Uptake and effects of lead and zinc on alfalfa (Medicago sativa L.) seed germination and seedling growth: Role of plant growth promoting bacteria

Abstract

The present study was conducted to screen bacterial strains most effective in increasing alfalfa growth and metal accumulation in the presence of toxic levels of lead (Pb) and zinc (Zn). Results show that, compared to root and shoot growth, alfalfa seed germination is less sensitive to high levels of Pb and Zn. Pb and Zn concentrations of 4.4 and 7.9 mM, respectively, were required to give rise to a 50% reduction in in vitro seed germination. Root growth in alfalfa seedlings is, however, completely suppressed at a Pb concentration of 4 mM or a Zn concentration of 6 mM in the agar plate medium. Inoculation of the bacterial strains capable of producing indole-acetic acid and siderophore positively affected the plant growth parameter in the metal contaminated mediums. Root and/or shoot growths of alfalfa seedlings are significantly stimulated by the seven inoculated bacterial strains investigated, among which Bacillus filamentosus YSP110 is found to be the most effective. Inoculation of alfalfa plants with B. filamentosus strain YSP110 grown in a vermiculate medium is also seen to increase Pb accumulation in plant root and shoot by 18.0 and 72.4%, respectively. Bacillus cereus YSP4 is the bacterial strain most effective in stimulating root and shoot growth in alfalfa seedlings cultured in a Zn-contaminated medium. Compared to the non-inoculated plants, alfalfa seeds inoculated with B. cereus YSP4 also exhibit increased Zn accumulation in their roots and shoots by 43.2 and 48.7%, respectively. Results suggest that B. filamentosus YSP110 and B. cereus YSP4 offer a great potential for promoting plant growth and metal accumulation, which makes them good candidates for microbe-assisted phytoextraction of metal-contaminated soils.