Comparative transcriptomics and cellular analysis reveal the tolerance mechanisms of Candida tropicalis towards phenol as the representative phenolic inhibitors from lignocellulosic hydrolysis

Abstract

Background: Phenolics derived from lignocellulose severely influence cell growth and fermentation ability in yeast. Due to its high tolerance for various stresses, Candida tropicalis has recently emerged as a promising strain for the degradation of industrial sewage. Great efforts have been made on studying the degradation machineries of phenol by C. tropicalis, but the global mechanisms underlying its tolerance of high concentrations of phenolics remain ambiguous. Since the substructure and toxicity of phenol are similar to those of other phenolic compounds derived from lignin hydrolysis, we treated C. tropicalis cells with phenol as a representative substance for the phenolic compounds. Results: We found that after treatment with 0.5 g/l, 1.0 g/l, and 2.0 g/l phenol, the growth of C. tropicalis SHC-03 was inhibited, but recovered completely after a lag phase. Although we found that C. tropicalis could degrade phenol when phenol was used as the sole carbon source, it could not degrade phenol in YPD medium, implying that increased tolerance for phenol contributed to the recovery of cell growth. Morphological observation demonstrated that phenol could induce reactive oxygen species (ROS) accumulation, mitochondrial membrane damage, and damage to the endoplasmic reticulum (ER), but did not affect the structural stability of chromatin. On the basis of transcriptome data and cell wall susceptibility analysis, a number of genes related to DNA repair, DNA replication, heat shock protein (HSP)-mediated proteasomal degradation, autophagy, accumulation of fatty acids, cell wall remodeling, and MDR/MXR transporters were found. These genes may play key roles in the increased tolerance of C. tropicalis to phenol stress. Conclusion: C. tropicalis appears to prevent phenol-induced cell damage through maintenance of high-fidelity DNA replication, intracellular protein homeostasis, organelle integrity, and keep the intracellular phenol concentration at a low level through cell-wall remodeling and removal of excess phenol via MDR/MXR transporters. The knowledge obtained from this research provided us with a global understanding of phenolic tolerance mechanisms in C. tropicalis and will promote the genetic modification of yeast strains to develop biological products using lignocellulosic hydrolysates as a carbon resource.