Toxic metal accumulation, responses to exposure and mechanisms of tolerance in plants

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

Osmotic Stress Signaling and Osmoadaptation in Yeasts

Specific Aquaporins Facilitate the Diffusion of Hydrogen Peroxide across Membranes

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca2+ signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.

Cell Wall Integrity Signaling in Saccharomyces cerevisiae

Membrane transport of hydrogen peroxide

https://www.jbc.org/content/274/23/16040.full.pdf

Yap1 and Skn7 Control Two Specialized Oxidative Stress Response Regulons in Yeast

https://www.jbc.org/content/276/11/8469.full.pdf

A Proteome Analysis of the Cadmium Response in Saccharomyces cerevisiae

A New Antioxidant with Alkyl Hydroperoxide Defense Properties in Yeast

Yeast, and especially S. cerevisiae, is a unique eukaryotic model organism for studying oxidative stress and its cellular responses. S. cerevisiae has become a very powerful tool to decipher the complexity of these biologically important responses, because it offers the relative simplicity of a single celled eukaryotic organism that enables the combination and integration of genetic, biochemical, physico-chemical, cell biological, and genome-wide experimental approaches. We introduce the subject with basic concepts about reactive oxygen species (ROS) and explain their cellular toxicity and the experimental approaches that have been adapted to their analysis. Subsequently, we summarize some of the knowledge obtained in the yeast model of the cellular effects of ROS, the antioxidant and thiol redox control systems, and the cellular responses to elevated ROS concentrations. Cellular responses to ROS concentrations are often referred to as adaptive oxidative stress responses. Special attention is given to the signal transduction pathways that regulate these responses in S. cerevisiae, S. pombe, and other fungi. Emphasis is given to the systems that sense elevated ROS concentrations. Important links between oxidative stress responses, carbohydrate metabolism, respiration and the mitochondria, iron and copper metabolism, cell cycle control and yeast apoptotic-like responses are also dealt with here. The data reviewed in this article illustrates how much our understanding of the biology of oxidants has advanced, and how much research is still needed to unravel its complexity.

Daniel Spector

Papers

Yap1 and Skn7 Control Two Specialized Oxidative Stress Response Regulons in Yeast

A Proteome Analysis of the Cadmium Response in Saccharomyces cerevisiae

A New Antioxidant with Alkyl Hydroperoxide Defense Properties in Yeast

Oxidative stress responses in yeast