Environmental stress acts as selective agent that alter gene frequencies in a population and causes genetic divergence within and among populations, but it also changes the gene expression and regulation modifying both transcription and translation. The importance of studying how the protein patterns change in hyperaccumulators in metalliferous or non-metalliferous soils as compared with non-hyperaccumulator or tolerant species has been paramount to explain this phenotype (Fig. 3). Comparison of hyperaccumulator and closely related species differing in other environmental proprieties showed how the threshold of protein pattern perturbation can be raised or lowered according to the need of adaptation. Most of the studies reported and discussed in this chapter on proteome's variation in hyperaccumulators are evidence that, among all the species considered (though only a few), there is not a behavior that can be considered as a major one. Rather there are many different responses which may have something in common. A modulation of protein levels associated with the energy metabolism and oxidative stress response reveals a tendency to a new trade-off between the genotype and the environment in the hyperaccumulators (Fig. 2). Because the cost of adaptation increases this is paid by a surplus in energy production. Another common feature in many hyperaccumulators is the enhanced accumulation of stress responsive proteins (for both biotic and abiotic stress) and of detoxifying proteins against ROS or reactive toxic intermediates of the hyperaccumulation response (Fig. 2). This is also explained in term of the new trade-off because the signal transduction chain, that is triggered to permit the hyperaccumulation, also has elements in common with the general stress response mechanism and because the hyperaccu-mulation is certainly more exposed to the deleterious effect of other stresses, especially the primary ones (Levitt 1980). Finally, hyperaccumulation in terms of specific proteome variation so far detected has been below the expectations in the sense that few specific proteins were identified, whose presence or absence could be clearly correlated with the hyperaccumulation of a specific metal respect to non-hyperaccumulator or tolerant species. Indeed there is a certain percentage (15%) of proteins of un-known functions that can contribute to explain the hyperaccu-mulation behavior (Fig. 2).
In conclusion the hyperaccumulator is a very complex phenotype affected by many genetic and environmental causes. From the proteome point of view, it seems more the result of phenotypic plasticity. For this reason the proteome analysis is interesting behind the specific curiosity of these plants, because it can be a paradigmatic case to discuss the relative roles that genetic and phenotypic plasticity play in plant evolution and adaptation.
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