Arsenic toxicity to rice (Oryza sativa L.) in Bangladesh

Natural contamination of groundwater with arsenic (As) occurs around the world but is most widespread in the river basin deltas of South and Southeast Asia. Shallow groundwater is extensively used in the Bengal basin for irrigation of rice in the dry winter season, leading to the possibility of As accumulation in soils, toxicity to rice and increased levels of As in rice grain and straw. The impact of As contaminated irrigation water on soil-As content and rice productivity was studied over two winter-season rice crops in the command area of a single tubewell in Faridpur district, Bangladesh. After 16–17 years of use of the tubewell, a spatially variable build up of As and other chemical constituents of the water (Fe, Mn and P) was observed over the command area, with soil-As levels ranging from about 10 to 70 mg kg^?1. A simple mass balance calculation using the current water As level of 0.13 mg As L^?1 suggested that 96% of the added arsenic was retained in the soil. When BRRI dhan 29 rice was grown in two successive years across this soil-As gradient, yield declined progressively from 7–9 to 2–3 t ha^?1 with increasing soil-As concentration. The average yield loss over the 8 ha command area was estimated to be 16%. Rice-straw As content increased with increasing soil-As concentration; however, the toxicity of As to rice resulted in reduced grain-As concentrations in one of the 2 years. The likelihood of As-induced yield reductions and As accumulation in straw and grain has implications to agricultural sustainability, food quality and food security in As-affected regions throughout South and Southeast Asia.     

Phosphorylation upon cold stress in rice (Oryza sativa L.) seedlings

The response of plants to cold stress is not well understood at the biochemical level, although it has been studied extensively at the ecological level. To investigate whether protein phosphorylation may play an important role in cold stress, we exposed rice seedlings to low temperatures, prepared protein extracts from the leaves and incubated these in the presence of [γ-³²P]ATP. The proteins were then separated by two-dimensional polyacrylamide gel electrophoresis. While several proteins were found to be phosphorylated upon cold stress one protein, pp35, which has an isoelectric point of 8.0, was more phosphorylated than the others. The pp35 protein was found to be phosphorylated when rice seedlings were incubated for 6 h at 5°C before the leaf protein extract was prepared and radioactive labeling was performed. The pp35 was, however, significantly more phosphorylated in cold-tolerant rice varieties. Antibodies were raised against purified pp35 in adult rabbits. Using this pp35 antibody, which can recognize the RuBisCO large-chain subunit (LSU), and from amino acid sequencing of pp35, we were able to identify and confirm the pp35 protein as the fragment of RuBisCO LSU (EC 4.1.1.39). Phosphorylation of the RuBisCO LSU may be important in cold tolerance. Received: 7 July 1998 / Accepted: 19 December 1998     

Metabolic responses of etiolated rice (Oryza sativa L.) seedlings to streptomycin

This paper reports experiments performed to investigate the influence of various concentrations of streptomycin sulphate on a few parameters of importance in the metabolism of rice (Oryza sativa L.) seedlings. It was shown that respiration rate was accelerated by streptomycin. The specific activities of catalase and peroxidase decreased whereas IAA oxidase increased with increasing streptomycin concentrations. Increased activities of all these enzymes were apparent on a dry weight basis suggesting increased succulence caused by streptomycin treatment. There was a considerable rise in the water soluble protein content following streptomycin application. It may be suggested that growth inhibition by streptomycin results from reduction in the auxin level owing to enhanced auxin destruction.     

Shoot tolerance mechanisms to iron toxicity in rice (Oryza sativa L.)

Iron toxicity frequently affects lowland rice and leads to oxidative stress via the Fenton reaction. Tolerance mechanisms were investigated in contrasting genotypes: the intolerant IR29 and the tolerant recombinant inbred line FL483. Seedlings were exposed to 1000 mg L^‐1 ferrous iron, and the regulation of genes involved in three hypothetical tolerance mechanisms was investigated (I) Iron uptake, partitioning and storage. The iron concentration and speciation in different plant tissues did not differ significantly between genotypes. Sub‐cellular iron partitioning genes such as vacuolar iron transporters or ferritin showed no genotypic differences. (II) Antioxidant biosynthesis. Only one gene involved in carotenoid biosynthesis showed genotypic differences, but carotenoids are unlikely to scavenge the reactive oxygen species (ROS) involved in Fe toxicity, i.e. H₂O₂ and hydroxyl radicals. (III) Enzymatic activities for ROS scavenging and antioxidants turnover. In shoots, glutathione‐S‐transferase and ascorbate oxidase genes showed genotypic differences, and consistently, the tolerant FL483 had lower dehydroascorbate reductase and higher ascorbate oxidase activity, suggesting that high rates ascorbate reduction confer sensitivity. This hypothesis was confirmed by application of exogenous reduced ascorbate or L‐galactono‐1,4‐lactone, which increased lipid peroxidation under iron toxic conditions. Our results demonstrate in planta pro‐oxidant activity of reduced ascorbate in the presence of iron.     

