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

Water Stress
25
min, respectively (Brouquisse et al., 1989; Burnet et al., 1995) and 1 mM and 12 µmol/mg
protein/min, respectively (Ikuta et al., 1977). Moreover, the specificity (V max
/K m
) of the
enzymatic reaction of Arthrobacter CO is 50 times higher than that of spinach CMO,
suggesting that Arthrobacter CO is superior to spinach CMO with respect to glycine
betaine production if the expressed protein levels of both genes are the same. Celery
synthesizes both mannitol and sucrose as translocation sugars in source leaves. Mannitol
is synthesized from fructose 6-phosphate, an intermediate of gluconeogenesis for
sucrose synthesis in the cytosol, and thereby pathways of mannitol and sucrose syntheses
compete for carbons from photosynthesis (Stoop et al., 1996). The accumulation
of mannitol is accomplished partly by sucrose-induced suppression of the mannitol-catabolizing
enzyme NAD-dependent mannitol dehydrogenase (MTD) (Stoop et
al., 1996), the transcript level of which is also down-regulated by sucrose (Williamson et
al., 1995; Prata et al., 1997; Zamski et al., 2001). Under non-stressful growth conditions,
celery plants convert half their fixed CO 2
into mannitol, while the other half is used to
produce sucrose; it is possible they preferentially use sugars to support central metabolism.
Such sugar repression during mannitol degradation would allow large amounts
of mannitol to be stored as a reserve carboxyhydrate. On the other hand, when plants
experience stress, sucrose synthesis is accelerated and MTD activity is inhibited. In
addition to this direct effect of sucrose, transcript levels of MDH are also reduced by
ABA. Reducing equivalents not utilized under stress are transferred to the cytosol via
the triose phosphate shuttle to promote reduction of mannose-6-phosphate to mannitol-1-phosphate,
which is then converted to mannitol (Gao and Loescher, 2000).
3.3. Transgenic Plants
An increasing number of reports have documented successful creation of compatible
solute-forming transgenic plants. These trials are important in furthering our understanding
of the functions of compatible solutes and elucidation of their accumulation
mechanisms. Model plants such as Arabidopsis and tobacco as well as crop plants
such as rice and potatoes have been common recipients of the genes necessary for
synthesis of compatible solutes. Resistance against various kinds of stresses such as
drought, low and high temperatures, and high salt concentrations have been achieved
through such experiments (Chen and Murata, 2002; Hare et al., 1998; Nuccio et al.,
1999).
Stress-tolerant transgenic plants have been created by over-expressing genes
of enzymes absent or rate-limited in the metabolic pathway. These genes are obtained
from organisms that naturally accumulate the compatible solute in question, and are
sometimes engineered to lose feedback regulation for massive accumulation (Hare et
al., 1998; Nuccio et al., 1999). However, not all transgenic plants accumulate sufficient
amounts of compatible solutes nor attain the positive trait, particularly in the field
(Chen and Murata, 2002; Serraj and Sinclair, 2002). To overcome these hurdles, we need
to know more about the regulatory mechanism of compatible solute synthesis. Infor-