Water stress tolerance

Bohnert, H.J., and Jensen, R.G., 1996, Strategies for engineering water-stress tolerance in plants. 

The movement onto land and the subsequent evolutionary divergence involved a number of innovations including the development of mechanisms to enable plants to withstand atmospheric drought including cuticle, xylem, stomata, and intercellular spaces. All of these innovations occurred during the late Silurian and early Devonian around 400 million years ago (Raven, 1977). Evidently coinciding with these structural innovations came the evolution of compounds that aided both protection from UV-B and water-stress tolerance. 

Table 2 is a summary of effects of water-related stresses on photosynthesis. Water-related stresses appear not to affect the chloroplast reactions until well after the growth of plants has been affected. Photosynthesis is reduced and lack of photosynthate could limit plant growth. However, the reduced rate of photosynthesis results from stomatal closure, a response of plants to information about their environment. Evidence indicates that there is no weak link in photosynthesis which if reengineered, would allow water-stress-sensitive plants to become water-stress-tolerant. 

Rower, D.J. Ludlow, M.M. (1986). Contribution of osmotic adjustment to the dehydration tolerance of water-stressed pigeon pea (Cajanus cajan (L.) Millsp.) leaves. Plant, Cell and Environment, 9, 33-40. 

Polyols are present in desiccation tolerant lichens and liverworts, although not in mosses (Lewis, 1984). More generally starch hydrolysis and sugar accumulation occur in many plants experiencing severe water deficits (Hsiao, 1973). It is tempting to speculate that the accumulation of low molecular weight solutes in reponse to water stress represents a mechanism for the protection of membranes and proteins in the dry state. 

Cells selected for tolerance to water stress have been demonstrated in a number of plant species (Handa et al., 1983 Heyser Nabors, 1981 Stavarek Rains, 1984b). High molecular weight PEG is commonly used to induce water deficits (Hasegawa et al., 1984). This provides an opportunity to evaluate intracellular osmotic adjustment of plants exposed to water stress. If information is desired on the effect of specific osmotic substances on cellular adjustment to osmotic stress PEG can be used as an independent osmotic regulator and/or in combination with an absorbable osmoticum. In these situations the interaction of osmoticum produced by the cell with the osmoticum absorbed by the cell can be evaluated. 

A more comprehensive review of the significance of these organic osmotica in the tolerance of plant cells to water stress can be found in a paper by Hasegawa et al. (1987). The significance of organic osmotica in tolerance processes of intact plant systems is discussed jn Chapters 7 and 8. 

Plant cells selected for tolerance to stress show varied responses to the imposed osmotic gradients. In adapted cells, tolerance to salinity or to water stress was not found to increase proportionately with increases in turgor (Handa et al., 1983 Binzel et al., 1985). It was suggested from these observations and from studies by Heyser Nabors (1981) that no relationship existed between turgor and growth and that stress adaptation may alter the relationship between turgor and cell expansion (see also Chapter 6). 

Ben-Hayyim, G. (1987). Relationship between salt tolerance and resistance to polyethylene glycol-induced water stress in cultured citrus cells. Plant Physiology, 85, 430-4. 

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. 

Iuchi, S., M. Kobayashi et al. (2000). A stress-inducible gene for 9-cw-epoxycarotenoid dioxygenase involved in abscisic acid biosynthesis under water stress in drought-tolerant cowpea. Plant Physiol. 123(2) 553-562. 

Several roles of endophytic fungi for the host plant have been postulated. These include acting to increase access to mineral nutrients (a mycorrhizal function), to increase access to organic soil N, P and C, to increase drought and stress tolerance, to improve water uptake, protection from herbivory (mammals, insects), and for protection from plant pathogenic fungi, bacteria, nematodes, and other parasites. We should not be surprised that endophytic fungi are such common plant symbioses. 

Piatkowski, D., Schneider, K., Salamini, F. Bartels, D. (1990). Characterization of five abscisic acid-responsive cDNA clones isolated from the desiccation-tolerant plant Craterostigma plantagineum and their relationship to other water-stress genes. Plant Physiology 94, 1682-8. 

Subaerial rock communities metabolize under conditions of limited water availability and high solar and cosmic irradiation levels and can be found even on desert rocks and at high altitudes. In these places, they have found an ideal environment that allows for a stressful but less competitive existence. In more favourable conditions, these communities are quickly succeeded by more developed but less stress-tolerant symbiotic lichen or... 

Table 4 Microbial tolerance to matric-controlled (T ni) water stress.

---