Meccanismi di adattamento delle piante allo stress da metalli pesanti: possibili implicazioni per la tecnologia denominata fitorimedio

Responsabili di progetto

Massimo Zacchini, Vladimir Kuznetsov

Accordo

RUSSIA - RAS old - Russian Academy of Sciences old

Bando

CNR/RAS 2011-2013

Dipartimento

Terra e Ambiente

Area tematica

Scienze del sistema Terra e tecnologie per l'ambiente

Stato del progetto

Nuovo

Proposta di ricerca

In the last years, the release of heavy metals in water and soils, due to the enhanced industrial activities and the intensive farming, has posed a relevant threat for the food chains and more in general for the ecosystem survival. To face the elevated metal concentration often present in the environment, plants have evolved an array of resistance mechanisms, including avoidance or exclusion processes, that reduce metal accumulation in the cells, and tolerance mechanisms, which allows plants to survive while accumulating high concentrations of metals. The adaptive responses to high concentration of metals in plants involve biochemical processes that reduce the metabolic impact of metal ions on cell physiological activity, like metal sequestration and chelation. Metal chelators can be grouped following the electron donor element, namely sulphur, oxygen and nitrogen. Sulphur contaning ligands  (Thiols) are involved in metal trasport and detoxification and are rapidly induced in plants following metal exposure. Phytochelatines and metallothionein, and their precursors cysteine, g-glutamylcysteine and glutathione (GSH), are characterised by a remarkable affinity for metals, especially Zn, Cd, Pb, Cu,  Hg e Ag. Also oxygen-containing molecules such as organic acids (OAS, especially citric, malic and oxalic acids) are claimed to participate in metal binding and transportation from roots to shoots. An increase of OA levels in plant tissues or an extrusion by plants roots to complexate metals and to reduce their damaging effects was reported. For that regards N-compounds, aminoacids, such as hystidine, and polyamines (PAs) are reported to be involved in metal chelation and detoxification. As reported for other kind of stress, an increase of the three main PAs (Putrescine, spermidine and spermine) is commonly described after metal treatment. An involvement of  PAs in metal binding and in metal storage in vacuole was reported. It was also showed that in rapeseed  plants a pre-treatment with PAs induced a  three fold  increase of  Ni accumulation  in the above ground organs reducing the toxic effects exerted by this metal. Moreover, a capability of PAs to act as antioxidants and radical scavengers and to stabilise and protect cell membrane under stress condition was showed.
The capability of plants to tolerate, absorb and concentrate metals in their organs is a basic characteristic to be exploited to utilise them for the remediation of metal polluted substrates. This natural technique, called phytoremediation, can allow to reduce or get rid of the mild metal contamination of agricultural soils without the heavy impacts on the ecosystems and the high costs characterising the traditional techniques. Phytoextraction of heavy metals, that is a part of the phytoremediation processes, relies on the ability of plants to efficiently absorb the pollutants by roots and translocate them in the above ground organs while tolerating. Metal confinement in the roots, in order to protect the photosynthetic process, is a common process carried by plants that express a  good metal tolerance. This physiological mechanism results in a reduced metal translocation to the above-ground organs, i.e. the plants parts that can be easily harvested for a disposal. Then, it represents one of the  constraints to overcome to achieve plants with a superior potentiality to phytoremediate polluted soils. In this context, a better understanding of the physiological mechanisms underlying the metal tolerance, absorption, accumulation and translocation in plants is currently requested to make phytoremediation a bio-technology more and more attractive for the reclamation of polluted areas. The aim of this project is to study the physiological, biochemical and molecular mechanisms of plant adaptation to high concentrations of heavy metals like Ni, Cd, Pb, Cu. Research planning will be devoted to perform a screening among metal hyperaccumulating plants (belonging to the family of Brassicaceae, Amaranthaceae and others) investigating on the tolerance response and in particular on the role of protective compounds such as PAs, thiols and organic acids in this process. Research will be also focused to better understand the implication of these compounds in metal translocation to aboveground organs. For this purpose, treatment with exogenous metal binding and protective compounds will be carried out in selected plants in order to increase metal removal from contaminated substrates. Experiments will be conducted to characterise the oxidative stress status of plants (by measuring MDA levels, membrane redox activity and H2O2 levels), the antioxidant activity (evaluating the activities of enzyme like APX, GR, CAT, GPX and the non enzymatic tissue activities by DPPH assay), and the physiological performances of plants (by measuring photosynthetic, transpiration and respiratory gas exchanges, and chlorophyll "a" fluorescence by IMAGING-PAM fluorometer). Metal accumulation will be evaluated on digested materials by an Atomic Absorption Spectrophotometer and by an XRF spectrometer. HPLC analysis of PAs, thiols and organic acids will be conducted to determine the levels of this compounds in plant tissues. For that regards PAs, the catabolism of these molecules will be characterised by analysing the activity of  diamine oxidase (DAO) and polyamine oxidase (PAO). It is expected that the obtained results can contribute to clarify the role of the above reported substances in the physiological processes associated to metal tolerance, accumulation and translocation. That can allow to individuate the genetic determinants regulating these processes and to characterise some traits useful to screen out for genotypes with an enhanced ability to extract metals from contaminated substrates, making the phytoremediation technique more efficient and attractive.  
 
 
 

Obiettivi della ricerca

The present proposal is aimed at gaining more insight into the role of polyamines and other metal ligands and oxidative stress protectors in the tolerance, accumulation and above ground translocation of heavy metals by plants. The expansion of knowledge in this field represents a fundamental aspect to select plants with an enhanced ability to remove pollutants from contaminated substrates. That can allow the bio-technology called phytoremediation to become more efficient and then more attractive for large applications. For this purpose it is expected that the project  produce several important outcomes such as the characterisation of the tolerance abilities in selected heavy metal hyperaccumulating plant species by means of physiological, biochemical and molecular analysis, the individuation of some physiological traits associated to heavy metal tolerance, measured through non-destructive analysis, that allow to perform a rapid screening of plant individuals and genotypes, the clarification of the mechanisms by which  polyamines act as protective compounds towards oxidative stress exerted by metals, a correlation between polyamine content and metal accumulation in plant organs,  the elucidation of the role played by polyamines in the metal translocation to above ground plant organs, the identification, characterisation and quantification of a polyamine-metal complex in plants.

Ultimo aggiornamento: 18/05/2025