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

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A.R. Reddy and A.S. Raghavendra
The ascorbate-glutathione cycle plays a crucial role in scavenging superoxide
and H 2
O 2
in plant cells. Reduced glutathione (GSH) acts as an electron donor to regenerate
ascorbate from its oxidized form, dehydroascorbate. The redox ratio of
dehydroascorbate/ ascorbate and GSH/GSSG are significant for the optimized operation
of ascorbate glutathione cycle under conditions of photooxidative stress. Masi et
al. (2002) reported increased turnover rates of glutathione in UV stressed maize leaves.
GSH/GSSG ratios were higher in stress adapted plants indicating strong activation of
ascorbate-glutathione cycle under photooxidative stress. However, Tausz (2004) postulates
that the response of glutathione system to photooxidative stress will be differ-
Figure 4 . Ascorbate-Glutathione cycle as a defense system
under photooxidative stress
ential depending upon the environmental stress. In the initial reaction phase there may
be a transient shift of the GSH/GSSG redox state towards slightly more oxidized value,
while in an acclimation reaction, ROS triggers the production of glutathione by enhancing
the enzyme activities associated with glutathione biosynthesis thus increasing the
scavenging capacity of ascorbate-glutathione cycle (Baena-Gonzalez et al., 2001; Noctor
et al., 2002; Neill et al., 2003). However, if the acclimatory responses of plants are too
slow or too weak, photooxidative stress gradually depletes the glutathione content and
the balance between oxidative load and the scavenging will be disproportionate, which
leads to progressive oxidation and degradation of ascorbate and glutathione pools,
eventually leading to senescence and plant death. Increased activities of ascorbateglutathione
cycle increased resistance to chilling related photooxidative stress in cot-