Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2011
Publication Date: 6/1/2011
Publication URL: http://handle.nal.usda.gov/10113/54320
Citation: Adams, J.P., Adeli, A., Hsu, C., Harkess, R., Page, G., Depamphilis, C., Schultz, E., Yuceer, C. 2011. Poplar maintains zinc homeostasis with heavy metal genes HMA4 and PCS1. Journal of Experimental Botany. 62:3737-3752. Interpretive Summary: Heavy metal contamination of soils poses serious problems both for human health and agriculture. With growing interests in bioremediation of contaminated soils, several mechanisms in heavy metal uptake and sequestration have been identified and studied in hyper accumulating annual plants such as Thlaspi caerulescens. These plants accumulate large quantities of heavy metals such as Zn (zinc) and Cd (cadmium) but their use in remediation is limited by their small size. Perennial, woody species, with high biomass production have the potential to sequester heavy metals such as Zn while avoiding potential toxicity. The mechanisms in high-biomass, non-hyper accumulating perennial species such as poplar (Populus species), a fast growing tree, are unknown. Poplar contains genes with sequence homology to genes from other species, including hyper-accumulators, which are involved in regulation of heavy metal transport and detoxification indicating basic mechanism conservation. However, poplar does not have a hyper accumulating phenotype. Instead, poplar has a common indicator phenotype in which heavy metal accumulation is proportional to environmental concentrations but excesses are guarded against. This phenotype is affected by a large array of genes with sequence homology to the HMA and PCS families that have emerged through whole-genome duplications. These findings are providing us a better understanding of heavy metal uptake and sequestration by perennial woody species which ultimately will be beneficial in the phytoremediation of heavy metal contaminated soils.
Technical Abstract: This project has identified two large gene families, HMA and ZIP, in poplar that have been greatly expanded by the latest Salicoid genome duplication in poplar. In other species, these two families have been shown to be central in both hyperaccumulators and non-hyperaccumulators In poplar, many of the genes in the HMA4/PCS related pathway down-regulated in response to higher concentrations of Zn. Also, over-expressed PtHMA4 and PtPCS1 genes result in varying Zn phenotypes in poplar; specifically, there is a doubling of Zn accumulation in leaf tissues in over-expressed PtPCS1. The genomic complement and regulation of poplar highlighted in this study points to a large gene/protein pathway interacting congruently with the full nutrient complement in dictating its phenotype. There may be specific genes that can be altered in poplar to change its interaction with heavy metals, particularly Zn. Similarly, both studied genes in the ZIP family, PtZNT1and PtZIP1, down-regulate in response to higher Zn accumulation. Down-regulation of transporter mechanisms and relatively stable PCS production seem to be an evolved regulatory system allowing adequate absorption of Zn while inhibiting excess. Altering the natural expression of the genes may provide a means for potential creation of a hyperaccumulating phenotype in poplar. Poplar lines of over-expressed PtHMA4 and PtPCS1 show differing responses to Zn. Overall, this project has increased the knowledge of poplar’s genomic foundation for Zn interaction mechanisms. Using the increased understanding of these mechanisms, one line of poplar has been created that accumulates Zn at over two times the amount of control plants.