Silicon induces phytochelatin and ROS scavengers facilitating cadmium detoxification in rice

Cadmium (Cd) is detrimental to crops and the environment. This work examines the natural mechanisms underlying silicon- (Si-)directed Cd detoxification in rice plants. The addition of Si to plants under Cd stress caused significant improvements in morphological parameters, chlorophyll score, F_v /F_m and total soluble protein concentration compared to controls, confirming that Si is able to ameliorate Cd-induced damage in rice plants. This morpho-physiological evidence was correlated with decreased cell death and electrolyte leakage after Si application. The results showed no critical changes in root Cd concentration, while shoot Cd decreased significantly after Si supplementation in comparison with Cd-stressed rice. Additionally, expression of Cd transporters (OsNRAMP5 and OsHMA2) was significantly down-regulated while the concentration of phytochelatin, cysteine and glutathione, together with expression of OsPCS1 (phytochelatin synthase) in roots of Cd-stressed rice was significantly induced when subjected to Si treatment. This confirms that the alleviation of Cd stress is not only limited to the down-regulation of Cd transporters but also closely related to the phytochelatin-driven vacuolar storage of Cd in rice roots. The enzymatic analysis further revealed the role of SOD and GR enzymes in protecting rice plants from Cd-induced oxidative harm. These findings suggest a mechanistic basis in rice plants for Si-mediated mitigation of Cd stress.

Zinc deficiency tolerance in maize is associated with the up-regulation of Zn transporter genes and antioxidant activities

Zinc (Zn) is an essential micronutrient for the growth and development of plants. However, Zn deficiency is a common abiotic stress causing yield loss in crop plants. This study elucidates the mechanisms of Zn deficiency tolerance in maize through physiological and molecular techniques. Maize lines tolerant (PAC) and sensitive (DAC) to Zn deficiency were examined physiologically and by atomic absorption spectrometry (AAS). Proteins, H₂ O₂ , SOD, POD, membrane permeability and gene expression (using real-time PCR) of roots and shoots of both maize lines were assessed. Zn deficiency had no significant effect on root parameters compared with control plants in PAC and DAC but showed a substantial reduction in shoot parameters in DAC. AAS showed a significant decrease in Zn concentrations in both roots and shoots of DAC but not PAC under Zn deficiency, implying that Zn deficiency tolerance mechanisms exist in PAC. Consistently, total protein and membrane permeability were significantly reduced in DAC but not PAC in both roots and shoots under Zn deficiency in comparison with Zn-sufficient plants. Real-time PCR showed that expression of ZmZIP1, ZmZIP4 and ZmIRT1 transporter genes significantly increased in roots of PAC, but not in DAC due to Zn deficiency compared with controls. The H₂ O₂ concentration dramatically increased in roots of DAC but not PAC. Moreover, tolerant PAC showed a significant increase in POD and SOD activity due to Zn deficiency, suggesting that POD- and SOD-mediated antioxidant defence might provide tolerance, at least in part, under Zn deficiency in PAC. This study provides an essential background for improving Zn biofortification of maize.

Biochemical and molecular changes in rice seedlings (Oryza sativa L.) to cope with chromium stress

Chromium (Cr) is very toxic to both humans and plants. This investigation aimed to understand the physiological and molecular responses of rice seedlings to Cr stress. Cr toxicity did not significantly affect morphological features and Cr accumulation in roots and shoots in Pokkali but not in BRRI 51, although there was a reduction in chlorophyll concentration in leaves of both genotypes. These results imply that Pokkali has mechanisms to cope with Cr supplementation. We therefore performed quantitative real-time PCR on the expression pattern of two chelator genes, OsPCS1 and OsMT1, but there were no significant changes in expression in roots and shoots of Pokkali and BRRI 51 following Cr stress. This suggests that there was no metal sequestration following heavy metal stress in roots of these genotypes. Moreover, no expression of two heavy metal transporter genes, OsHMA3 and OsNRAMP1, was induced after Cr stress in roots and shoots, suggesting that these transporter genes are not induced by Cr stress or might not be involved in Cr uptake in rice. We also performed a targeted study on the effect of Cr on Fe uptake mechanisms. Our studies showed a consistent reduction in Fe uptake, Fe reductase activity and expression of Fe-related genes (OsFRO1 and OsIRT1) under Cr stress in both roots and leaves of Pokkali. In contrast, these parameters and genes were significantly increased in Cr-sensitive BRRI 51 under Cr stress. The results confirm that limiting Fe uptake through the down-regulation of Fe reductase and Fe transporter genes is the main strategy of Cr-tolerant Pokkali to cope with Cr stress. Finally, increased CAT, POD and GR activity and elevated glutathione and proline synthesis might provide strong antioxidant defence against Cr stress in Pokkali. Taken together, our findings reveal that Cr stress tolerance in rice (Pokkali) is not related to metal sequestration but is associated with reduced Fe transport and increased antioxidant defence.