Osmotic stresses, plant responses

Hi) Poly amines. In many respects the role of poly amines in plant functioning is still mysterious after many years work. They are almost certainly involved in the control of growth and development through their interactions with nucleic acids and membranes (Smith, 1985). There is increasing circumstantial evidence for their involvement, especially of putrescine, in plant responses to a wide range of stresses including pH, Mg deficiency, osmotic shock, cold, SO2 pollution, and cadmium and ammonium toxicity (Smith, 1985). It remains to be determined, however, how, and indeed whether, putrescine accumulation in response to these diverse stresses is beneficial. 

Growth characteristics of cells exposed to water stress mimic some of the structural responses of organised plant tissues. A frequently observed response of plants exposed to water stress is a reduction in cell size (Cutler, Rains Loomis, 1977). This cellular phenomenon was observed in tomato cells stressed with PEG (Handa et al., 1983). Concomitantly with a decrease in cell size with increasing osmotic stress was a reduction in fresh weight. In contrast the dry weight was not affected. 

Expression of ERF3 and ERF4 from sugarcane and cotton, respectively, rapidly increases response to exogenous ethylene and ABA, salt, cold, and drought, whereas their overexpression in transgenic plants enhances tolerance to drought and osmotic stress. ... 

Ackerson RC. 1981. Osmoregulation in cotton in response to water stress. II. Leaf carbohydrate status in relation to osmotic adjustment. Plant Physiol 67 489-493. 

Skriver, K. Mundy, J. (1990). Gene expression in response to abscisic acid and osmotic stress. The Plant Cell 2, 503-12. 

The loss of productivity of arable land due to desertification has induced intensive research into understanding plant responses to drought and salt stress. Osmotic stress causes changes in cell volume that can impact multiple levels of cellular organization and function including plasma membrane... 

A wider perspective on inositols and their metabolites in abiotic and biotic stress responses has been documented by Taji, Takahashi and Shinozaki (Chapter 10). Inositol and its metabolites function as both osmolytes and secondary messengers under biotic and abiotic stresses. The accumulation of different osmolytes during osmotic stress is an ubiquitous biochemical mechanism found in different organisms from bacteria, fungi and algae to plants and animals. Plants accumulate many types of inositol derivatives during abiotic... 

More than 30 years ago it was reported that flaxseed homogenates convert 13-HpOTrE into a- and -y-ketols, and the enzyme responsible for this reaction was called hydroperoxide isomerase [54]. Later studies indicated that this reaction was actually a two-step process, consisting of an enzyme-catalysed dehydration of the hydroperoxy fatty acid to a rather unstable allene oxide followed by a non-enzymic hydrolysis [55,56]. The enzyme responsible for allene oxide synthesis has been purified and shown to be a cyt E-450 isoform [57]. As an alternative to hydrolysis, the allene oxide may undergo enzymic cyclization forming 12-oxo-phytodienoic acid. This compound is subsequently converted into jasmonic acid, a phytohormone implicated in the reaction of higher plants to a number of stimuli, such as wounding, fungal elicitation, mechanical forces and osmotic stress [54]. 

Fujita Y, Fujita M, Shinozaki K, Yamaguchi-Shinozaki K (2011) ABA-mediated transcriptional regulation in response to osmotic stress in plants. J Plant Res 124 509-525 Galiba G, Kocsy G, Kaur-Sawhney R, Sutka J, Galston AW (1993) Chromosomal localization of osmotic and salt stress-induced differential alterations in polyamine content in wheat. Plant Sci 92 203-211... 

The common response of both cultured cells and cells comprising the body of a plant when these two systems are exposed to water stress is the requirement for osmotic adjustment. It seems reasonable then to expect that information obtained at the cellular level should enhance our understanding of the biochemical and physiological response of plants exposed to water stress. 

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). 

A number of chapters in this volume (especially Chapters 5 and 6) provide a more thorough discussion of osmotic adjustment by intact plants and tissues in response to environmental stress and the role of osmotically active solutes in this response. The following section focuses on the role of organic osmotica in the response of plant cells to salt stress. Cultured plant cells offer the opportunity to evaluate the effect of both internally synthesised and externally administered organic osmotica. 

A frequently observed response of plant cells exposed to saline stress is the accumulation of proline. Two cell lines of tobacco, one resistant and the other sensitive to growth inhibition by NaCl, accumulated proline when exposed to 1.5% w/v NaCl in the growth media (Dix Pearce, 1981). The NaCl sensitive line accumulated proline more rapidly than did the resistant line, though the levels accumulated were not adequate to provide osmotic protection against salt stress. The authors suggested that proline accumulation may have a protective role other than osmoregulation and may be symptomatic of stress injury, the nature of which was not discussed. 

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