Tolerance stress

There are several aspects of different environmental stresses that either have common features or the plant responses or adaptations to those stresses may have common components or indicate general principles. It is an objective of this volume to identify such features where they exist so as to help in the development of stress-tolerant crop plants by making the best use of the newer techniques of molecular biology. Particular examples will be discussed in more detail in succeeding chapters. 

The techniques of molecular biology have particular potential for rapidly introducing small numbers of single genes. Unfortunately there is strong evidence that the complex compensation mechanisms that exist in plants, and the interactions between different whole-plant and biochemical responses to stress, will make the direct improvement of environmental stress tolerance in crop plants by genetic engineering rather more difficult... 

Fig. 1. (a) Model describing the various equilibria between competition, stress and disturbance in vegetation and the location of primary and secondary strategies. C, competitor S, stress tolerator R, ruderal C-R, competitive-ruderal S-R, stress-tolerant ruderal C-S, stress-tolerant... 

Molecular biology application to studies of stress tolerance... 

In this chapter we first discuss the development of systems for delivering DNA to plant cells. The remainder of the chapter then outlines the ways in which molecular genetics can be applied to research on stress tolerance, giving examples where these techniques are being used. 

For a detailed discussion of DNA delivery systems, readers are referred to the recent very comprehensive review by Walden (1988). Here we concentrate on the two methods most used in stress tolerance studies. 

As this suggests, the major problem we face is the recognition and isolation of genes which confer stress tolerance. Thereafter, we may use reverse genetics to confirm the adaptive significance of these genes. Other approaches to gene isolation will now be discussed. 

In some sequences we may have a genetic understanding of stress tolerance to the point where we know that it, or some aspect of it, appears to segregate as a single gene. In this case, although there is little reason for... 

A further opportunity for the use of stress-responsive promoters and enhancers is as probes to isolate other stress-responsive genes, the activity of which is not manifest by protein synthesis. As regards the manipulation of stress tolerance as a breeding tool, it is likely that the stress-responsive promoters and enhancers will have a role to play in controlling the expression of adaptive genes when these are transplanted over great evolutionary distances. 

The use of plants from extreme environments Wild plants from extreme environments may possess genes and gene combinations which confer stress tolerance. We must realise, however, that many of their characteristics, e.g. leaf pubescence and succulence in drought-resistant plants, are incompatible with the high yield potential required for crop plants. In addition, most of these species contain compounds such as phenolics and mucilages which interfere with conventional molecular biology techniques. 

For future research in this field, in addition to physiological and biochemical approaches, genetic analysis will be essential in the establishment of causal relationships between the induction of a stress protein and the establishment of tolerance to the stress condition. In most cases it is not difficult to detect the induction of new proteins during stress. However, the induction of new proteins does not necessarily establish stress tolerance it may well be the consequence of damage caused by stress conditions. Thus, genetic mutants will be necessary to test the physiological role of a stress protein. 

A more general role of ABA in stress tolerance has been found in carrot cells. When a suspension culture of carrot cells was exposed to ABA and then selected for tolerance to freezing, the ABA-treated cells were found to be more tolerant to the stress (Reaney Gusta, 1987). These results provide further evidence for the presence of common mechanisms conveying tolerance to many of the environmental stresses. 

The use of protoplasts in studies of stress physiology and biochemistry expands the advantages of cell culture systems discussed in the preceding sections. Additional applications are related to the fusion of protoplasts. Intraspecifie and interspecific protoplast fusion greatly enhance genetic variability of the fused protoplasts (Kumar Cocking, 1987). The resulting somatic hybrids provide cells which can be used for selection of specific traits (e.g. environmental stress tolerance) provided by one or both donor cells and for basic studies on cytoplasmic and nuclear inheritance of desired characteristics. 

The literature is less extensive on the use of protoplasts in stress-tolerance investigations however, some applications have been attempted. For example, in one study protoplasts were isolated from the leaves of a wild relative of tomato shown to be salt tolerant and from a salt-sensitive, cultivated species (Rosen Tal, 1981). In the presence of NaCI the plating efficiency (number of surviving cells/number of cells applied to the plate) of the wild relative was greater than the cultivated, sensitive cultivar. Proline, when added to the culture media, was found to enhance the plating efficiency of the salt-sensitive cultivar but not the wild, salt-tolerant relative. These results suggest that traits related to salt tolerance are expressed by the isolated protoplasts and that the response of protoplasts to environmental stress can be manipulated, i.e. the proline response. 

A more significant body of literature focuses on the use of protoplasts in understanding processes related to stress tolerance. The role of Ca in salt toleranee has been evaluated using maize root protoplasts. Exposure of the plasmalemma directly to external media revealed a non-specific replacement of Ca by salt. Sodium was found to replace Ca though this could be reversed by adding more Ca (Lynch, Cramer Lauchli, 1987). This approach assists in understanding the role of specific ion interaction in enhancing salt tolerance and is potentially applicable to studies on the molecular basis for ion specificity of plant membranes. 

Castleberry, R.M. (1983). Breeding programs for stress tolerance in corn. In Crop Reactions to Water and Temperature Stresses in Humid, Temperature Climates, ed. C.D. Raper Jr and P.J. Kramer, pp. 277-88. Boulder, Colorado Westview Press. 

Vaadia, Y. (1987). Salt and drought tolerance in plants regulation of water use efficiency in sensitive and tolerant species. In NATO Conference on Biochemical and Physiological Mechanisms Associated with Environmental Stress Tolerance in Plants, University of East Anglia, Norwich, 2-7 August 1987. 

Flowers, T.J. Yeo, A.R. (1989). Effects of salinity on plant growth and crop yields. In Biochemical and Physiological Mechanisms Associated with Environmental Stress Tolerance in Plants, ed. J. Cherry. Berlin Springer-Verlag, (in press). 

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