Arsenic transport in plants

Arsenic (As), a metalloid occurring ubiquitously in nature in organic and inorganic forms, is classified as a potent carcinogen. Among the inorganic forms which are more toxic, AsIII has a high affinity to bind with sulfhydryl groups of the amino acid cysteine, affecting many key metabolic processes such as fatty acid metabolism and glutathione production. AsV is a phosphate analogue and can substitute the inorganic phosphate affecting nucleotide synthesis and energy homeostasis of the cell. Much of the research on As in plants focuses on rice as it is the major source of dietary As intake and is often grown in areas with aquifers containing high amounts of As which are prevalent in south east Asian countries. Currently there is a great need to understand how plants deal with As, both from the perspective as potential sources of dietary As but also as potential mechanisms for phytoremediation. We are therefore using various approaches to identify and characterise plant membrane proteins involved in transport of As.
NIPs (nodulin like intrinsic proteins) are a subgroup of plant aquaporins reported to be involved in bidirectional transport of AsIII. Loss of function mutants in Arabidopsis NIPs (nip5;1, nip6;1 and nip7;1) were identified and analysed for their role in As uptake, efflux and translocation. The data showed that nip5;1 and nip6;1 may be involved in the efflux of As. The data for total As concentration in root and shoot tissues showed that among these mutants only nip6;1 has a higher fraction of total As in the shoots compared to wild type. Lower efflux and more translocation suggest that this isoform (nip6;1) may be involved in vacuolar sequestration of As. Interestingly, nip7;1 showed a higher efflux of AsIII compared to other mutants and wild type. This suggests that NIP7;1 might have a role in the vacuolar sequestration or translocation because the loss of function resulted in more cytosolic AsIII due to lower sequestration or translocation, which made more AsIII available for the efflux.
The Saccharomyces cerevisiae gene ACR3 is involved in AsIII efflux from the cytosol. We have transformed it into Arabidopsis and rice to assess if this can improve plant As tolerance. The results showed that expression of ACR3 affects plant growth. It appears that both in Arabidopsis and rice ACR3 may be involved in the efflux and translocation of As, as was shown by the results at the cellular, seedling and mature plant levels. ACR3 expression could be a potential means for phytoremediation of As in Arabidopsis because it increases As translocation to shoot tissue.
In addition, yeast was used as a heterologous expression system to screen cDNA libraries from rice (Oryza sativa) and Arabidopsis thaliana using the yeast strains (ycf1∆ and acr3∆) to identify (new) transporters that are involved in As transport in plants. No growth tolerant/sensitive phenotype was observed in any yeast strain with both expression libraries suggesting the absence of putative new transporters/proteins involved in As transport.
The information obtained from this study can be used in future research. ACR3-like genes involved in As tolerance from other organisms can be potentially useful in plants. Based on the results from NIPs, the isoforms involved in As efflux (nip5;1 and nip6;1) can be used to generate double knock out mutants to see if these have an additive/synergistic effect on As transport that will add to the knowledge of As transport in planta.