▪ Abstract Plant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of stress sensing. We make extensive use of comparisons with model organisms, halophytic plants, and yeast, which provide a paradigm for many responses to salinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and membrane transport of ions for maintenance and re-establishment of homeostasis. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly based on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pathways. We summarize evidence for plant stress signaling systems, some of which have components analogous to t...

Plant cellular and molecular responses to high salinity.

Mutations at the URA3 locus of Saccharomyces cerevisiae can be obtained by a positive selection. Wild-type strains of yeast (or ura3 mutant strains containing a plasmid-borne URA3+ gene) are unable to grow on medium containing the pyrimidine analog 5-fluoro-orotic acid, whereas ura3- mutants grow normally. This selection, based on the loss of orotidine-5'-phosphate decarboxylase activity seems applicable to a variety of eucaryotic and procaryotic cells.

A positive selection for mutants lacking orotidine-5'-phosphate decarboxylase activity in yeast: 5-fluoro-orotic acid resistance.

Environmental stresses come in many forms, yet the most prevalent stresses have in common their effect on plant water status. The availability of water for its biological roles as solvent and transport medium, as electron donor in the Hill reaction, and as evaporative coolant is often impaired by environmental conditions. Although plant species vary in their sensitivity and response to the decrease in water potential caused by drought, low temperature, or high salinity, it may be assumed that all plants have encoded capability for stress perception, signaling, and response. First, most cultivated species have wild relatives that exhibit excellent tolerance to abiotic stresses. Second, biochemical studies have revealed similarities in processes induced by stress that lead to accumulated metabolites in vascular and nonvascular plants, algae, fungi, and bacteria (Csonka, 1989; Galinski, 1993; Potts, 1994). These metabolites include nitrogen-containing compounds (proline, other amino acids, quaternary amino compounds, and polyamines) and hydroxyl compounds (sucrose, polyols, and oligosaccharides) (McCue and Hanson, 1990). Accumulation of any single metabolite is not restricted to taxonomic groupings, indicating that these are evolutionarily old traits. Third, molecular studies have revealed that a wide variety of species express a common set of genes and similar proteins (for example, Rab-related proteins and dehydrins) when stressed (Skriver and Mundy, 1990; Vilardell et al., 1994). Although functions for many of these genes have not yet been unequivocally assigned, it is likely, based on their characteristics, that these proteins play active roles in the response to stress. Learning about the biochemical and molecular mechanisms by which plants tolerate environmentat stresses is necessary for genetic engineering approaches to improving crop performance under stress. By investigating plants under stress, we can learn about the plasticity of metabolic pathways and the limits to their functioning. Also, questions of an ecological and evolutionary nature need investigation. Are the genes that confer salt tolerance on halophytes and/or drought tolerance on xerophytes evolutionarily ancient genes that have been selected against in saltand drought-sensitive plants (glycophytes) for the sake of productivity? Or have some species obtained nove1 genes in their evolutionary history that have enabled them to occupy stressful environments? How will the

Adaptations to Environmental Stresses.

Elevated recombination rates in transcriptionally active DNA.

The increased use of antibacterial and antifungal agents in recent years has resulted in the development of resistance to these drugs. The significant clinical implication of resistance has led to heightened interest in the study of antimicrobial resistance from different angles. Areas addressed include mechanisms underlying this resistance, improved methods to detect resistance when it occurs, alternate options for the treatment of infections caused by resistant organisms, and strategies to prevent and control the emergence and spread of resistance. In this review, the mode of action of antifungals and their mechanisms of resistance are discussed. Additionally, an attempt is made to discuss the correlation between fungal and bacterial resistance. Antifungals can be grouped into three classes based on their site of action: azoles, which inhibit the synthesis of ergosterol (the main fungal sterol); polyenes, which interact with fungal membrane sterols physicochemically; and 5-fluorocytosine, which inhibits macromolecular synthesis. Many different types of mechanisms contribute to the development of resistance to antifungals. These mechanisms include alteration in drug target, alteration in sterol biosynthesis, reduction in the intercellular concentration of target enzyme, and overexpression of the antifungal drug target. Although the comparison between the mechanisms of resistance to antifungals and antibacterials is necessarily limited by several factors defined in the review, a correlation between the two exists. For example, modification of enzymes which serve as targets for antimicrobial action and the involvement of membrane pumps in the extrusion of drugs are well characterized in both the eukaryotic and prokaryotic cells.

