Overexpression of phytochelatin synthase (AtPCS) in rice for tolerance to cadmium stress

Research Advances in Cadmium Uptake, Transport and Resistance in Rice (Oryza sativa L.)

Rice (Oryza sativa L.) is one of the most important food crops, feeding half of the world’s population. However, rice production is affected by cadmium (Cd) toxicity. Due to an increase in Cd-contaminated soil and rice grains, and the serious harm to human health from Cd, research on Cd uptake, transport and resistance in rice has been widely conducted, and many important advances have been made. Rice plants absorb Cd mainly from soil through roots, which is mediated by Cd absorption-related transporters, including OsNramp5, OsNramp1, OsCd1, OsZIP3, OsHIR1, OsIRT1 and OsIRT2. Cd uptake is affected by soil’s environmental factors, such as the concentrations of Cd and some other ions in soil, soil properties, and other factors can affect the bioavailability of Cd in soil. Then, Cd is transported within rice plants mediated by OsZIP6, OsZIP7, OsLCD, OsHMA2, CAL1, OsCCX2, OsLCT1 and OsMTP1, from roots to shoots and from shoots to grains. To resist Cd toxicity, rice has evolved many resistance strategies, including the deposition of Cd in cell walls, vacuolar Cd sequestration, Cd chelation, antioxidation and Cd efflux. In addition, some unresolved scientific questions surrounding Cd uptake, transport and resistance in rice are proposed for further study.

Responses of glutathione and phytochelatins biosysthesis in a cadmium accumulator of Perilla frutescens (L.) Britt. under cadmium contaminated conditions

Screening new accumulators of heavy metal and identifying their tolerance, enrichment capacity of heavy metals are currently hot issues in phytoremediation research. A series of hydroponic experiments were conducted to analyze the effects of glutathione and phytochelatins in roots, stems, and leaves of Perilla frutescens under cadmium stress. The results showed that the non-protein thiols in roots and stems mainly existed in the form of GSH, PC₂, PC₃, and PC₄ under Cd stress condition, while in leaves they existed in the form of GSH, PC₂, and PC₃. Furthermore, the contents of GSH and PCs positively correlated with Cd, but negatively correlated with root vigor and chlorophyll content under Cd stress conditions. After 21 days of treatments, the contents of Cd in different parts of the plant were 1465.2-3092.9 mg· kg^-1 in the roots, 199.6-478.4 mg·kg^-1 in the stems and 61.3-96.9 mg· kg^-1 in the leaves at 2, 5, 10 mg·L^-1 Cd levels respectively, and the amount of Cd uptakes were up to 3547.7-5701.7 μg·plant^-1. Therefore, P. frutescens performed high capacity in Cd accumulation, and PCs played a key role in Cd tolerance. The application prospect of the plant in phytoremediation Cd polluted soil was also discussed.

Polyamines may influence phytochelatin synthesis during Cd stress in rice

Although the metabolism of phytochelatins and higher polyamines are linked with each other, the direct relationship between them under heavy metal stress has not yet been clarified. Two approaches were used to reveal the influence of polyamine content on cadmium stress responses, particularly with regard to phytochelatin synthesis: putrescine pre-treatment of rice plants followed by cadmium stress, and treatment with the putrescine synthesis inhibitor, 2-(difluoromethyl)ornithine combined with cadmium treatment. The results indicated that putrescine pre-treatment enhanced the adverse effect of cadmium, while the application of 2-(difluoromethyl)ornithine reduced it to a certain extent. These differences were associated with increased polyamine content, more intensive polyamine metabolism, but decreased thiol and phytochelatin contents. The gene expression level and enzyme activity of phytochelatin synthase also decreased in rice treated with putrescine prior to cadmium stress, compared to cadmium treatment alone. In contrast, the inhibition of putrescine synthesis during cadmium treatment resulted in higher gene expression level of phytochelatin synthase. The results suggest that polyamines may have a substantial influence on phytochelatin synthesis at several levels under cadmium stress in rice.

An eukaryotic translation initiation factor, AteIF5A-2, affects cadmium accumulation and sensitivity in Arabidopsis

Cadmium (Cd) is one of the most toxic elements and can be accumulated in plants easily; meanwhile, eIF5A is a highly conserved protein in all eukaryotic organisms. The present work tried to investigate whether eIF5A is involved in Cd accumulation and sensitivity in Arabidopsis (Arabidopsis thaliana L.) by comparing the wild-type Columbia-0 (Col-0) with a knockdown mutant of AteIF5A-2, fbr12-3 under Cd stress conditions. The results showed that the mutant fbr12-3 accumulated more Cd in roots and shoots and had significantly lower chlorophyll content, shorter root length, and smaller biomass, suggesting that downregulation of AteIF5A-2 makes the mutant more Cd sensitive. Real-time polymerase chain reaction revealed that the expressions of metal transporters involved in Cd uptake and translocation including IRT1, ZIP1, AtNramp3, and AtHMA4 were significantly increased but the expressions of PCS1 and PCS2 related to Cd detoxification were decreased notably in fbr12-3 compared with Col-0. As a result, an increase in MDA and H2 O2 content but decrease in root trolox, glutathione and proline content under Cd stress was observed, indicating that a severer oxidative stress occurs in the mutant. All these results demonstrated for the first time that AteIF5A influences Cd sensitivity by affecting Cd uptake, accumulation, and detoxification in Arabidopsis.