Research of determinants controlling cadmium accumulation in lettuce "Lactuca sativa".

Cadmium is an extremely toxic pollutant causing a great number of diseases. It is a widespread metal which concentration rises in cultivated soils, thus exposing food or feed crops to it and ultimately causing potential major sanitary problems as a consequence of its entry in the food chain. Among the vegetable species used for food, lettuce shows a higher capacity of accumulating cadmium.Our objective was to identify molecular determinants controlling cadmium accumulation in lettuce.We aim at characterizing mechanisms controlling cadmium accumulation in lettuce, which is a food crop showing one of the highest capacities to accumulate this toxic compound.Starting from a phenotypic and transcriptomic analysis of 18 lettuce genotypes representing the genetic diversity of the species, we identified trends linking cadmium and zinc accumulation in roots an shoots, cadmium translocation from roots to shoot, cadmium tolerance, and expression of a dozen of candidate genes proposed to be involved in cadmium transport. A positive correlation linked cadmium accumulation and the mRNA accumulation of an heavy metal efflux transporter. Interestingly, measurements of 109Cd influx in roots and 109Cd efflux from roots revealed that the genotype displaying the least cadmium accumulation could be discriminated from the genotype displaying the highest one by its markedly increased ability to efflux cadmium from the roots to the culture medium. Progeny analysis from crosses between the genotypes displaying extreme performances for cadmium accumulation or cadmium translocation from roots to shoots revealed that these traits could be supported by single genetic determinism.In conclusion, starting from the analysis of cadmium accumulation in a large set of lettuce genotypes, a major determinant limiting cadmium accumulation in this species was revealed, that is the ability to efflux cadmium from the root to the culture medium. A candidate gene has been identified for this function, which paves the way towards the breeding for lettuce with reduced cadmium accumulation in leaves.As a non-essential element for plants, cadmium has been assumed to be taken up by making use of non-specificity of essential elements transporters. Numerous physiological studies have described that cadmium uptake is negatively associated with iron pathway, by competing for absorption sites in roots. The principal candidate for this transport in lettuce is a possible “orthologue” of Arabidopsis thaliana IRT1. In fact, AtIRT1 is a major transporter responsible for iron uptake from the soil, and is known to have a broad substrate range, including cadmium. IRT1-overexpressing A. thaliana plants were then shown to accumulate more cadmium in root tissue. In this work, NCBI sequence libraries were analysed and two lettuce “orthologues” of AtIRT1 were found (LsIRTa and LsIRTb). Cloning “orthologues” of AtIRT1 was carried out for five lettuce varieties. Moreover, LsIRTs expression in plants from 14 lettuce accessions were analysed by PCRQ and were compared to cadmium accumulation in plants. Interestingly, our results revealed that expression of LsIRTa is significantly correlated with root cadmium content. LsIRTa seems to be involved in the control of cadmium accumulation in roots.This result has been supported by tow physiological experiments. The first experiment showed that increasing iron in the growth medium resulted in a strong decrease in root cadmium content. And conversely, the second experiment revealed that root iron content decrease in response to increasing concentrations of cadmium in the medium. These results suggest that cadmium competes with iron for uptake through ion transporters in lettuce root cells. As mentioned above, a candidate gene for this transport has been identified (LsIRTa), which paves the way towards the breeding for lettuce with reduced cadmium accumulation.