Comparative analysis of expansin gene codon usage patterns among eight plant species.

Expansins are essential components of plant cell wall and play an important role in plant growth, development, and stress resistance via loosening function. To understand the codon usage pattern of expansin genes, we gained the sequence data of expansin genes from eight plant species. Statistics analysis showed obvious codon characteristics between monocot and dicot plants. Comparably, expansin genes in monocot plants had really higher GC content, more high-frequency codons, and more optimal codons than that in dicot plants. Several monocot plants performed somehow as dicot plants in a few characters. Codon information of expansin genes might contribute to the understanding of the relationship and evolution clues between monocot and dicot plants. It further gained insight into the improvement of the gene expression and roles.

Correction: Transcriptome analysis of Phytolacca americana L. in response to cadmium stress.

[This corrects the article DOI: 10.1371/journal.pone.0184681.].

Diversity in cadmium accumulation and resistance associated with various metallothionein genes (type III) in Phytolacca americana L.

Metallothioneins (MTs) are known for their heavy metal deoxidation during phytoremediation. To estimate their roles in the cadmium (Cd) hyperaccumulator Phytolacca americana L., three MT genes, PaMT3-1, PaMT3-2 and PaMT3-3, belonging to the MT3 subfamily were cloned. They separately encoded 63, 65 and 65 amino acids, containing12, 10 and 11 cysteines (Cys), respectively. Each gene was individually transformed and expressed in Escherichia coli cells. A Cd-resistance assay showed that the recombinant strains had enhanced survival rates, especially those containing PaMT3-1 and PaMT3-3. Additionally, the recombinant strains were high Cd accumulators, with the recombinant PaMT3-1's maximum accumulation being 2.16 times that of the empty vector strains. The numbers of cysteines and the structures of MT proteins were associated with the Cd enrichment and resistance capabilities. PaMT3-1 could be an effective gene resource in future plant Cd remediation-related breeding programs.

Over-expression of the poplar expansin gene PtoEXPA12 in tobacco plants enhanced cadmium accumulation.

As important cell wall proteins in plants, expansins are involved in a serious of abiotic stress resistance such as drought, heat, salt, even heavy metals. To understand the role of expansins in cadmium (Cd) stress, we analyzed the expression patterns of 36 expansin genes in Populus tomentosa. A Cd-induced expansin gene, PtoEXPA12, was identified, cloned, and transformed into tobacco plants. After treatment with Cd, the transgenic plants showed stronger symptoms of Cd toxicity as to the wild-type tobacco plants. Further physiological tests showed that the transformants had higher relative electrolyte leakage and superoxide dismutase activity, more malondialdehyde and H2O2 content, and lower chlorophyll content in Cd stress. Cd content measurement showed it is 1.40-2.07-fold higher and 1.29-1.38-fold higher separately in roots and shoots of transgenic plants than those in wild-type plants, while the transfer coefficient value kept invariably even decreased. Therefore, PtoEXPA12 was really involved in Cd uptake and accumulation, and led to Cd toxicity of cells. It would be a potentially applicable part in phytoremediation system.

Transgenic tobacco plants expressing a P1B-ATPase gene from Populus tomentosa Carr. (PtoHMA5) demonstrate improved cadmium transport.

Heavy metal ATPase (HMA) plays an important role in phytoremediation via long-distance transportation from root to shoot. In this report, we identified a heavy metal ATPase gene, PtoHMA5, from Populus tomentosa Carr. Its encoded peptide consists of 967 amino acids and has eight trans-membrane motifs inside. Tobacco plants were transformed with this gene via Agrobacterium tumefaciens-mediated method. After exposure to 50mg/LCdCl2 for 10d, the transgenic lines displayed higher cadmium accumulation in leaves than did the wild-type plants with an absolute increase of 25.04%, while the transfer coefficient increased by 16.01%-43.25%. Physiological testing including assessment of relative electrolytic leakage (REL), malondialdehyde (MDA) content, and chlorophyll content revealed that the transgenic lines were seriously affected when compared with the wild-type plants. In summary, PtoHMA5 is really involved in cadmium transport from root to shoot but is not associated with the removal of cadmium toxicity.

Transcriptome analysis of Phytolacca americana L. in response to cadmium stress.

Phytolacca americana L. (pokeweed) has metal phytoremediation potential, but little is known about its metal accumulation-related genes. In this study, the de novo sequencing of total RNA produced 53.15 million reads covering 10.63 gigabases of transcriptome raw data in cadmium (Cd)-treated and untreated pokeweed. Of the 97,502 assembled unigenes, 42,197 had significant matches in a public database and were annotated accordingly. An expression level comparison between the samples revealed 1515 differentially expressed genes (DEGs), 923 down- and 592 up-regulated under Cd treatment. A KEGG pathway enrichment analysis of DEGs revealed that they were involved in 72 metabolism pathways, with photosynthesis, phenylalanine metabolism, ribosome, phenylpropanoid biosynthesis, flavonoid biosynthesis and carbon fixation in photosynthetic organisms containing 24, 18, 72, 14, 7 and 15 genes, respectively. Genes related to heavy metal tolerance, absorption, transport and accumulation were also identified, including 11 expansins, 8 nicotianamine synthases, 6 aquaporins, 4 ZRT/IRT-like proteins, 3 ABC transporters and 3 metallothioneins. The gene expression results of 12 randomly selected DEGs were validated using quantitative real-time PCR, and showed different response patterns to Cd in their roots, stems and leaves. These results may be helpful in increasing our understanding of heavy metal hyperaccumulators and in future phytoremediation applications.