Populus spp. strategies to counteract Zn excess: Phytoremediation perspectives

Environmental pollution with metals is one of the most widespread problem. Phytoremediation technologies use the plant efficiency in acquiring and concentrating nutrients as well as numerous metabolic activities. Because of their extensive root system, high biomass production and low-input cultivation, trees are attractive phytoremediators. The recent genome sequencing, together with the development of genomics tools and the ease of genetic transformation of poplar, have opened up new avenues for the use of trees in phytoremediation. In our studies we have reported about morpho-physiological, biochemical and molecular responses of hybrid poplar clones to several heavy metals (Zn, Cd, Cu, etc.), in different growing conditions. For ultrastructural and molecular determinations, we concentrated our attention on Zn, a heavy metal that is also a micronutrient. Using the poplar hybrid clone I-214 as a model system, we obtained interesting results for the understanding of tolerance mechanisms to excess and nutrient impairments. The preferential accumulation of Zn in the older leaves can be explained as a defence or tolerance mechanism against the excess uptake of the metal by the plant: restriction of Zn transport towards young tissues and storage in the old ones. In the roots, the Zn localization by cryo-SEM/EDXMA has revealed that the epidermis and cortex represent a physical barrier against the Zn excess uptake within the stele and into the xylem flux. For the identification and study of specific genes involved in I-214 (and Populus) responses to Zn and other metals we choose the microarray technology. The Affymetrix GeneChip Poplar Genome Array containing more than 61,000 probe sets (over 56,000 transcripts and gene predictions) permitted us to analyse the transcriptomic changes in Zn-exposed poplar leaves. The 40% of the genes involved were over-expressed, while the 60% under-expressed. Among these, the more representative functions were: active transmenbrane transporter activity, transition metal ion binding, oxidoreductase activity and carbohydrate metabolic processes. Isolation and functional characterization of these genes are actually in progress.