The pathway of transmembrane cadmium influx via calcium-permeable channels and its spatial characteristics along rice root.

To develop elite crops with low cadmium (Cd), a fundamental understanding of the mechanism of Cd uptake by crop roots is necessary. Here, a new mechanism for Cd2+ entry into rice root cells was investigated. The results showed that Cd2+ influx in rice roots exhibited spatially and temporally dynamic patterns. There was a clear longitudinal variation in Cd uptake along rice roots, with the root tip showing much higher Cd2+ influx and concentration than the root mature zone, which might be due to the much higher expression of the well-known Cd transporter genes OsIRT1, OsNRAMP1, OsNRAMP5, and OsZIP1 in the root tip. Both the net Cd2+ influx and the uptake of Cd in rice roots were highly inhibited by ion channel blockers Gd3+ and TEA+, supplementation of Ca2+ and K+, and the plasma membrane H+-ATPase inhibitor vanadate, with Gd3+ and Ca2+ showing the most inhibitory effects. Furthermore, Ca2+- or Gd3+-induced reduction in Cd2+ influx and Cd uptake did not coincide with the expression of Cd transporter genes, but with that of two Ca channel genes, OsAAN4 and OsGLR3.4. These results indicate that Cd transporters are in part responsible for Cd2+ entry into rice root, and provide a new perspective that the Ca channels OsAAN4 and OsGLR3.4 might play an important role in rice root Cd uptake.

Physiological and proteomic alterations in rice (Oryza sativa L.) seedlings under hexavalent chromium stress.

MAIN CONCLUSION: Rice plants employ two strategies to cope with Cr toxicity: immobilizing Cr ions into cell walls to reduce its translocation and activating antioxidant defense to mitigate Cr-induced oxidative stress. The investigation aimed at understanding the physiological and proteomic responses of rice seedlings to hexavalent chromium (Cr(6+)) stress was conducted using two rice genotypes, which differ in Cr tolerance and accumulation. Cr toxicity (200 µM) heavily increased the accumulation of H2O2 and [Formula: see text], enhanced lipid peroxidation, decreased cell viability and consequently inhibited rice plant growth. Proteomic analyses suggest that the response of rice proteome to Cr stress is genotype- and Cr dosage-dependent and tissue specific. Sixty-four proteins, which show more than fourfold difference under either two Cr levels, have been successfully identified. They are involved in a range of cellular processes, including cell wall synthesis, energy production, primary metabolism, electron transport and detoxification. Two proteins related to cell wall structure, NAD-dependent epimerase/dehydratase and reversibly glycosylated polypeptide were greatly up-regulated by Cr stress. Their enhancements coupled with callose accumulation by Cr suggest that cell wall is an important barrier for rice plants to resist Cr stress. Some enzymes involved in antioxidant defense, such as ferredoxin-NADP reductase, NADP-isocitrate dehydrogenase, glyoxalase I (Gly I) and glutamine synthetase 1 (GS1) have also been identified in response to Cr stress. However, they were only detected in Cr-tolerant genotype, indicating the genotypic difference in the capacity of activating the defense system to fight against Cr-induced oxidative stress. Overall, two strategies in coping with Cr stress in rice plants can be hypothesized: (i) immobilizing Cr ions into cell walls to reduce its translocation and (ii) activating antioxidant defense to mitigate Cr-induced oxidative stress.

Alleviation of chromium toxicity in rice seedlings by applying exogenous glutathione.

The effect of exogenous reduced glutathione (GSH) on alleviation of hexavalent chromium (Cr(6+)) toxicity to rice seedlings and its physiological mechanisms were comprehensively investigated in a series of experiments. Our results showed that growth and nutrient uptake of rice seedlings were dramatically reduced under 100 µM Cr(6+) stress, and the reduction was significantly alleviated by exogenous GSH. Cr(6+) stress also reduced cell viability in root tips and damaged ultrastructure of both chloroplasts and root cells, while the addition of GSH alleviates those negative effects. Cr-induced toxicity and GSH-caused Cr alleviation differed significantly between Cr-tolerant Line 117 (L117) and Cr-sensitive Line 41 (L41). Under Cr(6+) stress, cystine content was increased and GSH content was decreased in rice plants, exogenous GSH, however, mitigated the Cr-toxicity by reversing the Cr-induced changes of the two compounds. The types of Cr-induced secretion of organic acids varied between the genotypes, where reduction in the contents of acetic and lactic acids and tartaric and malic acids were observed in L117 and L41, respectively. The addition of GSH alleviated the reduction of secretion of these organic acids. Exogenous GSH also altered the forms of Cr ions in the rhizosphere and the fraction of distribution at subcellular level in both shoots and roots. It may be concluded that the alleviation of Cr(6+) toxicity by exogenous GSH is directly attributed to its regulation on forms of Cr ions in rhizosphere and their distribution at subcellular levels.

