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

46 Z . Dajic
The strategy of salt exclusion relies on the selective release of Na + into the
xylem and its resorption from the xylem stream. Net accumulation of sodium ions in the
plant is dependent on the balance between passive influx and active efflux. Salt exclusion
operates at the cellular and whole plant level (Munns et al., 1983) and is to a great
extent related to regulation of K/Na selectivity (Jeschke and Hartung, 2000). According
to Munns et al. (2002), the ability of plants to regulate the uptake and transport of salts
is dependent on the following mechanisms: a) selectivity of uptake by root cells, b)
preferential loading of K + rather than Na + into the xylem by the cells of the stele, c)
removal of salts from the xylem in the upper parts of roots, the stem and leaf sheaths,
based upon exchange of K + for Na + , and d) loading of the phloem.
Passive movement of ions into roots and shoots is the consequence of the
transpirational stream (Flowers and Yeo, 1992). However, at the endodermis, radial movement
of solutes must be via the symplast, and the extent to which the symplastic
pathway of ion transport regulates reduced delivery of ions into the xylem is still not
known (Clarkson, 1991). The endoderm zone represents the main barrier of passive flow
of ions towards the shoot. Thus, in some halophytic species the level of suberization of
the endoderm cell walls reaches 50-100%, compared with non-halophytes with 27-40%
(Osmond et al., 1980).
Besides the role of the cortex and endodermis, transfer root cells are also
important in the process of control of sodium ions movement (Greenway and Munns,
1980). It has been suggested that in order to minimize Na + delivery to the shoot in the
apoplast of the xylem, cells in the outer half of the root need to suppress the influx from
and/or increase efflux to the soil solution, in contrast to the cells of the inner half of the
root, which should maximize influx from and/or minimize delivery of sodium ions to the
xylem (Tester and Davenport, 2003).
Radial transport of sodium from the soil solution into the xylem vessels might
be genetically controlled, according to results obtained in Arabidopsis sas1 mutants
(for sodium over-accumulation in the shoot) compared with the wild-type plants (Nublat
et al., 2001). Apart from the regulation of xylem loading, controlled by Na + /H + antiporters,
retrieval of ions from the xylem may also operate, probably due to the Na + -permeable
channel of xylem parenchyma cells (Wegner and Raschke, 1994).
The salt tolerance in species that exclude salts is achieved by changes between
sodium and calcium ions, rather than changes in osmotic potential, since adsorption
of calcium ions on membranes of root cells leads to reduced penetration of monovalent
cations (Munns et al., 1983). This was demonstrated for wheat where inhibition of
non-directional Na + influx occurred following the addition of external Ca 2+ (Reid and
Smith, 2000). Involvement of both Ca 2+ sensitive and Ca 2+ insensitive pathways (regulated
mainly by non-selective cation channels) in the control of Na + entry into the root
has been proposed (Tester and Davenport, 2003).
The search for mechanisms of active sodium extrusion in root cells has to
continue, as, in contrary to algae, advanced protocols for the isolation of rhizodermal
protoplasts and PM vesicles are needed (Gimmler, 2000). An important goal of salt