Is it possible to improve the efficiency of bioremediation technologies? The use of mixed cultures of bacteria and fungi inoculated at the rhizosphere level could promote the growth of the associated hyperaccumulating plant species and increase the absorption of metals in polluted soils, broadening new horizons on bioremediation purposes. This work investigates interactions between Ni-tolerant plant growth-promoting bacteria and fungi (BF) isolated from the rhizosphere of a hyperaccumulating plant. The aim is to select microbial consortia with synergistic activity to be used in integrated bioremediation protocols. Pseudomonas fluorescens (Pf), Streptomyces vinaceus (Sv) Penicillium ochrochloron (Po), and Trichoderma harzianum group (Th) were tested in mixes (Po-Sv, Po-Pf, Th-Pf, and Th-Sv). These strains were submitted to tests (agar overlay, agar plug, and distance growth co-growth tests), tailored for this aim, on Czapek yeast agar (CYA) and tryptic soy agar (TSA) media and incubated at 26 ± 1°C for 10 days. BF growth, shape of colonies, area covered on plate, and inhibition capacity were evaluated. Most BF strains still exhibit their typical characters and the colonies separately persisted without inhibition (as Po-Sv) or with reciprocal confinement (as Th-Sv and Th-Pf). Even if apparently inhibited, the Po-Pf mix really merged, thus obtaining morphological traits representing a synergic co-growth, where both strains reached together the maturation phase and developed a sort of mixed biofilm. Indeed, bacterial colonies surround the mature fungal structures adhering to them without any growth inhibition. First data from in vivo experimentation with Po and Pf inocula in pot with metalliferous soils and hyperaccumulator plants showed their beneficial effect on plant growth. However, there is a lack of information regarding the effective co-growth between bacteria and fungi. Indeed, several studies, which directly apply the co-inoculum, do not consider suitable microorganisms consortia. Synergic rhizosphere BFs open new scenarios for plant growth promotion and soil bioremediation.
The soil–root interface is the micro-ecosystem where roots uptake metals. However, less than 10% of hyperaccumulators’ rhizosphere has been examined. The present study evaluated the root and shoot response to nickel in hyperaccumulator and non-hyperaccumulator species, through the analysis of root surface and biomass and the ecophysiological response of the related aboveground biomass. Ni-hyperaccumulators Alyssoides utriculata (L.) Medik. and Noccaea caerulescens (J. Presl and C. Presl) F.K. Mey. and non-hyperaccumulators Alyssum montanum L. and Thlaspi arvense L. were grown in pot on Ni-spiked soil (0–1000 mg Ni kg−1, total). Development of root surfaces was analysed with ImageJ; fresh and dry root biomass was determined. Photosynthetic efficiency was performed by analysing the fluorescence of chlorophyll a to estimate the plants’ physiological conditions at the end of the treatment. Hyperaccumulators did not show a Ni-dependent decrease in root surfaces and biomass (except Ni 1000 mg kg−1 for N. caerulescens). The non-hyperaccumulator A. montanum suffers metal stress which threatens plant development, while the excluder T. arvense exhibits a positive ecophysiological response to Ni. The analysis of the root system, as a component of the rhizosphere, help to clarify the response to soil nickel and plant development under metal stress for bioremediation purposes.
The co-growth and synergistic interactions among fungi and bacteria from the rhizosphere of plants able to hyper accumulate potentially toxic metals (PTMs) are largely unexplored. Fungi and bacteria contribute in an essential way to soil biogeochemical cycles mediating the nutrition, growth development, and health of associated plants at the rhizosphere level. Microbial consortia improve the formation of soil aggregates and soil fertility, producing organic acids and siderophores that increase solubility, mobilization, and consequently the accumulation of nutrients and metals from the rhizosphere. These microorganism consortia can both mitigate the soil conditions promoting plant colonization and increase the performance of hyperaccumulator plants. Indeed, microfungi and bacteria from metalliferous soils or contaminated matrices are commonly metal-tolerant and can play a key role for plants in the phytoextraction or phytostabilization of metals. However, few works deepen the effects of the inoculation of microfungal and bacterial consortia in the rhizosphere of metallophytes and their synergistic activity. This mini-review aimed to collect and report the data regarding the role of microbial consortia and their potentialities known to date. Moreover, our new data had shown an active fungal-bacteria consortium in the rhizosphere of the hyperaccumulator plant Alyssoides utriculata.
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