Zinc ameliorates copper-induced oxidative stress in developing rice (Oryza sativa L.) seedlings

Rice (Oryza sativa L.) seedlings were treated with different concentrations of copper (Cu) either in presence or absence of zinc (Zn), and different events were investigated to evaluate the ameliorative effect of Zn on Cu stress. In presence of high Cu concentration, growth of both root and shoots were considerably reduced. Decline in elongation and fresh mass was observed in root and shoot. Zn alone did not show any considerable difference as compared to control, but when supplemented along with high concentration Cu, it prompted the growth of both root and shoot. After 7 days, root growth was 9.36 and 9.59 cm, respectively, at 200 and 500 μM of Cu alone as compared to 10.59 and 12.26 cm at similar Cu concentrations, respectively, in presence of Zn. Cu accumulation was considerably high after 7 days of treatment. In absence of Zn, significant accumulation of Cu was observed. Zn supplementation ameliorated the toxic impact of Cu and minimized its accumulation. Cu treatment for 1 and 7 days resulted in a dose-dependent increase in hydrogen peroxide (H₂O₂). When Cu was added in presence of Zn, the H₂O₂ production in root and shoot was reduced significantly. The increase in H₂O₂ production under Cu stress was accompanied by augmentation of lipid peroxidation. In absence of Zn, Cu alone enhanced the malondialdehyde (MDA) production in both root and shoot after 1 and 7 days of treatment. The MDA content drastically reduced in root and shoot as when Zn was added during Cu treatment. The activities of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), and guaiacol peroxidase (GPX) were elevated under Cu stress both in root and shoot. Addition of Zn further stimulated the activities of these enzymes. Both ascorbate (AsA) and glutathione (GSH) contents were high under Cu stress either in presence or absence of Zn. The results suggests that Zn supplementation improves plant survival capacity under high Cu stress by modulating oxidative stress through stimulation of antioxidant mechanisms and restricts the accumulation of toxic concentrations of Cu.     

Straighthead disease of rice (Oryza sativa L.) induced by arsenic toxicity

Straighthead disease is a physiological disorder of rice (Oryza sativa L.) characterized by sterility of the florates/spikelets leading to reduced grain yield. Though the exact cause of straighthead is unknown, a glass house experiment was conducted to investigate the effect of inorganic arsenic on straighthead disease in rice (Oryza sativa L.). BRRI dhan 29, a popular Bangladeshi rice strain, was grown in soils spiked with arsenic (prepared from sodium arsenate, Na₂HAsO₄·7H₂O) at the rate of 10, 20, 30, 40, 50, 60, 70, 80 and 90 mg of As kg^?1 and one control treatment was also run to compare the results. Although there may be some other soil physico-chemical factors involved, arsenic concentration was found to be closely associated with straighthead of rice. With the increase of soil arsenic concentration, the severity of straighthead increased significantly. Up to the 50 mg of As kg^?1 soil treatments, the severity of straighthead incidences were not prevalent. Straighthead resulted in sterile florets with distorted lemma and palea, reduced plant height, tillering, panicle length and grain yield. Straighthead caused approximately 17–100% sterile florates/spikelets formation and about 16–100% loss of grain yield. Straighthead also causes the reduction of panicle formation and panicle length significantly (p < 0.01). In the present study, panicle formation was found to be reduced by 21–95% by straighthead.     

Effect of phosphorus,potassium and zinc fertilizers on iron toxicity in wetland rice (Oryza sativa L.)

