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

Water Stress
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concentrations, and consequently, a reduction in stomatal aperture are observed (Figure
2) (Schroeder et al., 2001). Guard cells integrate external and internal environmental
information through ABA-dependent and independent pathways to modulate stomatal
aperture (Fan et al., 2004). These external and internal stimuli include light (quality and
quantity), CO 2
level, air moisture, and leaf-air vapor pressure difference (Mott and
Parthurst, 1991; Assmann et al., 2000).
Biotinated ABA induces stomatal closure by binding to the outer surface of
guard cell plasma membranes (Yamazaki et al., 2003), strongly suggesting that the receptor
of ABA is located on the surface of the plasma membranes. A 42-kDa protein
has been identified as an ABA-binding protein from membrane fractions of the leaf
epidermis of Vicia fava (Zhang et al., 2002), but whether this protein functions as a
receptor of ABA under drought stress remains unknown.
Calcium ions act as a second messenger in intracellular signal transduction
during ABA signaling (Schroeder et al., 2001). In-flow of calcium ions into the cytosol
from the vacuole and extracellular space increases the cytosolic concentration of calcium
ions in ABA-treated guard cells. The level of calcium ions oscillates at intervals of
several minutes. This increase in calcium concentration is not observed in the ABAinsensitive
mutants abi1 and abi2 (Allen et al., 1999). Calcium ions suppress inward
potassium channels and activate inward anion channels; thereby playing a central role
in stomatal closure (Blatt, 2000).
Figure 2. The ABA signaling pathway in guard cells. Ion channels involved
in stomatal closure are shown by a pair of black boxes, and protein factors
involved in signal transduction are depicted in rounded squares. Abbreviations:
ABA, abscisic acid; NOS, nitric oxidase synthase; NO, nitric oxide;
InsP6, myo-inositol hexakisphosphate; InsP3, inositol 1,4,5-trisphosphate;
SPHK, sphingosine kinase; S1P, sphingosine-1-phosphate; GPA,
heterotrimetric G protein α subunit; [Ca 2+ ]cyt, cytosolic Ca 2+