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Production of osmoprotective compounds The accumulation of low molecular mass osmoprotectants and osmopytes (such as quaternary amines, amino acids and sugar alcohols) is considered to be an important strategy that plants use to overcome environmental stress. However, some species are able to accumulate such compounds more efficiently than others. For example, rice potato and tobacco accumulate limited amounts of the potent osmoprotective compound glycine betaine. This makes them excellent targets for introducing osmoprotectant/osmolyte-producing enzyme systems. Two mechanisms are thought to lie behind the activity of these substances:
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Improved membrane flexibility There is a strong correlation between chilling sensitivity and the degree of unsaturation of fatty acids in plastid membranes of various higher plants. The presence of centrally positioned cis-double bonds in the membrane lipid lowers the phase-transition temperature to approximately 0oC. This hypothesis has been tested in two reports (Kodama, H. et al ´94 and Ishizaki-Nishizama, O. et al ´96). |
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Stress-induced proteins The transcription of genes encoding the late-embryogensis-abundant (LEA) proteins (first characterised in seed embryos) is activated under abiotic stress. It has been hypothesised that these proteins have a protective effect, and this was tested by introducing the HVA1 gene, encoding a group 3 LEA protein from barley, into rice (Xu, D. et al ´96). |
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Scavenging reactive intermediates Several reports have shown that salt, freezing and drought stress are also accompanied by the formation of reactive oxygen intermediates. These toxic molecules damage membranes, membrane-bound structures and macromolecules, especially in the mitochondria and chloroplast, resulting in oxidative stress. Evidence for this is that freezing- and salinity-tolerant plants also have well-developed antioxidant defences, and by pretreating plants with one form of stress it is often possible to increase the tolerance to a different stress factor. Transgenic plants have been used more recently to study the relationship between abiotic stress tolerance and functional antioxidant-defence system. Antioxidant systems in plants consist of enzymes that can scavenge oxygen radicals, such as superoxide dismutases (SODs), peroxidases, catalases and glutathionc reductases. The SODs are essential components in almost all plant antioxidant defences, catalysing the dismutation of two superoxide radicals into oxygen and hydrogen peroxide. |
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Hypoxia- and anoxia-reducing proteins Oxygen stress is often caused by flooding. Oxygen is involved in respiration and several crucial biosynthetic pathways such as the synthesis of chlorophylls. Plants have evolved different strategies to cope with hypoxia: altered root architecture; increased internal oxygen transport by a larger aerenchyma; radial loss of soluble substances by altered cortex structure, induction of enzymes, export mechanisms and enzyme systems to avoid toxication by fermentation end-products; and production of oxygen binding proteins. Plant haemoglobins were first discovered in the nitrogen-fixing root nodules, occupied by Rhizobium or Drankia-type bacteria, of leguminous plants. It was later suggested that a haemoglobin gene may be a component of all plants, and that the role for non-legume haemoglobin was not to facilitate oxygen diffusion, but rather to function as an oxygen-sensing device. However, characterisation of the non-symbiotic haemoglobin genes of Casuatine glauca has increased speculation that the haemoglobin may facilitate oxygen diffusion (Jacobsen-Lyon, K. et al ´95). |
Foreign genes expressed in transgenic plants
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