Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Salt Stress
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6.2. Nitrogen Fixation and Salt Stress
Legumes represent a very significant group of crops in agriculture, and therefore their
responses to salt stress are described in several reports (e.g. Katerji et al., 2001, Lachaal
et al., 2002). Tolerant varieties and accessions within the legumes have been revealed,
such as for soybean (Essa, 2002) and among both cultivated and wild Phaseolus species
(Bayuelo-Jimenez et al., 2002). Breeding and genetic engineering programs of
legumes must be directed to optimise their nitrogen fixation and growth in saline conditions.
Establishment of symbiosis is highly sensitive to salt stress, whereas fully
developed nodules that had been formed under salt stress can continue to fix nitrogen
(Singleton and Bohlool, 1984). Host tolerance was a major factor for nodulation and
nitrogen fixation in genotypes of faba bean (Cordovilla et al., 1995). Rhizobia can survive
under much higher salinity than its host legume (Nair et al., 1993). Bacterial ability
to adapt to salt stress is important for the bacteroid nitrogen-fixing function inside the
legume nodule. Among several Rhizobium species tested for salt stress tolerance, R.
meliloti has expressed a much higher ability to survive in the saline medium, comparing
to R. leguminosarum and, especially R. japonicum (Bernard et al., 1986).
Salt stress tolerance in rhizobia is at least partially associated with osmoregulation
achieved by accumulation of compatible solutes (Imhoff, 1986). Inn conditions of
salt stress, pea plants treated with boron and calcium exhibited enhanced cell and
tissue invasion by Rhizobium leguminosarum and increased nodule number (El-
Hamdaoui et al., 2003). Furthermore, enzymes involved in ammonium assimilation in
root nodules exhibited a significant sensitivity to salt stress and should not be considered
as reliable criteria for selection of salt tolerance in faba bean (Cordovilla et al., 1995)
and pea (Cordovilla et al., 1999). Activity of nitrogenase, nodule number and dry matter
accumulation in soybean (Abdalla et al., 1998) and alfalfa (Serraj and Drevon, 1998)
were affected under salt stress.
Eight genes of Rhizobium tropici were involved in salt tolerance and establishment
of symbiosis with beans. These were classified into three groups: a) two genes
responsible for regulation of gene expression and regulation of nitrogen metabolism, b)
genes related to synthesis or maturation of proteins, and c) genes associated with
potassium uptake and polysaccharide biosynthesis (Nogales et al., 2002).
7. WATER REGIME AND PHOTOSYNTHESIS UNDER SALT STRESS
The effects of drought and salt stress on plants are tightly related, since the first
responses of plant cells under these stress conditions are induced by osmotic shock.
Thus, upon exposure to osmotic stress, plants exhibit many common adaptive reactions
at the molecular, cellular and whole-plant level (Greenway and Munns, 1980; Yeo,
1998; Bohnert et al., 1995; Zhu et al., 1997). These include morphological and anatomical
alterations (life cycle, xeromorphic features, increased root/shoot ratio), and physi-