Antifungal Agents: Mode of Action, Mechanisms of Resistance, and Correlation of These Mechanisms with Bacterial Resistance

A membrane polypeptide involved in K+ transport in a higher plant was cloned by complementation of a yeast mutant defective in K+ uptake with a complementary DNA library from Arabidopsis thaliana. A 2.65-kilobase complementary DNA conferred ability to grow on media with K+ concentration in the micromolar range and to absorb K+ (or 86Rb+) at rates similar to those in wild-type yeast. The predicted amino acid sequence (838 amino acids) has three domains: a channel-forming region homologous to animal K+ channels, a cyclic nucleotide-binding site, and an ankyrin-like region.

https://science.sciencemag.org/content/sci/256/5057/663.full.pdf

Cloning and expression in yeast of a plant potassium ion transport system.

Summary
An Arabidopsis thaliana cDNA bank has been constituted in a Saccharomyces cerevisiae expression vector based on the phosphoglycerate kinase (PGK) promoter and terminator. This bank was used to complement eight S. cerevisiae auxotrophic markers. All of them were corrected. These results confirm the quality of the bank and the feasibility of cloning plant genes by yeast mutant complementation. The cDNA complementing the ura1 yeast mutant was sequenced, analysed and shown to encode a dihydroorotic (DHO) dehydrogenase sequence.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-313X.1992.t01-38-00999.x

Complementation of Saccharomyces cerevisiae auxotrophic mutants by Arabidopsis thaliana cDNAs.

The fine structure map of the yeast URA 3 gene was established by meiotic recombination, and amber nonsense mutations were located at different points on the map. The effect of the length of the labeling time on the specific radioactivity of ura 3 messenger RNA and on its repartition between poly(A)-RNA and RNA not containing poly(A) has been followed in nonsense mutants. Nonsense mutations reduce the messenger level without lowering its instantaneous rate of synthesis. The strength of the reduction depends on the position of the nonsense codon within the locus and concerns essentially the accumulation of polyadenylylated ura 3 mRNA.

https://www.pnas.org/content/pnas/76/10/5134.full.pdf

Interference of nonsense mutations with eukaryotic messenger RNA stability

Mutants of Saccharomyces cerevisiae capable of growth on a minimal medium supplemented with ureidosuccinic and glutamic acids have been isolated from a pyrimidineless strain. One of these mutants consistently yielded a non-Mendelian meiotic segregation. Moreover, the mitotic transmission of the mutation was very high. It is suggested that the mutation is nonchromosomal and could be mitochondrial. However, this mutation behaves very differently from other mitochondrial mutations.

Non-Mendelian Mutation Allowing Ureidosuccinic Acid Uptake in Yeast

Biochemical steps of the pyrimidine pathway have been found to be the same in yeast as in bacteria, and all except one step have been characterized. The activities of the first two enzymes, carbamoyl phosphate synthetase and aspartic transcarbamylase, are simultaneously controlled by feedback inhibition and repression. Moreover, these enzymes are coded by the same genetic region (ura-2) and seem to form a single enzymatic complex. The enzymes that follow later in the pathway are induced in a sequential way by the intermediary products and are insensitive to pyrimidine repression. The corresponding genes (ura-4, ura-1, ura-3) are not linked to each other or to ura-2, the gene for carbamoyl phosphate synthetase and aspartic transcarbamylase. Mutants that have simultaneously lost feedback inhibition by uridine triphosphate for carbamoyl phosphate synthetase and for aspartic transcarbamylase have been found and mapped in the gene ura-2.

François Lacroute

Papers

Cloning and expression in yeast of a plant potassium ion transport system.

Complementation of Saccharomyces cerevisiae auxotrophic mutants by Arabidopsis thaliana cDNAs.

Interference of nonsense mutations with eukaryotic messenger RNA stability

Non-Mendelian Mutation Allowing Ureidosuccinic Acid Uptake in Yeast

Regulation of Pyrimidine Biosynthesis in Saccharomyces cerevisiae