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Final Activity Report Summary - MYCOREMED (Role of arbuscular mycorrhizal fungi on the accumulation of radiocaesium by plants)

MYCOREMED aimed to elucidate the mechanisms involved in the transport of radiocesium by arbuscular mycorrhizal fungi (AMF) to the root cells, its uptake by root cells and translocation to shoot cells in mycorrhizal plants. Three principal plant models were used, namely medicago truncatula, plantago lancelolata and nicotiana tabacum, and the experiments were conducted in vitro with root organ cultures and arbuscular mycorrhiza - plant (AM-P) in vitro culture system, and in vivo, with pot cultures and within the field.

Overall, it was confirmed that AMF were able to take up, translocate and transfer radiocaesium to their host plant. However, transfer to shoots was in most cases restricted as compared to non-mycorrhizal controls, suggesting rather an accumulation in the roots than a transfer to the shoots. This was confirmed by X-ray microanalysis showing the accumulation of 133Cs in cortical and vascular tissues of mycorrhizal roots. This accumulation seemed to differ between AMF, further suggesting that the plant and AMF combination might be important for radiocaesium accumulation in shoots. AMF were also shown capable to transfer radiocaesium between plants. Even though this transfer was quantitatively low, it suggested that shared mycorrhizal networks could contribute to the redistribution of this radionuclide in the environment, which otherwise would be restricted both in time and space.

Several factors, both internal, at the plant level, and environmental, at the soil level, played key roles in the level of radiocaesium transferred to the plant. Under specific controlled in vitro culture conditions, it was shown that the level of abcisic acid was higher in colonised m. truncatula plants as compared to the non-mycorrhizal controls, suggesting, as had been earlier hypothesised, that ABA could be involved in the reduced root to shoot translocation of radiocaesium. Potassium (K) and phosphorus (P) alone and in combination showed to impact the transport of radiocaesium by the AMF. Under increased concentration of K, the amount of radiocaesium transported to the plant was decreased. The reverse was observed for P. Identically, AMF in combination with K reduced the radiocaesium uptake and its subsequent transfer to plant shoots under non-K starvation conditions and under K-deficient conditions, glomus sp. did not influence Cs accumulation by m. truncatula grown in vitro.

Caesium (Cs) was shown to affect gene expression in m. truncatula. Furthermore, roots seemed to harbour a K+ sensor which under K+ starvation and root colonisation by AMF detected higher internal K+ concentrations as compared to the controls. This further suggested that K+ transport to the root of mycorrhizal plants might primarily be performed by the AMF. Interestingly, nearly 25 years after the Chernobyl accident, roots of p. lanceolata collected in the exclusion zones of Chernobyl, containing different level of 137 Cs, still showed the differences in colonization by AMF and dark septated endophytes. Identically, macromycetes collected from Opole region in Poland showed that still nowadays fruiting bodies contained large doses of radiocaesium. This demonstrated again that fungi were radiocaesium accumulators.

As a conclusion, AMF were intimately associated to the uptake, translocation and transfer of radiocaesium by their host plant. Internal factors such as ABA and environmental factors such as K and P modulated the level of radiocaesium accumulation which could further vary between the plant and AMF combinations. Gene expression was intimately associated with the transport and accumulation of radiocaesium in the couple plant and AMF. Finally AMF, by linking most plants below ground were able to redistribute radiocaesium in the ecosystem, even if this transfer was quantitatively low.

These results were obtained by four PhD students embedded in a consortium of four teams. They were offered a strong mobility and training scheme that resulted in several publications, conferences participations with oral and poster presentations and summer schools, in order to finally succeed in their PhD defences.

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