Phenotypic characterization of microbes in the rhizosphere of Alyssum murale

Metal hyperaccumulator plants like Alyssum murale are used for phytoremediation of Ni contaminated soils. Soil microorganisms are known to play an important role in nutrient acquisition for plants, however, little is known about the rhizosphere microorganisms of hyperaccumulators. Fresh and dry weight, and Ni and Fe concentrations in plant shoots were higher when A. murale was grown in non-sterilized compared to sterilized soils. The analysis of microbial populations in the rhizosphere of A. murale and in bulk soils demonstrated that microbial numbers were affected by the presence of the plant. Significantly higher numbers of culturable actinomycetes, bacteria and fungi were found in the rhizosphere compared to bulk soil. A higher percent of Ni-resistant bacteria were also found in the rhizosphere compared to bulk soil. Percentage of acid producing bacteria was higher among the rhizosphere isolates compared to isolates from bulk soil. However, proportions of siderophore producing and phosphate solubilizing bacteria were not affected by the presence of the plant. We hypothesize that microbes in the rhizosphere of A. murale were capable of reducing soil pH leading to an increase in metal uptake by this hyperaccumulator.