The Pb-hyperaccumulator aquatic fern Salvinia minima Baker, responds to Pb2+ by increasing phytochelatins via changes in SmPCS expression and in phytochelatin synthase activity

The relationship between accumulation of Pb2+ and the activation of chelation and metal sequestration mechanisms mediated by phytochelatins (PC) was analyzed in the Pb2+ hyperaccumulator aquatic fern Salvinia minima, after exposure to 40 μM Pb(NO3)2. The tissue accumulation pattern of lead and the phytochelatin biosynthesis responses were analyzed in both, S. minima submerged root-like modified fronds (here named “roots”), and in its aerial leaf-like fronds (“leaves”). S. minima roots accumulated a significantly higher concentrations of Pb+2 than leaves did. Accumulation of Pb2+ in roots was bi-phasic with a first uptake phase reached after 3 h exposure and a second higher uptake phase reached after 24 h exposure. In leaves, a single delayed, smaller uptake phase was attained only after 9 h of exposure. In roots lead accumulation correlated with an increased phytochelatin synthase (PCS) activity and an enhanced PC production. A higher proportion of polymerized PC4 was observed in both tissues of exposed S. minima plants relative to unexposed ones, although a higher concentration of PC4 was found in roots than in leaves. PCS activity and Pb2+ accumulation was also higher in roots than in leaves. The expression levels of the S. minima PCS gene (SmPCS), in response to Pb2+ treatment, were also evaluated. In S. minima leaves, the accumulation of Pb2+ correlated with a marked increase in expression of SmPCS, suggesting a transcriptional regulation in the PCS activation and PC accumulation in this S. minima tissue. However, in roots, the basal expression of SmPCS was down-regulated after Pb2+ treatment. This fact did not correlate with the later but strong increase in both, PCS activity and PC production; suggesting that the PC biosynthesis activation in S. minima roots occurs only by post-translational activation of PCS. Taken together, our data suggest that the accumulation of PC in S. minima is a direct response to Pb2+ accumulation, and phytochelatins do participate as one of the mechanism to cope with Pb2+ of this Pb-hyperaccumulator aquatic fern.