MULTILEVEL REGULATION OF GLUTATHIONE HOMEOSTASIS 545
Figure 1 Evidence for the coordination between cysteine and GSH synthesis pathways. Arabidopsisplants
grown in liquid culture were treated with 100 M JA for the indicated time or for 3 hr with the indicated con-
centrations of JA and with 5 mM H 2 O 2 for the indicated time or for 3 hr with the indicated concentrations of
H 2 O 2. Total RNA isolation and gel blot analysis were performed as described [20]. The filter was sequentially
probed with^32 P-labeled cDNA for APR1, APR2, and gsh1. Ethidium bromide–stained gel is shown for RNA
equal loading.
Figure 2 Multilevel regulation of GSH homeostasis. GSH homeostasis is dynamically established through a
concerted interplay of synthesis, transport, utilization, and degradation in plant cells. GSH is synthesized by -
EC synthetase and GSH synthetase using Glu, Cys, Gly, and ATP as substrates. GSH is utilized to combat ox-
idative stress by the ascorbate (Asc)/GSH cycle, to synthesize phytochelatins (PCs) for detoxification of heavy
metals and to detoxify xenobiotics by GSH S-transferases (GSTs). GSH homeostasis is regulated at multiple
levels. At the transcriptional level, heavy metal Cu and JA use parallel signaling pathways to up-regulate GSH1.
At the translational level, a redox-sensitive RNA-binding complex specific for the 5 UTR of GSH1mRNA
modulates the translation of GSH1mRNA. The redox-sensitive nature gives this RNA-binding complex the
ability to sense the change in GSH/GSSG ratio, an indicator of oxidative stress, and to modulate the translation
accordingly. At the metabolic level, GSH feedback inhibits -EC synthetase. In addition, transport and turnover
of GSH contribute to GSH homeostasis. As the transport and storage form of organic sulfur, GSH is exported
from source tissue to sink tissue, where it is degraded to release cysteine for protein synthesis and other uti-
lization. Sulfate uptake and assimilation pathways that make cysteine the key substrate for GSH synthesis are
likely to be coordinately regulated with the pathway for GSH synthesis. The circled plus sign indicates positive
effects, and the minus sign indicates negative regulation. Dashed lines indicate uncertain but possible regula-
tory mechanisms. The stop sign indicates the RNA-binding factor bound to the 5 UTR of GSH1mRNA.