Diversity of Arbuscular Mycorrhizal Fungi in seleniferous soils and their role in selenium sequestration and biotransformation

Abstract

The present research aimed to investigate the molecular diversity of Arbuscular Mycorrhizal Fungi associated with plants growing in natural seleniferous soils and their role in protecting plants from Se toxicity. Arbuscular mycorrhizal fungal spores were isolated from natural seleniferous soil. Their morphological identification was done and the role of AMF inoculation to host plants in Se stress amelioration, plant growth promotion and improvement in bioactive and antioxidant potential was studied. Further, Se uptake and biotransformation in various plant parts of maize were studied via nursery trial and field experiment in natural selenium contaminated soil. For molecular identification, the genomic DNA extracted from maize roots grown in seleniferous and non-seleniferous regions was amplified using AMF specific primers by nested PCR approach. The 1.5 kb amplicon spanning pSSU-ITS-pLSU of 18S rRNA of AMF was deciphered using Illumina MiSeq Next Generation Sequencing (NGS) technique. A total of 17 AMF species from the seleniferous region and 18 AMF species from the non-seleniferous region were identified. The number of reads of Glomus irregularis, Glomus custos and Glomus intraradices was higher in seleniferous soil than non-seleniferous soil. A consortium of Se tolerant AMF inoculum was prepared and inoculated to maize seeds to be grown in seleniferous soils. AMF-inoculated plants were in good shape with higher root, shoot and grain biomass than non-AMF-inoculated plants. AMF inoculation lead to higher Se accumulation in roots but lesser Se accumulation in shoots and seeds of inoculated maize plants as compared to control plants. Nursery trial in seleniferous soil revealed that AMF inoculated plants had 2.3-times, 2.3-times and 1.9-times higher root, shoot and maize cob biomass respectively as compared to control plants. Se uptake studies through fluorescence spectrometry revealed that AMF inoculation led to 1.1 times higher Se accumulation in mycorrhizal maize roots as compared to control roots, but lesser translocation to shoots and seeds i.e. 0.3-times and 0.5-times lesser. Evaluation of growth parameters after field trial revealed that the root length and shoot length of AMF plants were of 2.13 times and 1.92 times respectively that of control plants. There was an upsurge of 2.35 folds, 2.32 folds and 1.93 folds in root biomass, shoot biomass and maize cob biomass respectively in AMF plants as compared to control plants. Bioaccumulation studies revealed that there was 7.80% higher Se accumulation in AMF roots as compared to control roots but at the same time, there was 71.83% and 49.92% lesser Se accumulation in shoots and grains of AMF plants. AMF inoculation increased Total phenolic (1.6-times) and Total flavonoid content (2.3-times) in host maize plants as compared to control plants. Antioxidant studies revealed that AMF inoculation also led to significant rise in catalase (1.4 & 2.2-times), superoxide dismutase (1.7 & 2.0-times), ascorbate peroxidase (1.2 &1.4-times) enzyme activities and DPPH radical scavenging activities (1.3 & 1.3-times) respectively in both shoots and roots of AMF inoculated plants. The study of selenium speciation in various plant tissues of AMF and control plants from field trial carried out using X-Ray absorption near edge spectroscopy technique revealed that the toxic inorganic selenium species present in control plant roots were replaced by reduced elemental selenium in AMF inoculated plants. Volatile organic forms of selenium i.e. dimethyl selenide and dimethyldiselenide present in higher proportion in AMF inoculated plant tissues, escape from the plant and act as active response of the inoculated plants to evade Se toxicity. Present research results suggest that AMF impart phytostablization potential to inoculated maize plants which means that such plants can stabilize or hold Se inside its roots along with its reduced translocation to shoots and grains. AMF further protect plants from selenium toxicity by biotransformation of Se to methylated volatile Se derivatives and improve plant growth promotion in maize plants by lesser Se translocation to above ground tissues while thriving under selenium stressed environment.