Integrated approach on the rhizosphere response to Nickel in a facultative hyperaccumulator species

The contamination of metals like Nickel (Ni) in the soil represents a serious threat worldwide. To counteract this phenomenon, hyperaccumulator plant species, able to remove metal from soil and store it at high concentration in shoots, are employed for metal phytoremediation purposes. Native microbial communities occurring in the rhizosphere of hyperaccumulators often promote plant growth and metal uptake. So far, each abiotic and biotic rhizospheric components (soil, root system and microbiota) have been used without considering the reciprocal interactions and the responses to Ni stress as a whole. The present study aims to develop for the first time an innovative and multidisciplinary approach to examine the rhizosphere of Ni-hyperaccumulators as a holistic model, promoting the plant development and the Ni uptake. This integrated system is feasible owing to the collaboration with the Laboratory of Micology and the Laboratory of Microbiology of Department of Earth, Environmental and Life Sciences (DISTAV), University of Genoa. Among metalliferous soils, specific attention was given to serpentinitic soil which display extremely hostile conditions (nutrient shortage and concentration of metals - e.g., Ni - highly toxic) for most plants except for some hyperaccumulator species. Early response to Ni in plant development was assessed with micro- and mesocosm germination tests under Ni stress in the Ni-hyperaccumulator species Alyssoides utriculata (L.) Medik, Noccaea caerulescens (J. Presl & C. Presl) F. K. Mey. and Odontarrhena bertolonii (Desv.) L. Cecchi & Selvi and on the related non-accumulator species Alyssum montanum L. and Thlaspi arvense L., used for comparison. Afterwards, the response to increasing Ni concentrations in terms of root surface area, root and shoot biomass and photosynthetic efficiency was evaluated. Subsequently, A. utriculata was selected as a good candidate to study rhizospheric components because of its Ni-facultative hyperaccumulation traits and its ability to thrive in harsh metalliferous soils. Related rhizosphere and bare soil samples were collected from serpentine and non-serpentine sites. Plant and soil samples were processed and analysed with specific attention to isolation and identification of culturable microbiota, then selected for their Ni-tolerance and Plant Growth Promoting (PGP) traits. Later, most performing Ni tolerant bacterial and fungal strains were tested by means of co-growth methods to estimate their reciprocal behaviour in a mixed culture to be used as inoculum in the rhizosphere of A. utriculata. Results demonstrate that increasing Ni concentrations can induce marked inhibition of germination in hyperaccumulator species, despite their accumulation ability. However, hyperaccumulator species exhibit a positive response in terms of root surface area, biomass and photosynthetic efficiency, compared to non-hyperaccumulator species in which there is a dose-response effect by Ni, except for T. arvense in pot test. In particular, A utriculata reveals an increased aboveground biomass and sample vitality in pot test, suggesting an adaptation to harsh environmental conditions. Microbiota isolates are more abundant in non-serpentinitic and rhizospheric soil, without selectivity between microorganisms and Ni. Some bacterial and fungal strains (Pseudomonas sp. SERP1, Streptomyces sp. SERP4 and Penicillium ochrochloron Biourge Serp03S, Trichoderma harzianum Rifai Serp05S respectively) reveal high Ni tolerance (up to 20 nM) and PGP traits. In particular SERP1 and Serp03S display a mutual synergism in co-growth methods and they could be promising candidates as natural chelators in the rhizosphere of A. utriculata, to enhance plant development and Ni uptake. This research represents the first step of integrated plant-microbiota tool, in the perspective to improve Ni uptake from polluted soil, using native Ni-hyperaccumulator species and associated rhizobiota, although further investigations are required to ascertain the efficiency of the field application.