During the period January–August 1996, an investigation was carried out in La Mata, Cotuí, Dominican Republic with the objective to study the effect of P, K and Zn fertilizers on Fe toxicity in the rice varieties JUMA-57 (sensitive to Fe toxicity), ISA-40 and PSQ-4 (both tolerant to Fe toxicity). The rate of fertilizer application was 22 and 62 kg P ha^–1; 58 and 116 kg K ha^–1; 3 and 7 kg Zn ha^–1 and a constant dose of 140 kg N ha^–1 and 40 kg S ha^–1 on all fertilized plots. The control received no fertilizer. JUMA-57 was the only variety that showed symptoms of Fe toxicity. The observed symptoms showed a yellow to orange colour. Symptoms of Fe toxicity appeared first one week after transplanting (WAT), decreased at the fourth WAT, but returned six WAT and continued until the end of the experiment. Fertilizer application reduced symptom intensity and increased grain yield in all varieties, but only JUMA-57 did not reach the maximum yield typical for that variety. Fertilizer application did not completely overcome the toxicity effect, i.e. in symptom intensity and grain yield. The positive effect of fertilizer application could not be attributed to a specific nutrient. Intensity of symptoms was not related to Fe concentration in the leaves. The average Fe concentration of 108 mg kg^–1 was not high enough to be considered toxic. Symptoms could not be explained through Mn toxicity (average Mn concentration in the leaves was 733 mg kg^–1) nor Zn deficiency (average Zn concentration in the leaves was 20 mg kg^–1). There was a clear relationship, though, between soil DTPA extractable Fe and symptom intensity or grain yield. The toxic effect was observed when the DTPA extractable Fe in the flooded soil was above 200 mg kg^–1. From these results, we concluded that the Fe toxicity resulted from high Fe in the root zone and not from high Fe concentrations in the leaves.     

Heterosis breeding in rice (Oryza sativa L.)

Summary Studies conducted at the International Rice Research Institute (IRRI) during 1980 and 1981 have shown up to 73% heterosis, 59% heterobeltiosis and 34% standard heterosis for yield in rice. The latter was estimated in comparison to commercial varieties: IR36 and IR42 (yield 4–5 t/ha in wet season trials and 7–8 t/ha in dry season trials). Generally speaking, absolute yield was lower and extent of standard heterosis was higher in wet season than in dry season with some exception. Yields up to 5.9 t/ha (22% standard heterosis) in the wet season and 10.4 t/ha (34% standard heterosis) in the dry season were obtained. Most of the hybrids performed better in some season while some performed better in both seasons. Hybrids showed better lodging resistance although they were 5–10 cm taller. F₁ hybrids had significant positive correlations with the parental traits viz., yield (r = 0.446), tillering (r = 0.746), height (r = 0.810) and flowering (r = 0.843). Selection of parents among elite breeding lines on the basis of their per se yield performance, diverse origin and resistance to insects and diseases should give heterotic combination. Yield advantage of hybrids was due primarily to increase in number of spikelets per unit area even though tiller number was reduced. Grain weight was either the same or slightly higher. High yielding hybrids also showed significant heterosis and heterobeltiosis for total dry matter and harvest index. For commercial utilization of heterosis in rice, effective male sterility and fertility restoration systems are available and up to 45% natural outcrossing on male sterile lines has been observed. Consequently, F₁ rice hybrid have been successfully developed and used in China. Prospects of developing hybrid rice varieties elsewhere appear bright especially in countries that have organized seed production, certification and distribution programs and where hybrid seed can be produced at a reasonable cost.     

Role of abscisic acid in cadmium tolerance of rice (Oryza sativa L.) seedlings

Changes in abscisic acid (ABA) contents in Cd-treated rice (Oryza sativa L.) seedlings of two cultivars were investigated. On treatment with CdCl2, the ABA content rapidly increased in the leaves and roots of Cd-tolerant cultivar (cv. Tainung 67, TNG67) but not in the Cd-sensitive cultivar (cv. Taichung Native 1, TN1). The reduction of transpiration rate of TN1 caused by Cd was less than that of TNG67. Exogenous application of ABA reduced transpiration rate, decreased Cd content, and enhanced Cd tolerance of TN1 seedlings. Exogenous application of the ABA biosynthesis inhibitor, fluridone, reduced ABA accumulation, increased transpiration rate and Cd content, and decreased Cd tolerance of TNG67 seedlings. Fluridone effect on Cd toxicity of TNG67 seedlings was reversed by the application of ABA. The roles of endogenous ABA in Cd tolerance of rice seedlings are discussed and suggested.     

