Identification of up-regulated genes from the metal-hyperaccumulator aquatic fern Salvinia minima Baker, in response to lead exposure

Lead (Pb) is one of the most serious environmental pollutants. The aquatic fern Salvinia minima Baker is capable to hyper-accumulate Pb in their tissues. However, the molecular mechanisms involved in its Pb accumulation and tolerance capacity are not fully understood. In order to investigate the molecular mechanisms that are activated by S. minima in response to Pb, we constructed a suppression subtractive hybridization library (SSH) in response to an exposure to 40μM of Pb(NO3)2 for 12h. 365 lead-related differentially expressed sequences tags (ESTs) were isolated and sequenced. Among these ESTs, 143 unique cDNA (97 were registered at the GenBank and 46 ESTs were not registered, because they did not meet the GenBank conditions). Those ESTs were identified and classified into 3 groups according to Blast2GO. In terms of metabolic pathways, they were grouped into 29 KEGG pathways. Among the ESTs, we identified some that might be part of the mechanism that this fern may have to deal with this metal, including abiotic-stress-related transcription factors, some that might be involved in tolerance mechanisms such as ROS scavenging, membrane protection, and those of cell homeostasis recovery.To validate the SSH library, 4 genes were randomly selected from the library and analyzed by qRT-PCR. These 4 genes were transcriptionally up-regulated in response to lead in at least one of the two tested tissues (roots and leaves). The present library is one of the few genomics approaches to study the response to metal stress in an aquatic fern, representing novel molecular information and tools to understand the molecular physiology of its Pb tolerance and hyperaccumulation capacity. Further research is required to elucidate the functions of the lead-induced genes that remain classified as unknown, to perhaps reveal novel molecular mechanisms of Pb tolerance and accumulation capacity in aquatic plants.