Abstract
Ectopic expression in tobacco (Nicotiana tabacum v. Xanthi) of the export protein AtHMA4 (responsible in Arabidopsis for the control of Zn/Cd root to shoot translocation) resulted in decreased Cd uptake/accumulation in roots and shoots. This study contributes to understanding the mechanisms underlying this Cd-dependent phenotype to help predict the consequences of transgene expression for potential phytoremediation/biofortification-based strategies. Microarray analysis was performed to identify metal homeostasis genes that were differentially expressed in roots of Cd-exposed AtHMA4-expressing tobacco relative to the wild type. It was established that down-regulation of genes known to mediate Cd uptake was not responsible for reduced Cd uptake/accumulation in AtHMA4 transformants. The transcript levels of NtIRT1 and NtZIP1 were higher in transgenic plants, indicating an induction of the Fe and Zn deficiency status due to AtHMA4 expression. Interestingly, upon exposure to Cd, genes involved in cell wall lignification (NtHCT, NtOMET, and NtPrx11a) were up-regulated in transformants. Microscopic analysis of roots demonstrated that expression of AtHMA4 caused an induction of cell wall lignification in the external cell layers that was accompanied by enhanced H2O2 accumulation. Further study showed that the concentration of other elements (B, Co, Cu, Ni, Mo, and Zn) was reduced in AtHMA4 transformants in the presence of Cd. In conclusion, due to ectopic expression of 35S::AtHMA4, the physical apoplastic barrier within the external cell layer developed, which is likely to be responsible for the reduction of Cd uptake/accumulation.
Highlights
Cadmium (Cd) is a heavy metal that is widespread in the environment, occurring both naturally and as a result of anthropogenic activities (Clemens et al, 2013)
Microarray analysis was conducted to screen for metal homeostasis genes that were differentially expressed in tobacco transformed with AtHMA4 relative to the wild type
In agreement with the results presented by Siemianowski et al (2011), Cd concentrations in the roots and the shoots were significantly lower in AtHMA4 transformants compared with the wild type (Supplementary Fig. S1 available at JXB online)
Summary
Cadmium (Cd) is a heavy metal that is widespread in the environment, occurring both naturally and as a result of anthropogenic activities (Clemens et al, 2013). It is highly toxic to living organisms, even at low concentrations. It is a non-essential element it is taken up by plants from the soil and enters the food chain, posing a threat to human health (Williams and Salt, 2009). There are various types of phytoremediation, and phytoextraction has potential for pollutants such as Cd; this involves uptake and accumulation of the pollutant into the plant biomass from the environment. It is predicted that plant species useful for metal phytoremediation/phytoextraction would have the ability to take up the metal from the soil, and efficiently translocate and accumulate it in the shoot, which could be harvested. Translocation of Cd (and other heavy metals) from the roots to the shoots is under tight control.
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