Plasticity of the response of repetitive/non coding DNA to the heavy-metal stress

Previous works have shown that genome reacts to heavy-metal stress by selective amplification of (G+C)-rich highly repetitive/ non coding DNA. Such phenomenon takes place similarly both in protonemal and in leafy shoot phases. However, when metals are removed from culture medium, if stress has taken place during the leafy shoot phase new-formed DNA is eliminated from nucleus; and vice versa, if stress has taken place during protonemal phase, the new-formed DNA remains inside genome of the target organism.Our idea is that such different behaviour of metal-induced repetitive/non coding DNA could be linked to different role of these two phases: in fact, while in protonemal phase protective role of repetitive/non-coding DNA may have a particular importance, conversely, in leafy shoot phase preservation of new generation from genome’s changes may have the priority. Based on these considerations, we have analyzed the behavior of metal-induced repetitive DNA during transition from protonemal to leafy shoot phase in different kind of situations. Our results have showed that: (i) most of repetitive/non coding DNA sequences triggered by heavy-metal stress in protonemal phase, even if apparently it should be stabilized in the genome, it is not actually transmitted to leafy shoots phase; (ii) in the leafy shoots resulting from stressed protonemata, answer of repetitive DNA to metal stress is similar but much less meaningful compared to the one trigged by the same conditions in the leafy shoots resulting from unstressed protonemata; (iii) in the leafy shoots resulting directly from those from mine through vegetative way, heavy-metal stress doesn’t trigger any type of DNA amplification. Some hypotheses are discussed on the basis of observed morphological and functional modifications.