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

Salt Stress
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A significant number of halophytes are C 4
species, which are characterized by
their higher requirements for sodium ions compared with C 3
species (Brownell and
Crossland, 1972). In conditions of osmotic stress and high temperatures, C 4
plants have
an advantage in comparison with C 3
plants, because of their ability to carry on photosynthesis
when stomata are to a large extent closed, coupled with the absence of
photorespiration in the mesophyll cells (Larcher, 1995). Photosynthetic responses to
salinity in the halophytic tribe Salsoleae (family Chenopodiaceae) have been reviewed,
with particular attention paid to relations between the C4 NAD-ME (malic enzyme)
Salsoloid type of carboxylation and the chloroplast structure (Voznesenskaya et al.,
1999).
Abscisic acid is well recognized as an important stress hormone. The concentration
of ABA increases when water deficits occur, with its de novo synthesis beginning
in the roots, in response to sensing an insufficient supply of water (Zhang et al.,
1989). In halophytes, which grow in conditions of “physiological drought”, due to low
water potential in the root medium, the lowest concentrations of ABA were found
under salinity concentrations optimum for growth (Clipson et al., 1988). Such case was
reported for the highly tolerant halophytic species Suaeda maritima, which exhibited
the lowest seasonal range of ABA contents (from 649.4 ng g -1 to 835.6 ng g -1 dry
weight) in comparison with several other species, where higher ABA concentrations
were correlated with increased sodium content of the shoot (Dajic et al., 1997a).
In glycophytes, salinity leads to the accumulation of ABA (Asch et al., 1995),
as in tomato (Chen and Plant, 1999; Yurekli et al., 2001) and wheat (Aldesuquy and
Ibrahim, 2002). In bush bean plants exposed to 75 mM NaCl, inhibition of leaf expansion
was mediated by ABA rather than by Na + or Cl - toxicity, and the increase of ABA
induced by a salt-pretreatment limited the accumulation of Na + and Cl - in the leaves,
resulting in adaptation to salinity stress (Montero et al., 1997). Besides the significant
role of ABA, favorable effects of other hormones in plant responses to salinity, such as
cytokinins (Kuiper and Steingrover, 1991) and gibberellins (Kaur et al., 1998; Ashraf et
al., 2002) have been documented.
8. MOLECULAR BASIS OF SALT TOLERANCE
According to Hasegawa et al. (2000) determinants of salt stress tolerance include effector
molecules that enable adaptive reactions and mechanisms of plants in saline environments
and regulatory molecules that control these pathways. Effectors are proteins
and metabolites involved in ion homeostasis (membrane proteins involved in regulation
of ionic transport), osmotic adjustment and water regime regulation (osmolytes)
and toxic radical scavenging (mainly enzymes), while regulatory molecules are cellular
signal pathway components and transducers of long-distance response coordination
(hormones, mediators, transcription factors and regulatory genes).