Metallothioneins from a Hyperaccumulating PlantProsopis julifloraShow Difference in Heavy Metal Accumulation in Transgenic Tobacco

Overexpression studies in plants have demonstrated the ability of metallothioneins (MTs) to hyperaccumulate heavy metals, but comparative analysis of heavy metal accumulation by different types of MTs from the same species is largely unavailable. Our previous study on three types of MTs (PjMT1, PjMT2, PjMT3) from heavy metal accumulating phreatophyte Prosopis juliflora reported that PjMT1 showed enhanced binding to cadmium, copper and zinc than other two types in E. coli. The present study, an extension of the previous work, compares the ability of PjMT1 and PjMT2 to impart heavy metal tolerance in transgenic tobacco plants. PjMT1 and PjMT2 were cloned separately in plant transformation vector under constitutive promoter along with GUS screening marker and transformed into Nicotiana tabacum. When subjected to 0.3 mM CdSO4, both PjMT1 and PjMT2 expressing transgenic plants demonstrated better survival and higher accumulation of Cd than wild type plants. Atomic absorption spectrometry revealed PjMT1 and PjMT2 transformed tobacco plants exhibiting nine fold and five fold higher Cd accumulation respectively, in comparison to non-transgenic plants. Measurement of chlorophyll degradation upon cadmium stress also indicated more chlorophyll retention in PjMT1 and PjMT2 transgenics compared to wild type plants. The results of the study identify PjMT1 as a better candidate gene for phytoremediation of cadmium.