Metal ion induced unscheduled DNA synthesis in Petunia pollen

Summary Al³⁺, Fe³⁺, V²⁺ or Be²⁺ when added to shaken suspensions of Petunia hybrida pollen in 10% sucrose —0.01% H₃BO₃ induce a strong unscheduled DNA synthesis as measured by incorporation of ³H-thymidine into pollen DNA. The metal ions (added in most cases as the chloride) gave maximum effect at approximately 2 mM. Weaker reactions are given by Ca²⁺, Zn²⁺, Mn²⁺, Cd²⁺ and Cr³⁺ in decreasing order of effectiveness, while twelve other metal ions were shown to be ineffective or to give very low reaction. The unscheduled DNA synthesis induced by Al³⁺ was not altered by hydroxyurea, nicotinamide, caffeine or cycloheximide. It was markedly affected by the pH of the medium, the optimum pH being 5.0, where there could be a tendency for some base-binding of the metal (in contrast to phosphate binding) at the high Al³⁺ to DNA mole ratio used. It was considered that the DNA synthesis induced by the metal ions represents a repair synthesis. A DNA polymerase activity was detected in pollen extracts. It showed a preference for Mn²⁺ over Mg²⁺ and was estimated to have more than enough activity to account for the unscheduled DNA synthesis in pollen given by the most effective inducer, Al³⁺.