Impact of cadmium on young plants of Populus euphratica and P. x canescens, two poplar species that differ in stress tolerance

Populus euphratica is a salt tolerant species, and Populus x canescens is a salt sensitive species. Because stress tolerance is mediated by an array of overlapping defense mechanisms, we hypothesized that P. euphratica would confer co-tolerance to heavy metal stress. To test this hypothesis, both poplar species were exposed to 50 mu M Cd for 24 h in hydroponic solutions. For the analyses of stress symptoms in roots and leaves, we used nutrient element concentrations and transcriptional responses of 28 stress- and defense-related genes. The roots showed strong Cd accumulation, but less in P. euphratica than in P. x canescens. In contrast, the leaves of P. euphratica accumulated 10-times more Cd than those of P. x canescens. Cd-stressed leaves of P. euphratica displayed water loss and wilting. Both species revealed contrasting transcriptional responses of defense genes to Cd stress. In P. euphratica, glutathione reductase, dehydroascorbate reductase and glutathione S-transferases of the theta and tau classes showed increases in transcript levels in response to Cd, pointing to oxidative stress and a strong activation of detoxification mechanisms. In contrast, in P. x canescens transcript levels of genes involved in signaling, activation of, and protection from drought stress were significantly increased including glutathione S-transferases of the phi class (early response to dehydration). Our data show that P. euphratica salt tolerance does not confer Cd tolerance and suggest that failure to activate early protection contributed to higher Cd sensitivity in P. euphratica than in P. x canescens.