Arsenic hyperaccumulation in ferns: Understanding an adaptation through functional genomics

Background/Question/Methods

Arsenic is a naturally-occurring metalloid with a high degree of toxicity and carcinogenicity.  While most plants are susceptible to arsenic, Chinese brake fern (Pteris vittata L.) is resistant to it.  The brake fern efficiently takes up arsenic from the soil and hyperaccumulates it in its frond, thus of great use in arsenic phytoremediation.  Because of its arsenic hyperaccumulation ability, the brake fern represents a unique ecosystem.  We asked (a) whether arsenic hyperaccumulation in brake fern has a biological advantage for the fern, (b) does the brake fern provide an ecosystem to arsenic-resistant microflora?  and (c) whether the genetic basis of arsenic tolerance in the fern be identified?  

Results/Conclusions

Our studies using nymphs of the grasshopper Schistocerca americana (Drury) showed that arsenic hyperaccumulation in brake fern could be an adaptation to deter herbivores.  Microbiological investigations indicated that the brake fern harbors arsenic-resistant microflora with unusual properties including proteobacteria with a capacity to reduce arsenate to arsenite.  To understand the mechanisms of brake fern’s arsenic resistance, we developed an efficient high throughput method to identify brake fern cDNAs based on their abilities to increase arsenic resistance in bacteria.  Brake fern glutaredoxin PvGrx5 cDNA encoded a functional glutaredoxin, a glutathione-dependent oxidoreductase.  Using a combination of genetic and biochemical tools, we implicated PvGrx5 to have a role in resistance to arsenic and oxidative stress. Analyses of protein sequences of PvGrx5 and related glutaredoxins identified specific amino acid residues likely important for the glutaredoxin’s role in arsenic resistance.