Different effects of calcium and lanthanum on the expression of phytochelatin synthase gene and cadmium absorption in Lactuca sativa

Phytochelatins (PCs) have an important role in metal detoxification in plants, however the relationship of the expression of Lactuca sativa phytochelatin synthase gene, its catalyzing products-PCs and the absorption of cadmium (Cd) was still not clear. To investigate this relationship, a full-length cDNA of a phytochelatin synthase gene (LsPCS1) was cloned from L. sativa and the physiological changes were observed. When L. sativa (cabbage lettuce) plants were exposed to toxic cadmium, growth was retarded. Though L. sativa plants cadmium tolerance was enhanced when synchronously treated with lanthanum (La3+) or calcium (Ca2+), the effect on the expression of phytochelatin synthase gene and cadmium absorption was different. The transcript level of LsPCS1 and the PCs content in the plants grown under normal conditions were very low, and the Cd absorption was minimal. In comparison with these plants, the expression level of LsPCS1 and the absorption of Cd were enhanced both in leaf and root under Cd2+, Cd2+ + Ca2+ and Cd2+ + La3+ treatments. In comparison with Cd2+ treatment, the accumulated mRNA level of LsPCS1, PCs content and the absorption of Cd were enhanced with Cd2+ + Ca2+ treatment. Though Cd2+ + La3+ treatment showed a similar expression profile with Cd2+ + Ca2+ treatment, the absorption of Cd of Cd2+ + La3+ treatment was less than that of Cd2+ treatment. Ca2+ alone has little effect on the expression of LsPCS1, whereas La3+ itself enhances the mRNA and PCs accumulation. It is suggested that Ca2+ increases the expression of PC synthase gene under Cd2+ stress, which in turn enhances the plant tolerance and Cd accumulation. Thus, fertilization with Ca2+ appears to be a promising strategy for increasing phytoremediation capacity. While La3+ could decrease the accumulation of Cd, La3+ may play an important role on reducing the accumulation of Cd of plants grown in contaminated soil.