Glutathione-mediated alleviation of chromium toxicity in rice plants.

A hydroponic experiment was conducted to determine the possible effect of exogenous glutathione (GSH) in alleviating chromium (Cr) stress through examining plant growth, chlorophyll contents, antioxidant enzyme activity, and lipid peroxidation in rice seedlings exposed to Cr toxicity. The results showed that plant growth and chlorophyll content were dramatically reduced when rice plants were exposed to 100 µM Cr. Addition of GSH in the culture solution obviously alleviated the reduction of plant growth and chlorophyll content. The activities of some antioxidant enzymes, including superoxide dismutase, catalase (CAT) and glutathione reductase in leaves, and CAT and glutathione peroxidase in roots showed obvious increase under Cr stress. Addition of GSH reduced malondialdehyde accumulation and increased the activities of these antioxidant enzymes in both leaves and roots, suggesting that GSH may enhance antioxidant capacity in Cr-stressed plants. Furthermore, exogenous GSH caused significant decrease of Cr uptake and root-to-shoot transport in the Cr-stressed rice plants. It can be assumed that GSH is involved in Cr compartmentalization in root cells.

The influence of pH and organic matter content in paddy soil on heavy metal availability and their uptake by rice plants.

The experiments were done to investigate the effect of soil pH and organic matter content on EDTA-extractable heavy metal contents in soils and heavy metal concentrations in rice straw and grains. EDTA-extractable Cr contents in soils and concentrations in rice tissues were negatively correlated with soil pH, but positively correlated with organic matter content. The combination of soil pH and organic matter content would produce the more precise regression models for estimation of EDTA-Cu, Pb and Zn contents in soils, demonstrating the distinct effect of the two factors on the availability of these heavy metals in soils. Soil pH greatly affected heavy metal concentrations in rice plants. Furthermore, inclusion of other soil properties in the stepwise regression analysis improved the regression models for predicting straw Fe and grain Zn concentrations, indicating that other soil properties should be taken into consideration for precise predicting of heavy metal concentrations in rice plants.

Differences in phytase activity and phytic acid content between cultivated and Tibetan annual wild barleys.

The Qinghai-Tibetan Plateau in China is considered to be one of the original centers of cultivated barley. At present, little is known about the phytase activity (Phy) or phytic acid content (PA) in grains of Tibetan annual wild barley. Phy and PA were determined in grains of 135 wild and 72 cultivated barleys. Phy ranged from 171.3 to 1299.2 U kg(-1) and from 219.9 to 998.2 U kg(-1) for wild and cultivated barleys, respectively. PA and protein contents were much higher in wild barley than in cultivated barley. Tibetan annual wild barley showed a larger genetic diversity in phytase activity and phytic acid and protein contents and is of value for barley breeding. There is no significant correlation between phytase activity and phytic acid or protein content in barley grains, indicating that endogenous phytase activity had little effect on the accumulation of phytic acid.

The effect of H2O2 and abscisic acid (ABA) interaction on beta-amylase activity under osmotic stress during grain development in barley.

The effects of exogenous abscisic acid (ABA) and polyethylene glycol (PEG 6000) treatments on grain H(2)O(2), ABA and beta-amylase activity were studied during grain development in the spike culture experiments with variety Triumph and its ABA-insensitive mutant TL43 as the plant materials. The results showed that during grain development the two genotypes were similar in the pattern of ABA concentration change, but differed greatly in the pattern of H(2)O(2) concentration and beta-amylase activity changes. The beta-amylase activity was positively correlated with H(2)O(2) concentration, negatively correlated with ABA concentration, and it is mainly closely associated with continued high levels of ABA with respect to H(2)O(2). Water stress (PEG treatment) induced beta-amylase was associated with H(2)O(2) concentration but not with ABA concentration. Exogenous application of H(2)O(2) and Ascorbic acid (AsA) increased beta-amylase activity in Triumph but reduced that of TL43. However, the endogenous H(2)O(2) concentration in grains was always consistent with beta-amylase activity. A novel model was hypothesized from the current results to illustrate the relationship between H(2)O(2), ABA and beta-amylase synthesis for the barley exposed to abiotic stresses.