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

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Z . Dajic
zyme has two main domains (peripheral and a membrane integral domain, Ratajczak,
2000) and three subunits: A, B and C, where subunit C transcripts increase in response
to salinity (Chen et al., 2002). Increases in the tonoplast H + -ATPase activity under
salinity conditions have been reported for different species (Mansour et al., 2003), in
contrast to an inhibition of vacuolar H + -pyrophosphatase by increased NaCl concentrations
(Blumwald et al., 2000). In the halophyte Sueada salsa the main strategy of salt
tolerance seems to be up-regulation of vacuolar H + -ATPase activity, while H + -pyrophosphatase
had a minor role (Wang et al., 2001). Nevertheless, an overexpression of
the AVP1 gene encoding the native vacuolar H + -translocating pyrophosphatase resulted
in an increased salinity tolerance in Arabidopsis compared with the wild-type
plants (Gaxiola et al., 2001). Thus, role and activity of H + -pyrophosphatase in responses
to salinity is still uncertain.
8.2.2. Determinants Involved in Control of Na + Uptake and Movement – Carriers and
Ion Channels
Carrier–type transport systems are characterized by exhibiting conformational changes
in the transport protein. The ion-coupled transport in salinity conditions operates as
symport and antiport. Carrier transport proteins act against a gradient and are energized
by coupling to an electrochemical gradient, such as high affinity K + accumulation
energized through coupling to the trans-membrane proton flux (Maathuis and Amtmann,
1999). The main carriers involved in sodium and potassium uptake and sodium influx
across the plasma membrane are high affinity potassium carriers (HKT) and low affinity
cation carriers (LCT) (Amtmann and Sanders, 1999, Blumwald et al., 2000). High affinity
K + uptake is related to two gene families: 1) KUP-HAK, extremely selective for K + and
blocked by Na + when present in mM concentrations (Santa-Maria et al., 1997, Kim et al.,
1998), and 2) HKT1 (Schachtman and Schroeder, 1994), which represents a putative
pathway for high-affinity K + uptake and low-affinity Na + uptake (Maathuis and Amtmann,
1999).
Transgenic wheat lines, expressing the HKT1, exhibited enhanced growth
under salinity, due to reduced Na + /K + ratios when compared with the control plants
(Laurie et al., 2002). In contrast to the wheat HKT1, homologous transporter from
Arabidopsis (AtHKT1) can mediate Na + and, to a small degree K + transport in heterologous
expression systems (Uozumi et al., 2000). This transporter may be involved in Na +
recirculation from shoot to root, probably by mediating Na + loading into the phloem in
shoots, and unloading in roots (Berthomieu et al., 2003). Similar transporters have been
isolated from the japonica rice: OsHKT1 and OsHKT2, acting as Na + transporter and
Na + - and K + -coupled transporter, respectively (Horie et al., 2001). Garciadeblas et al.
(2003) supposed that OsHKT transporters are involved in Na + movement in rice, and
that OsHKT1 specifically mediates sodium uptake in rice root in conditions of K + deficit.
Additionally, OsHKT4 was be the Na + transporter of a low affinity. Low affinity
cation carrier, encoded by LCT1, which was cloned from wheat (Schachtman et al.,