Responses of Carbonic Anhydrase to Cadmium in the Zinc/Cadmium Hyperaccumulator Picris divaricata Vant.

A number of higher plants are able to hyperaccumulate cadmium (Cd). However, it is unknown whether cadmium (Cd) plays a biological functional role in the carbonic anhydrase (CA) of hyperaccumulators. A hydroponic experiment was conducted to explore the potentially physiological function of Cd in CA and the accumulation and tolerance of Cd in the Zn/Cd hyperaccumulator Picris divaricata Vant. P. divaricata was exposed to nutrient solutions with six Cd concentrations (0, 5, 10, 25, 50 and 75 μmol L−1). After 12 d, plants were harvested for the analysis of plant biomass, Cd concentration and CA activity. The Cd concentrations in plant increased with the increasing Cd in nutrient solution, reaching 640 and 3 100 mg kg−1 in shoot and root, respectively, at the 75 μmol L−1 Cd treatment. Meanwhile, plant growth was enhanced by the Cd treatments at 5-25 L−1, but it was significantly inhibited when the plants were exposed to solutions with higher Cd concerntrations (50 and 75 μmol L−1). Exposure to Cd significantly increased the CA activity in P. divaricata, which reached a maximum value of 21.27 U mg−1 proteins at the 25 μmol L−1 Cd treatment, and the CA activity and shoot Cd concentration were positively correlated at solutions Cd of ≤ 25 μmol L−1. Moreover, two protein bands appeared on the denatured gel electrophoresis of purified CA, indicating that P. divaricata may have CA isomers with their respective molecular weights at around 60 and 55 kDa, at least one of which is Cd-bound. In addition, trace amounts of Cd in purified CA significantly increased with the supplied Cd concentration in nutrient solution (5-25 μmol L−1). The results suggested that Cd may play a biological role by enhancing the activities and forming the active Cd-specific CA in the hyperaccumulator P. divaricata.