Adaptation to Cd(II) and Zn(II), and the caffeine-potentiated override of the G₂ block induced by the checkpoint activated by DNA damage

ABSTRACT

Moderate and low concentrations of Zn(II) and Cd(II) were defined as those which depressed the rate of root elongation in Allium cepa L. to about 40 and 70% respectively of the control (17.3 ± 4.9 mm/day) at 25°C. At moderate concentrations, cells were detoxified from Cd(II), but not from Zn(II), by inducing the heavy metal chelators phytochelatins. Thus, root elongation further decreased (from 41 to 19% of the control) at moderate (0.05 mM) Cd(II) concentration upon addition of 0.25 mM L-buthionine-[S,R]-sulfoximine (BSO), a specific inhibitor of phytochelatin synthesis. On the other hand, cells were also detoxified from Zn(II) by an alternative mechanism, as the 42% inhibition displayed at 0.5 mM Zn(II) concentration was partially reversed (up to 79%) in the presence of BSO. Zn(II) activated the checkpoint pathway induced by DNA damage, as a transient G₂ block was produced; this block was partially cancelled by caffeine, so that chromosomal bridges (but no breaks) were observed in ana-telophase. On the other hand, Cd(II) did not activate the DNA damage checkpoint, as cells entered into anaphase with chromosomal breaks and bridges without any delay. Cd(II) may preclude the recognition of DNA damage by altering protein-DNA interactions, since 30% of the metaphases displayed clumped chromosomes. A minimum threshold was required to induce the adaptive responses described here, as BSO did not modify the reduction in root elongation rate recorded at low concentrations of both heavy metals.

KEY WORDS:

• heavy metals
• stress adaptation
• DNA damage-dependent checkpoint
• caffeine
• Allium cepa L.