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

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R.G. Trischuk, B.S. Schilling, M. Wisniewski and L.V. Gusta
phytohormones such as auxins as described above (Lu and Fedoroff, 2000) or in
combination with sucrose (Rolland et al., 2002). Wilen et al. (1994) observed bromegrass
suspension cells grown at 20 o C possessed little freezing tolerance in the presence
of R-ABA (the unnatural form) but were extremely freezing tolerant in the presence of S-
ABA (the natural form). Both enantiomers up regulated similar cold associated genes;
however, the major difference was S-ABA enhanced the uptake of sucrose from the
medium, whereas R-ABA did not. It is well documented an increase in sucrose concentration
during cold acclimation is intimately associated with the development of freezing
tolerance (Levitt 1980) and a combination of both stress associated proteins and
sucrose confer stress protection (Robertson et al. 1994). In addition, sucrose and other
sugars play a central role as signalling molecules that modulate the physiology, metabolism
and development of plants. (Koch, 1996, Coruzzi and Zhou, 2001, Arroyo et al.,
2003). There is ample evidence that several stress related genes contain sugar boxes,
that regulate expression (Atanassova et al., 2003) and that this sugar induction is
enhanced strongly by ABA (Cabir et al., 2003). Rook et al., (2001), working with two
sugar-induced genes involved in starch biosynthesis, demonstrated ABA strongly
enhances their sucrose-induced expression but has no effect in the absence of sucrose.
Wanner and Junttila, (1999) reported in the dark cold associated genes were upregulated
at low temperatures, but the plants did not increase in freezing tolerance. However, in
the light plants increased freezing tolerance with the accumulation of both sucrose and
upregulation of stress associated genes. The old school of thought assumed that
elevated levels of phytohormones were required to elicit gene changes; however, recent
evidence does not support this contention (Chiwocha et al., 2003). Since freezing
tolerance is a multigene event it is rather presumptuous to assume the single addition of
a phytohormone at non acclimating temperatures to actively growing plants would
have a major effect on cold acclimation.
The GA/ABA antagonism in regulating development is well established
(Chrispeels and Varner 1966, Gomez-Cadenas et al., 2001). The effect cytokinins have
on the development of freezing tolerance is poorly understood (Reany et al., 1989).
Cytokinins under non-stressful conditions promote growth and development. Under
stressful conditions, cytokinins are degraded by cytokinin oxidase, which is induced
by ABA (Brugiere et al., 2003). The balance between auxins and cytokinins controls the
formation of roots, shoots (Skoog and Miller, 1957), the outgrowth of shoot auxiliary
buds (Sachs and Thimann, 1967) and the formation of lateral roots (Wightman et al.
1980). Roots, the most sensitive tissue of plants, are killed between -6°C to -10°C and
do not survive overwintering and must be regenerated in the spring, when the seedlings
are recovering from overwintering injury (Chen et al. 1983).
Both auxin and cytokinins rapidly induce ethylene synthesis in many tissues
(Abeles 1966, Vogel et al., 1998); however, cytokinins in certain species inhibit ethylene
production (Coenen et al., 2003). Abiotic stress activates a MAPK cascade, (Sangwan
et al., 2002) which increases intracellular calcium (Moyen et al., 1998), calmodulin (Bergey
and Ryan 1999), and the activation of phospholiphase A 2
(Lee et al., 1997).