Regulation of sugar metabolism in rice (Oryza sativa L.) seedlings under arsenate toxicity and its improvement by phosphate

The effect of arsenate with or without phosphate on the growth and sugar metabolism in rice seedlings cv. MTU 1010 was studied. Arsenate was found to be more toxic for root growth than shoot growth and water content of the seedlings gradually decreased with increasing concentrations. Arsenate exposure at 20 μM and 100 μM resulted in an increase in reducing sugar content and decrease in non-reducing sugar content. There was a small increase in starch content, the activity of starch phosphorylase was increased but α-amylase activity was found to be decreased. Arsenate toxicity also affected the activities of different carbohydrate metabolizing enzymes. The activities of sucrose degrading enzymes viz., acid invertase and sucrose synthase were increased whereas, the activity of sucrose synthesizing enzyme, viz. sucrose phosphate synthase declined. The combined application of arsenate with phosphate exhibited significant alterations of all the parameters tested under the purview of arsenate treatment alone which was congenial to better growth and efficient sugar metabolism in rice seedlings. Thus, the use of phosphorus enriched fertilizers may serve to ensure the production of healthy rice plants in arsenic contaminated soils.     

Characterization of a vacuolar zinc transporter OZT1 in rice (Oryza sativa L.)

The CDF family is a ubiquitous family that has been identified in prokaryotes, eukaryotes, and archaea. Members of this family are important heavy metal transporters that transport metal ions out of the cytoplasm. In this research, a full length cDNA named Oryza sativa Zn Transporter 1 (OZT1) that closely related to rat ZnT-2 (Zn Transporter 2) gene was isolated from rice. The OZT1 encoding a CDF family protein shares 28.2 % ~ 84.3 % of identities and 49.3 % ~ 90.9 % of similarities with other zinc transporters such as RnZnT-2, HsZnT-8, RnZnT-8 and AtMTP1. OZT1 was constitutively expressed in various rice tissues. The OZT1 expression was significantly induced both in the seedlings of japonica rice Nipponbare and indica rice IR26 in response to Zn²⁺ and Cd²⁺ treatments. Besides, OZT1 expression was also increased when exposed to other excess metals, such as Cu²⁺, Fe²⁺ and Mg²⁺. Subcellular localization analysis indicated that OZT1 localized to vacuole. Heterologous expression of OZT1 in yeast increased tolerance to Zn²⁺ and Cd²⁺ stress but not the Mg²⁺ stress. Together, OZT1 is a CDF family vacuolar zinc transporter conferring tolerance to Zn²⁺ and Cd²⁺ stress, which is important to transporting and homeostasis of Zn, Cd or other heavy metals in plants.     

Effects of abscisic acid on ozone tolerance of rice (Oryza sativa L.) seedlings

Ozone is one of the major gaseous pollutants detrimental to crop growthand metabolism. The objective of this research was to study how ABA amelioratesthe effects of ozone on rice seedlings. Seedlings of two rice cultivars withdifferent sensitivities to ozone (Tainung 67, tolerant; and Taichung Native 1,sensitive) were treated with 400 ppb of ozone or ABA and 400ppb of ozone to determine their effect on growth, stomatalmovement, chlorophyll characteristics, and the activity of antioxidant enzymes.Activities of the enzymes SOD, APOD, GR and POD were significantly higher inthesensitive cultivar, TN 1, than in the tolerant cultivar, TNG 67. Seedlings ofthe sensitive cultivar pretreated with ABA (10 M) weresignificantly more tolerant of ozone than control seedlings. Pretreatment withABA effectively reduced stomatal conductance and the degree of injury. Abscisicacid also increased ascorbate peroxidase and glutathione reductase activity.Ozone increased peroxidase activity in sensitive seedlings, but ABA decreasedperoxidase activity. The sensitive cultivar had a higher density of stomata onits leaves than the tolerant cultivar. The results suggest that ABA inducedtolerance to ozone may be more associated with its effects on stomatal movementthan on the modulation of antioxidant enzyme activity.     

Variability and genetics of tolerance for aluminum toxicity in rice (Oryza sativa L.)

A study was undertaken to investigate the variability among lowland rice cultivars and the mode of gene action of aluminum (Al) toxicity tolerance in rice. Pregerminated seeds were grown in a nutrient solution containing 30 ppm Al and in normal nutrient solution, and relative root length (RRL) was determined at the 14-day-old stage to characterize genotypes for tolerance. Sixty-two traditional rice cultivars grown on lowland acid sulfate soil areas of Asia and West Africa were tested. Tolerant varieties Azucena, IRAT104, and Moroberekan, moderately sensitive IR29 and IR43, and sensitive IR45 and IR1552 were used to investigate the genetics of tolerance by diallel analysis. Of the 62 cultivars tested, only 3 were found to be sensitive to A l toxicity. Among the tolerant cultivars identified, 11 (Siyam Kuning, Gudabang Putih, Siyam, Lemo, Khao Daeng, Siyamhalus, Bjm-12, Ketan, Seribu Gantang, Bayer Raden Rati, and Padi Kanji) were found to possess higher levels of tolerance than the improved tolerant upland cultivar IRAT104. Diallel analysis revealed that high RRL is governed by both additive and dominance effects with a preponderance of additive effects. The trait exhibited partial dominance, and one group of genes was detected. Heritability was high, and environmenal effects were low. Findings suggest that when breeding for A1 toxicity tolerance, selection can be made in early generations. The pedigree method of breeding would be suitable. Combining ability analysis revealed the importance of both general combining ability (GCA) and specific combining ability (SCA) in the genetics of A1 toxicity tolerance in rice. GCA was more prevalent than SCA. Tolerant parens Azucena, IRAT104, and Moroberekan were the best general combiners. The presence of reciprocal effects among crosses suggested the proper choice of parents in hybridization programs. Results indicated that Azucena, IRAT 104, and Moroberekan should be used as the female in crosses for A1 toxicity tolerance.     

