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Specific and Unspecific Reactions to Stress

An organism that is stressed, for example, by elevated temperature, not only increases its metabolic rate, but other reactions occur which are usually not observed in the unstressed organism, or take place only to a very small degree. An example of this is the formation of "heat shock proteins" (see Chap, 1.3.4.2). The modification of the basic metabolism could be interpreted as an unspecific reaction, whilst the production of heat shock proteins would be considered a specific stress reaction of the organism. The differentiation of these two components of a stress reaction is based on the findings of Hans Selye (1973), a Canadian general practitioner, who, in the 1970s, summarised the various complexes of stress reactions of human beings as follows: "Everything which endangers life causes stress reactions and adaptive reactions. Both types of reactions are partly specific and partly unspecific." Contrary to plants there is, in humans, also a strong psychic-humoral stress component. The concept of both components of the stress reaction is complicated by the fact that even the specific reactions often lack specificity: The above-mentioned heat shock proteins also assist the folding of proteins during synthesis and after denaturing (see Chap. 1.3.4.2), not only by high-temperature stress, but also under other stresses. They are produced in high amounts, for example, under stress by xenobiotics (e.g. heavy metals). This does not exclude that there are in addition more specific responses by which an organism differentiates between stress by heat and by heavy metals (see Chap, 1 7.5).

There is yet another facet to the question of specificity of stress reactions which is described by the term cross-protection. Previous drought stress or salt stress (osmotic stress) is known to harden plants against temperature stress, and particularly cold stress (Fig. 1.1.4). Is this an unspecific stress response? The apparent lack of specificity of the adaptation is explained, on the one hand by considering the physiological effects of salt and drought stress on cells and, on the other, the effects of frost. All three factors lead to a partial dehydration of cells (in an ivy leaf at -7°C, ca. 90% of the total leaf water is frozen, forming ice, and thus is no longer available as free water; see Fig. 1.3.25). This causes problems with the stability of biomembranes in particular, as the lipid bilayers are stabilised by so-called hydrophobic interactions, which are disturbed if the availability of water, or the ion concentration at the surface of membranes, is drastically changed (see also Chap. 1.3.5.2). If too much water is removed from the aqueous environment of the biomembranes (by evaporation or freezing), the concentration of solutes increases, e.g. in the cytosol or the chloroplast stroma. Increase in the ion concentrations in turn changes the charges at the surface of membranes, and as a consequence the membrane potentials. This usually leads to déstabilisation of membrane structure. High charge densities, however, not only result from water deficiency, but also from excessive salt concentration. A general reaction to stress is the synthesis of hy-drophilic low molecular protectants, so-called compatible solutes (sugars, sugar alcohols and cyclitols, amino acids and betaines, see Chaps. 1.5.2.6 and 1.6.2.3), which replace water at the membrane surfaces and dislodge the ionic compounds upon loss of cellular water. Production of compatible solutes requires, of course, synthesis of respective enzymes, triggered by stress. Synthesis of these enzymes is often preceded by signals transmitted by certain phytohormones -particularly abscisic acid (ABA) or the stress hormone jasmonate, but also ethylene, may transiently change their concentration. One ex-

Salt treated plants T^ï_T

Control

0 1 2 3 4 5 Duration of salt treatment (days)

1" Salt treated * plants

1" Salt treated * plants

Fig. 1.1.4. Frost hardening through salt treatment. Cuttings of potato plants (Solanum commersonii Dun PI 458317) were grown in Murashige-Skoog medium to which NaCI was added (100 mM final concentration). A Frost hardiness of plants and b ABA content of plants. (After Ryu et al. 1995)

-2 -1 0 1 2 3 4 5 Duration of salt treatment (days)

-2 -1 0 1 2 3 4 5 Duration of salt treatment (days)

Fig. 1.1.4. Frost hardening through salt treatment. Cuttings of potato plants (Solanum commersonii Dun PI 458317) were grown in Murashige-Skoog medium to which NaCI was added (100 mM final concentration). A Frost hardiness of plants and b ABA content of plants. (After Ryu et al. 1995)

ample of such cross-protection is induction of frost hardening in wild potatoes by salt stress (Fig. 1.1.4). Potato plants treated with NaCl are able to tolerate lower temperatures than untreated controls. A transient increase in ABA concentration mediates this hardening reaction.

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