The 2011 Fukushima Daiichi Nuclear Power Plant accident dispersed radioactive cesium (Cs) into the environment [1], [2], so that land forest and fresh water ecosystems surrounding the Fukushima area in Eastern and northwest Japan have been heavily contaminated [3], [4]. In natural land forest ecosystems, this radiocesium accumulates in the deposited litter but also in the upper soil profile, including the humus layer [5], [6]. Wild edible mushrooms and other organically grown crops that rely on organic matter for nutrients [7] therefore become a potential risk to human health through exposure via the primary food chain [8], particularly within the metal-contaminant hyper accumulators such as Chinese cabbage and tomato, both grown in these contaminated areas in Ibaraki and Tohoku, respectively. As such, the preservation of these high accumulator crops from radionuclide accumulation becomes therefore of crucial importance.

Symbiotic microorganisms, such as mycorrhiza and dark septate endophytic fungi (DSE) are known to associate with most forest trees and crops [9], [10]. The potential of these microorganisms in the accumulation of metal contaminants, especially heavy metals, and radionuclide Cs has attracted much attention. Arbuscular mycorrhizal fungi (AMF), for example, showed high cesium accumulation, reaching 41.7% to 71.7% in the above-ground portion of three grass species inoculated with Glomus mosseae and Rhizophagus irregularis (G. intraradices) [11]. More recently, AMF showed the ability to improve both the biomass and Cs uptake of plants such as sunflower and cucumber [12], [13]. Contrary to mycorrhizas, the potential of these DSEs in the management of plant growth and Cs accumulation in high accumulator crops under contaminated environments remains widely untapped. DSEs are not very common in soil but are often associated with roots of many plants, forming “symbiotic” relationships [14]. They have little or no host specificity [10], [15] and can even live in symbiosis with non-mycorrhizal plants such as Chinese cabbage [16]. There was some striking variation among plant species, but Solanaceous and Brassica plants are known as plants with high affinity for DSEs with regards to the greatest number of DSE fungi with isolates obtained from Solanaceous and Brassica plant root compared to other plants [16], [17].

Interactions between DSEs and host plants have improved plant fitness through mechanisms such as nitrogen transfer and uptake of phosphorus, and nutrient provision to host plants [16], [18]. We predicted that DSEs would play key regulation roles in extreme environments, especially in Cs-polluted conditions.

We studied the interaction between DSEs and host plants in Cs-polluted environments using Chinese cabbage and tomato. Both crops are hyper accumulator vegetables and are cultivated in heavily polluted areas in Ibaraki and Tohoku prefectures. The former is a leafy vegetable consumed entirely and the latter is a fruity vegetable cultivated for fruit consumption.

The cycling of the radionuclides is not well understood in natural ecosystems due to high diversity [19]. It is well known that inoculation of endophytic bacteria to plants can improve biomass production and uptake/accumulation of Cs in contaminated environments [20]–[22]. However, although soil fungi have potential in radionuclide bioaccumulation [23], [24], how DSEs interact with plants with regard to growth and the accumulation of radioactive Cs in contaminated environments is poorly understood. In plants, Cs, especially at high levels, can affect plant growth by causing various negative effects, i.e., chlorosis, necrosis, phytotoxicity, reduction of root growth, etc. [25], [26].

In the present study, we demonstrated that DSE fungi also have the ability to enhance Chinese cabbage and tomato plants growth under Cs-contaminated conditions. Previously, some arbuscular mycorrhiza such as Glomus mosseae or Rhizophagus irregularis (Glomus intraradices) have been reported to promote the biomass of grass and sunflower species [11], [13] in Cs-contaminated environments. Endophytic bacteria also have been reported enhancing hyperaccumulator plants [27], [28]. Beside the use of plants and associated microorganisms, other techniques such as the use of elevated CO₂ were developed to trigger hyperaccumulation of Cs in sorghum [29].

Therefore, to our knowledge, this study is the first report involving DSEs in such environments. Three of the isolates used in the present study, i.e., H. velutinum isolate 41-1, V. simplex isolate Y34 and P. ibarakiensis isolate I.4-2-1, have already been reported to support plant growth under nutrient-deficient and other acidic pH conditions but not in contaminated conditions [30], [31], [32]. These results shed a light on a new field of use of these isolates. Interestingly, the biomass increase achieved with the present DSEs is greater than the 41.7% to 71.7% increase obtained with arbuscular mycorrhizal fungi [11]. Their ability to support plants growth combined to the high accumulation capacity they conferred to host plants suggest increased risk of contamination in the food chain, as the concentration of Cs in this food chain is of key importance for human health [33], [34]. However, they can be a viable means for solving the problem of small biomass production which is one of the major limiting factors for using tolerant plants in bioremediation [35], [36]. These DSEs can be selected as potential candidates to undergo further studies in real life conditions before they can be recommended (if conclusive) to farmers in the surrounding contaminated areas. Considering that Brassica plants are easily grown crops, their combination with DSEs can enhance phtyoremediation by means of Brassica plants including at least Chinese cabbage for extracting the Cs from contaminated soils. The DSE can even live in symbiosis with non-mycorrhizal plants such as Brassica plant at the present state [16]. We did not extend our study to other host plants and DSE species; however, other DESs might share the same abilities. Until now, DSE fungi in general have been mainly known to support plant growth under unfavorable conditions [14], [37].

Furthermore, our study demonstrated consistently the ability conferred to tomato plants by DSEs to decrease the accumulation of Cs under laboratory conditions. Previously, some studies reported improved uptake and/or accumulation of Cs by host plants using mycorrhiza, not DSE fungi [38], [39], [13]. For example, plant colonization by AM can increase Cs uptake [40]. Thus, to our knowledge, this study is the first report of DSE improving (decreasing) host Cs accumulation in contaminated environments.

These results demonstrate how DSE fungal symbionts are likely to function in radionuclide-polluted environments by regulating the bioavailability of Cs, confirming the hypothesis of the multifunctionality of DSE-plant symbioses [41]. The impact of these interactions goes beyond nutrient acquisition and the resultant positive host growth responses.

We are not able to explain the exact mechanism by which DSEs can promote plant growth or decrease the accumulation of Cs by their host plants, however, it might be related either to the sequestration of Cs in fungal hyphae or to the effects of ionizing radiation on the fungus. Exposure to ionizing radiation was recently proved to enhance the growth of melanized fungi [42]. The good performance of DSEs that also possess this melanin pigment in Cs-contaminated environments may also be related to this phenomenon. More studies are needed to support such hypotheses.

This finding is important as it shows another function of DSE fungi. Previously, these fungi were known to improve plant fitness through nutrient transfer and so on [16], [18]. This finding suggests that DSEs tend to display a multifunctional symbiosis with host plants, as is the case for mycorrhizal fungi and their host plants. These results can be used as a model in attempts to solve the crucial food chain contamination caused by these radionuclides. They can mainly be recommended to tomato growers in contaminated areas to reduce in a sustainable way the accumulation of Cs in the primary food chain by preceding tomato cultivation by Chinese cabbage cultivation as tomato represents a high demand for consumers. For future and more scientific perspectives, they can also serve as a starting point for more detailed investigations to clarify the mechanisms underlying beneficial plant-fungus interactions in DSE symbiosis.