthesized metabolites during the UBR fermentation which eventually led to the FUBR with potent melanogenesis inhibition activity. This study not only sheds light on the crucial functions of the culture starter in the FUBR on the production of melanogenesis inhibitors but also pave the way to initiate quality improvement of melanogenesis inhibition activity of the FUBR.