Avoidance and reduction of soil contamination with heavy metals is one of the most serious global challenges. Nowadays, science offers us new opportunities of utilizing plants to extract toxic elements from the soil by means of phytoremediation. Plant abilities to uptake, translocate, and transform heavy metals, as well as to limit their toxicity, may be significantly enhanced via genetic engineering. This paper provides a comprehensive review of recent strategies aimed at the improvement of plant phytoremediation potential using plant transformation and employing current achievements in nuclear and cytoplasmic genome transformation. Strategies for obtaining plants suitable for effective soil clean-up and tolerant to excessive concentrations of heavy metals are critically assessed. Promising directions in genetic manipulations, such as gene silencing and cis- and intragenesis, are also discussed. Moreover, the ways of overcoming disadvantages of phytoremediation using genetic transformation approachare proposed. The knowledge gathered here could be useful for designing new research aimed at biotechnological improvement of phytoremediation efficiency.

The increased frequency and intensity of drought periods is becoming a serious thread for agriculture, prompting the identification of crop species and cultivars with enhanced water stress tolerance. Drought responses were studied in four ornamental Sedum species under controlled greenhouse conditions, by withholding watering of the plants for four weeks. Determination of growth parameters (stem length, fresh weight) allowed establishing the relative degree of tolerance of the selected species as S. spurium > S. ochroleucum > S. sediforme > S. album. The levels of photosynthetic pigments (chlorophylls a and b and total carotenoids), oxidative stress [using malondialdehyde (MDA) as marker], non-enzymatic antioxidants (total phenolic componds and total flavonoids) and osmolytes (proline and total soluble sugars) were measured in leaves of control and stressed plants, to correlate drought tolerance with the activation of specific response mechanisms. The results obtained indicate that a higher tolerance to water deficit in Sedum is associated with: a) relatively lower stressinduced degradation of chlorophills and carotenoids, especially of the latter (which does not decrease in water-stressed plants of S. spurium, the most tolerant species); b) no increase in MDA levels; that is, lack of drought-induced oxidative stress and, consequently, no requirement to activate the synthesis and accumulation of antioxidant compounds; and c) higher proline levels in the non-stressed controls, which could be the basis of constitutive mechanisms of tolerance. However, proline concentrations are too low to have any significant osmotic effect, and its likely contribution to water deficit resistance in Sedumwould be due to its activity as 'osmoprotectant'. The identification of these biochemical markers of drought tolerance should help to develop rapid and efficient screening procedures to select Sedum taxa with enhanced tolerance when comparing different species within the genus, or different cultivars within a given species. Abbreviations Caro -total carotenoidsChl a -chlorophyll a Chl b -chlorophyll b MDA -Malondialdehyde (MDA),

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