Review article. Molecular biology of salt tolerance in the context of whole-plant physiology |
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Authors: | Yeo A |
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Institution: | Plant Stress Unit, School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK; e-mail: a.r.yeo@sussex.ac.uk |
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Abstract: | The halobacteria are the only organisms that are tolerant of salinity at
the molecular level. All other bacteria, all fungi, all plants, and all
animals avoid the need for salt tolerance for most of their macromolecules
by maintaining defined and conserved conditions in the cytoplasm. These
conditions favour potassium over sodium, the limitation of total inorganic
ion activity, and the supplementation of this where necessary with organic
solutes which are metabolically neutral osmolytes that may also be
osmoprotectant. The salt tolerance of an organism depends upon the range of
external salinity over which it is able to sustain these conditions in the
cytoplasm. There is substantial and increasing knowledge of the molecular
biology and molecular genetics of the processes of ion and organic solute
transport, solute synthesis, and compartmentation that underpin cell-based
tolerance. Much of recent research focuses on the identification of genes
and gene products that affect cell-based tolerance, commonly derived from
single-cell models. There is commonly the implicit or explicit assumption
that incorporation of these genes will benefit the salt tolerance of food
crop species. While this essential experimental approach is giving enormous
insight there should not be rash or premature expectations. The unique and
overriding consideration for the salinity tolerance of terrestrial plants
is the net flux of water due to transpiration and so resides at a higher
level of organization. Processes that are advantageous to a single cell in
an aqueous medium may be lethal to a cell in a leaf in the air. The likely
impact of single structural-gene changes in ion and solute transport upon
co-ordinated plant response is probably over-estimated, and recent views
consider regulatory processes and multiple gene transfers. While the
technical ability for plant transformation increases daily, the
practicality of using transgenic plants in complex breeding programmes
seems rarely to be given enough thought. If intervention at the molecular
level is to lead to salt-tolerant crop plants than it will be essential to
view this in the contexts of whole plants and of plant breeding. Recent
indications that a relatively small number of quantitative trait loci (QTL)
may govern complex physiological characters offer the most hope for the
future. |
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