An Alliance of Trifolium repens—Rhizobium leguminosarum bv. trifolii—Mycorrhizal Fungi From an Old Zn-Pb-Cd Rich Waste Heap as a Promising Tripartite System for Phytostabilization of Metal Polluted Soils

Abstract

The Bolesław waste heap in South Poland, with total soil Zn concentrations higher than 50,000 mg kg–1, 5,000 mg Pb kg–1, and 500 mg Cd kg–1, is a unique habitat for metallicolous plants, such as Trifolium repens L. The purpose of this study was to characterize the association between T. repens and its microbial symbionts, i.e., Rhizobium leguminosarum bv. trifolii and mycorrhizal fungi and to evaluate its applicability for phytostabilization of metal-polluted soils. Rhizobia originating from the nutrient-poor waste heap area showed to be efficient in plant nodulation and nitrogen fixation. They demonstrated not only potential plant growth promotion traits in vitro, but they also improved the growth of T. repens plants to a similar extent as strains from a non-polluted reference area. Our results revealed that the adaptations of T. repens to high Zn-Pb-Cd concentrations are related to the storage of metals predominantly in the roots (excluder strategy) due to nodule apoplast modifications (i.e., thickening and suberization of cell walls, vacuolar storage), and symbiosis with arbuscular mycorrhizal fungi of a substantial genetic diversity. As a result, the rhizobia-mycorrhizal fungi-T. repens association appears to be a promising tool for phytostabilization of Zn-Pb-Cd-polluted soils.

Potential of Rhizobia Nodulating Anthyllis vulneraria L. from Ultramafic Soil as Plant Growth Promoting Bacteria Alleviating Nickel Stress in Arabidopsis thaliana L.

“…Our previous studies on metalliferous waste confirmed the presence of effective legume-rhizobia symbiosis with the participation of rhizobia characterized by high tolerance to metals [ 12 , 13 , 14 ]. The plants growing on ultramafic soils, as well as their associated microsymbionts must have evolved mechanisms to cope with toxic levels of metals present in the soil [ 13 , 14 ]. Moreover, the rhizobia isolated from natural metal-contaminated sites show higher tolerance to metals than the bacteria from non-polluted habitats [ 29 , 59 , 60 ].…”

“…Rhizobia are the most beneficial group of bacteria mainly due to their ability to supply plants and soil with nitrogen during their symbiosis with legumes. This feature is crucial especially on poor, metal-polluted soils [ 12 , 13 , 14 ]. Moreover, many reports have shown that rhizobia can also interact with non-legume plant species (e.g., Arabidopsis thaliana ) and promote plant growth [ 19 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 55 ].…”

“…The metal-tolerant plants such as Anthyllis vulneraria L. ( Fabaceae ), which spontaneously colonize ultramafic soils, must adapt to the constant metal stress and nutrient deficiency. A symbiosis between legumes and nitrogen-fixing bacteria, named ‘rhizobia’, is a major reason why legumes can colonize nutrient-deficient soils and marginal lands contaminated with metals [ 12 , 13 , 14 , 15 ]. The legume-rhizobia symbiotic interactions lead to the formation of nodules that are essential organs providing microanaerobic conditions allowing differentiated rhizobia (enclosed in symbiosomes) to fix atmospheric nitrogen and convert it into ammonia used by the host plant [ 16 , 17 , 18 ].…”

“…The legume-rhizobia symbiotic interactions lead to the formation of nodules that are essential organs providing microanaerobic conditions allowing differentiated rhizobia (enclosed in symbiosomes) to fix atmospheric nitrogen and convert it into ammonia used by the host plant [ 16 , 17 , 18 ]. Our studies on legume-rhizobia symbiosis on ultramafic tailings indicate the presence of rhizobia not only effective in biological nitrogen fixation but also showing the ability to promote plant growth as plant growth promoting bacteria (PGPB) [ 14 ]. Apart from their interactions with legumes, rhizobia are known to form growth promoting associations with non-legumes, including monocots such as maize ( Zea mays ), wheat ( Triticum aestivum ), rice ( Oryza sativa ), and dicots such as radish ( Raphanus raphanistrum ), carrots ( Daucus carota ), tomato ( Lycopersicon esculentum ), and Arabidopsis thaliana [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ].…”

“…Application of PGP rhizobia increase plant survival in the presence of hazardous levels of metals mainly by counteracting biomass decrease by improved mineral nutrition and secretion of phytohormones, enhance the production of plant growth regulators (auxins, gibberellins, cytokinins), and stimulate photosynthesis and antioxidant activity [ 13 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. Moreover, rhizobia also support plant adaptation to metal-polluted sites through chelation of the metals in the soils [ 14 , 29 , 33 ]. It has been shown that ultramafic soil bacteria are tolerant to nickel [ 33 , 34 ], and they exhibit species-unique cellular response mechanisms under stress conditions [ 29 , 34 ].…”