Perception of Dehydration Stress
Water loss may cause the following changes: e shrinkage of the protoplast, » concentration of cellular solutions,
• decrease or loss of turgor,
• changes in the water potential gradient across membranes,
• in the worst cases, disintegration of biomembranes and denaturing of proteins.
Which of these phenomena play a role as cellular signals in plants has not yet been determined. Probably it is the change in the water potential or pressure gradient across membranes. Osmosensors are known from E. coli and yeast. Their functions were shown in complementation studies with mutants deficient in osmotic adaptation (Wurgler-Murphy and Saito 1997; see also Box 1.4.5). One of the transmembrane osmosensors of yeast, Sholp, forms a protein of four closely packed membrane-spanning peptides with a peripheral C-terminal domain that is responsible for triggering signal transduction. If the osmotic gradient across the plasma membrane is increased, Sholp activates a four-step MAP kinase cascade, leading finally to the production and accumulation of glycerol, which is an osmoprotectant of yeast. Successful complementation of the osmosensor-deficient mutant of yeast with a corresponding gene of Arabidopsis (ATHK1; Shinozaki and Yamaguchi-Shinozaki 1997) leads to the assumption that similar systems of drought perception are also effective in plants. Due to the multiplicity of stress responses of plants, it must be concluded that plant cells have several receptor systems which might render tissue specificity (Bonetta and McCourt 1998). Receptors for ABA should be differentiated from direct osmosensors, as
Biosynthesis, metabolism and inactivation of ABA
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