Response of sugarcane to increasing concentrations of copper and cadmium and expression of metallothionein genes.

Sugarcane (Saccharum spp.) offers the potential to be a phytoremediator species due to its outstanding biomass production, but its prospective metal accumulation and tolerance have not been fully characterized. Sugarcane plantlets were able to tolerate up to 100microM of copper in nutrient solution for 33 days, with no significant reduction in fresh weight, while accumulating 45mgCukg(-1) shoot dry weight. Higher levels of copper in solution (250 and 500microM) were lethal. Sugarcane displayed tolerance to 500microM Cd without symptoms of toxicity, accumulating 451mgCdkg(-1) shoot dry weight after 33 days, indicating its potential as Cd phytoremediator. DNA gel blot analyses detected 8 fragments using a metallothionein (MT) Type I probe, while 10 were revealed for the MT Type II and 8 for MT Type III. The number of genes for each type of MT in sugarcane might be similar to the ones identified in rice considering the interspecific origin of sugarcane cultivars. MT Type I gene appeared to present the highest level of constitutive expression, mainly in roots, followed by MT Type II, corroborating the expression pattern described based on large-scale expressed sequence tags sequencing. MT Type II and III genes were more expressed in shoots, where MT I was also importantly expressed. Increasing Cu concentration had little or no effect in modulating MT genes expression, while an apparent minor modulation of some of the MT genes could be detected in Cd treatments. However, the level of response was too small to explain the tolerance and/or accumulation of Cd in sugarcane tissues. Thus, cadmium tolerance and accumulation in sugarcane might derive from other mechanisms, although MT may be involved in oxidative responses to high levels of Cd. Sugarcane can be considered a potential candidate to be tested in Cd phytoremediation.