Ammonium nutrition increases water absorption in rice seedlings (Oryza sativa L.) under water stress

Water stress is a primary limitation on plant growth. In previous studies, it has been found that ammonium enhances the tolerance of rice plants to water stress, but how water is related to nitrogen form and water stress remains unknown. To study the effects of nitrogen form (NH ₄ ⁺ , NO ₃ ^? , and a mixture of NH ₄ ⁺ and NO ₃ ^? ) on the growth and water absorption of rice (Oryza sativa L.) seedlings, a hydroponic experiment with water stress, simulated by the addition of polyethylene glycol (PEG, 10% w/v, MW 6000), was conducted in a greenhouse. The results showed that, compared with non-water stress, under water stress, the fresh weight of rice seedlings increased by 14% with NH ₄ ⁺ nutrition, whereas it had decreased by about 20% with either NO ₃ ^? or mixed nitrogen nutrition. No significant difference was found in the transpiration rate of excised shoots or in xylem exudation of excised roots in NH ₄ ⁺ supply between the two water situations, whereas xylem flow decreased by 57% and 24% under water stress in NO ₃ ^? and mixed nutrition, and root hydraulic conductivity decreased by 29% and 54% in plants in NH ₄ ⁺ and NO ₃ ^? nutrition conditions, respectively. Although water absorption ability decreased in both NH ₄ ⁺ and NO ₃ ^? nutrition, aquaporin activity was higher in NH ₄ ⁺ than in NO ₃ ^? nutrition, regardless of water stress. We conclude that NH ₄ ⁺ nutrition can improve water handling in rice seedlings and subsequently enhance their resistance to drought.     

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 (Cr⁶⁺) 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 \({\text{O}}_{2}^{{ \cdot-}}\) , 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.     

Genetics of gel consistency in rice (Oryza sativa L.)

Inheritance of gel consistency in rice was studied in crossés involving highamylose, low-gelatinizalion temperature parents with hard, medium, and soft gel consistency. The results of single-grain analysis of parents, F₁, F₂, B₁F₁, B₂F₂, and their reciprocal crosses from a single-season harvest showed that the differences between hard and soft, hard and medium, and medium and soft gel consistency are under monogenic control and that modifiers affect the expression of the trait. Multiple alleles at the same locus, hereby designated asgec ^a for medium gel consistency andgec ^b for soft gel consistency, were recessive to the wild type allele for hard gel consistency andgec ^a was dominant overgec ^b. The results indicate that selection for desired gel consistency can effectively be done in early segregating generations.     

Low irradiance stress tolerance in rice (Oryza sativa L.)

The effect of low irradiance on three rice cultivars (shade tolerant cvs. Swarnaprabha and CO 43 and shade susceptible cv. IR 20) was studied. The large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase with molecular mass of 55 kDa was reduced in cv. IR 20 grown under low irradiance (LI). Native protein profile studied showed, under LI, reduction in the contents of proteins with RF values 0.03, 0.11 and 0.37. Analysis of chloroplast polypeptides revealed an induction of light-harvesting chlorphyll-protein 2 (LHCP2) under shade. The induction was more expressed in cv. CO 43 than in cv. IR 20. Under LI, in vivo labelled protein bands in the molecular range of 26 - 27 kDa were induced. These proteins were highly turned over in the LI-grown plants of cv. CO 43 than in cv. IR 20. A signal for rbcL gene sequences in EcoRI digested lanes was also found. Isozyme analysis of peroxidase showed an induction of a new band with RF 0.43 in cv. IR 20 subjected to LI. This revised version was published online in July 2006 with corrections to the Cover Date.