Mechanisms of cadmium toxicity and tolerance in Populus

Abstract

Cadmium (Cd), ranked the 7th most hazardous substance, is one of the most widespread pollutants of soil and water in industrialized nations. Its increased movement in soil-plant systems is posing a serious threat to human health. Cd, without any known functions in plants, was found to be toxic even at minute concentrations, leading to the development of symptoms such as leaf roll, chlorosis and root and shoot growth reduction. Phytoremediation is an emerging cost-effective and environment friendly technology that utilizes high biomass producing plants including Populus plants to remove, transform or stabilize contaminants in soils. The objectives of our study were to record phenotypic variation in a Populus pedigree to Cd exposure, to identify Cd tolerant and susceptible genotypes of Populus, to map QTLs (Quantitative Trait Loci - genomic regions responsible) for Cd tolerance and accumulation in Populus and to predict a mechanism for Cd toxicity and tolerance in susceptible and tolerant Populus genotypes.QTL mapping was accomplished by conducting a greenhouse hydroponic study in which 252 genotypes of a Populus pseudo-backcross progeny were grown for 40 days and treated with 25 µM Cd. Phenotypic variation in total dry weight was recorded on these genotypes and was used for identifying QTL for Cd tolerance. We identified genotypes with contrasting responses to the Cd treatment and conducted a microarray study to identify potential Cd tolerance mechanisms based on gene expression patterns. Significant variation was observed among genotypes in response to Cd treatment based on changes in total dry weight. Cd tolerant and susceptible genotypes were identified based on the least square mean differences (Control-Cd treated for each genotype) among all the genotypes. Three QTLs were identified for Cd tolerance and they accounted for approximately 25% of the phenotypic variation in Cd tolerance measured as total dry weights.In the microarray study, the Cd-susceptible genotype had higher expression of Fe-transporters compared to the tolerant genotypes, and Cd and Fe levels were significantly different in foliage. Even the susceptible genotype controls had higher, though not significant, Fe levels than tolerant genotype controls. We therefore hypothesize that part of the mechanism for Cd tolerance is determined by the differences in the activity of Fe transporters in genotypes with differential Fe homeostasis.