QTL mapping of a Brazilian bioethanol strain links the cell wall protein-encoding gene GAS1 to low pH tolerance in S. cerevisiae

General mechanisms of weak acid-tolerance and current strategies for the development of tolerant yeasts

Yeast cells are often subjected to various types of weak acid stress in the process of industrial production, food processing, and preservation, resulting in growth inhibition and reduced fermentation performance. Under acidic conditions, weak acids enter the near-neutral yeast cytoplasm and dissociate into protons and anions, leading to cytoplasmic acidification and cell damage. Although some yeast strains have developed the ability to survive weak acids, the complexity and diversity of stresses during industrial production still require the application of appropriate strategies for phenotypes improvement. In this review, we summarized current knowledge concerning weak acid stress response and resistance, which may suggest important targets for further construction of more robust strains. We also highlight current feasible strategies for improving the weak acid resistance of yeasts, such as adaptive laboratory evolution, transcription factors engineering, and cell membrane/wall engineering. Moreover, the challenges and perspectives associated with improving the competitiveness of industrial strains are also discussed. This review provides effective strategies for improving the industrial phenotypes of yeast from multiple dimensions in future studies.

The cell wall and the response and tolerance to stresses of biotechnological relevance in yeasts

In industrial settings and processes, yeasts may face multiple adverse environmental conditions. These include exposure to non-optimal temperatures or pH, osmotic stress, and deleterious concentrations of diverse inhibitory compounds. These toxic chemicals may result from the desired accumulation of added-value bio-products, yeast metabolism, or be present or derive from the pre-treatment of feedstocks, as in lignocellulosic biomass hydrolysates. Adaptation and tolerance to industrially relevant stress factors involve highly complex and coordinated molecular mechanisms occurring in the yeast cell with repercussions on the performance and economy of bioprocesses, or on the microbiological stability and conservation of foods, beverages, and other goods. To sense, survive, and adapt to different stresses, yeasts rely on a network of signaling pathways to modulate the global transcriptional response and elicit coordinated changes in the cell. These pathways cooperate and tightly regulate the composition, organization and biophysical properties of the cell wall. The intricacy of the underlying regulatory networks reflects the major role of the cell wall as the first line of defense against a wide range of environmental stresses. However, the involvement of cell wall in the adaptation and tolerance of yeasts to multiple stresses of biotechnological relevance has not received the deserved attention. This article provides an overview of the molecular mechanisms involved in fine-tuning cell wall physicochemical properties during the stress response of Saccharomyces cerevisiae and their implication in stress tolerance. The available information for non-conventional yeast species is also included. These non-Saccharomyces species have recently been on the focus of very active research to better explore or control their biotechnological potential envisaging the transition to a sustainable circular bioeconomy.

Crosstalk between Yeast Cell Plasma Membrane Ergosterol Content and Cell Wall Stiffness under Acetic Acid Stress Involving Pdr18

Acetic acid is a major inhibitory compound in several industrial bioprocesses, in particular in lignocellulosic yeast biorefineries. Cell envelope remodeling, involving cell wall and plasma membrane composition, structure and function, is among the mechanisms behind yeast adaptation and tolerance to stress. Pdr18 is a plasma membrane ABC transporter of the pleiotropic drug resistance family and a reported determinant of acetic acid tolerance mediating ergosterol transport. This study provides evidence for the impact of Pdr18 expression in yeast cell wall during adaptation to acetic acid stress. The time-course of acetic-acid-induced transcriptional activation of cell wall biosynthetic genes (FKS1, BGL2, CHS3, GAS1) and of increased cell wall stiffness and cell wall polysaccharide content in cells with the PDR18 deleted, compared to parental cells, is reported. Despite the robust and more intense adaptive response of the pdr18Δ population, the stress-induced increase of cell wall resistance to lyticase activity was below parental strain levels, and the duration of the period required for intracellular pH recovery from acidification and growth resumption was higher in the less tolerant pdr18Δ population. The ergosterol content, critical for plasma membrane stabilization, suffered a drastic reduction in the first hour of cultivation under acetic acid stress, especially in pdr18Δ cells. Results revealed a crosstalk between plasma membrane ergosterol content and cell wall biophysical properties, suggesting a coordinated response to counteract the deleterious effects of acetic acid.