30Heavy metals such as cobalt, copper, manganese, molybdenum, and zinc are essential in trace 31 amounts for growth by plants and other living organisms. However, in excessive amounts these 32 heavy metals have deleterious effects. Like other organisms, plants possess a variety of 33 detoxification mechanisms to counter the harmful effects of heavy metals. These include, the 34 restriction of heavy metals by mycorrhizal association, binding with plant cell wall and root 35 excretions, metal efflux from the plasma membrane, metal chelation by phytochelatins and 36 metallothioneins, and compartmentalization within the vacuole. Phytoremediation is an emerging 37 technology which uses plants and their associated rhizospheric microorganisms to remove 38 pollutants from contaminated sites. This technology is inexpensive, efficient and ecofriendly. 39 This review focuses on potential cellular and molecular adaptations by plants that are necessary 40 to tolerate heavy metal stress.41 42

This study explores the potential of lead resistant bacterium Acinetobacter junii Pb1 for adsorption/accumulation of lead using various techniques. In the present work, growth of A. junii Pb1 was investigated in the presence of a range of Pb(II) concentrations (0, 100, 250, 500, and 1000 mg l −1). Lead was found to have no toxic effect on the growth of A. junii Pb1 at 100 and 250 mg l −1 concentrations. However, further increase in Pb(II) concentration (500 mg l −1) showed increase in lag phase, though growth remained unaffected and significant growth inhibition was observed when concentration was increased to 1000 mg l −1. Same was confirmed by the observations of flow cytometry. Further, the effect of Pb(II) on A. junii Pb1 was evaluated by using fluorescence microscopy, spectrofluorimetry, and flow cytometry. The spectrofluorimetry and fluorescence microscopy results revealed the accumulation of Pb(II) inside the bacterial cells as evident by green fluorescence due to lead binding fluorescent probe, Leadmium Green AM dye. Flow cytometry observations indicate an increase in cell size and granularity of exposure to lead. Thus, present work provides a new understanding of Pb(II) tolerance in A. junii Pb1 and its potential use in remediation of lead from contaminated soil.

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