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.     

Silicon-mediated amelioration of zinc toxicity in rice (Oryza sativa L.) seedlings

Background and aims

Silicon (Si) was suggested to enhance plant resistance to toxic elements, and its beneficial role was mainly based on external and internal plant mechanisms. This work aimed at investigating the internal effect of Si on zinc (Zn) detoxification to rice (Oryza sativa L., cv. Tian You 116) seedlings.

Methods

In a hydroponic experiment, we examined the uptake, xylem loading and localization of Zn in rice seedlings under the condition of 200?μM Zn contamination with the additional silicate supply at three levels ( 0, 0.5 and 1.8?mM).

Results

The silicate addition significantly increased the seedling biomass, and decreased Zn concentration in both root and shoot of seedlings and in xylem sap flow. Zinpyr-1 fluorescence test and Energy-dispersive X-ray spectroscopy analysis showed the concentration of biologically active Zn²⁺ decreased, and Zn and Si co-localized in the cell wall of metabolically less active tissues, especially in sclerenchyma of root. The fractionation analysis further supported silicate supply increased about 10% the cell wall bound fraction of Zn.

Conclusions

This study suggests the Si-assisted Zn tolerance of rice is mainly due to the reduction of uptake and translocation of excess Zn, and a stronger binding of Zn in the cell wall of less bioactive tissues might also contribute to some degree.     

Arsenic uptake and accumulation in rice (Oryza sativa L.) irrigated with contaminated water

Long-term use of arsenic contaminated groundwater to irrigate crops, especially paddy rice (Oryza sativaL.) has resulted in elevated soil arsenic levels in Bangladesh. There is, therefore, concern regarding accumulation of arsenic in rice grown on these soils. A greenhouse pot experiment was conducted to evaluate the impact of arsenic-contaminated irrigation water on the growth and uptake of arsenic into rice grain, husk, straw and root. There were altogether 10 treatments which were a combination of five arsenate irrigation water concentrations (0–8 mg As l^–1) and two soil phosphate amendments. Use of arsenate containing irrigation water reduced plant height, decreased rice yield and affected development of root growth. Arsenic concentrations in all plant parts increased with increasing arsenate concentration in irrigation water. However, arsenic concentration in rice grain did not exceed the maximum permissible limit of 1.0 mg As kg^–1. Arsenic accumulation in rice straw at very high levels indicates that feeding cattle with such contaminated straw could be a direct threat for their health and also, indirectly, to human health via presumably contaminated bovine meat and milk. Phosphate application neither showed any significant difference in plant growth and development, nor in As concentrations in plant parts.     

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.     

Genotype and environment effects on rice (Oryza sativa L.) grain arsenic concentration in Bangladesh

Genetic analysis of 38 rice varieties released by the Bangladesh Rice Research Institute (BRRI) identified 34 as indica, 2 as admixed between indica and aus, and 4 as belonging to the aromatic/Group V subpopulation. Indica varieties developed for the two major rice-growing seasons, the wet monsoon (aman) and the dry winter (boro), were not genetically differentiated. The Additive Main Effect and Multiplicative Interaction (AMMI) model was used to assess the effect of genotype (G), environment (E) and genotype-environment interaction (GEI) on grain arsenic (As) concentration when these rice varieties were grown at ten BRRI research stations located across diverse agro-ecological zones in Bangladesh. G, E and GEI, significantly influenced grain As concentration in both seasons. Overall, E accounted for 69%–80%, G 9%–10% and GEI 10%–21% of the observed variability in grain As. One site, Satkhira had the highest mean grain As concentration and the largest interaction principle component analysis (IPCA) scores in both seasons, indicating maximum interaction with genotypes. Site effects were more pronounced in the boro than in the aman season. The soil level of poorly crystalline Fe-oxide (AOFe), the ratio of AOFe to associated As, soil phosphate extractable As and soil pH were important sub-components of E controlling rice grain As concentration. Irrespective of environment, the mean grain As concentration was significantly higher in the boro (0.290 mg As kg^?1) than in the aman (0.154 mg As kg^?1) season (p?<?0.0001), though the reasons for this are unclear. Based on mean grain As concentration and stability across environments, the variety BR3 is currently the best choice for the boro season, while BR 23 and BRRI dhan 38 are the best choices for the aman season. Popular varieties BR 11 (aman) and BRRI dhan 28 and 29 (boro) had grain As concentrations close to the mean value and were fairly stable across environments, while high-yielding, short-duration aman season varieties (BRRI dhan 32, 33 and 39) developed for intensified cropping had relatively high grain As concentrations. Results suggest that genetic approaches to reducing As in rice grain will require the introduction of novel genetic variation and must be accompanied by appropriate management strategies to reduce As availability and uptake by rice.     

Epigenetic responses to drought stress in rice (Oryza sativa L.)

Cytosine methylation polymorphism plays a key role in gene regulation, mainly in expression of genes in crop plants. The differential expression of cytosine methylation over drought stress response was analyzed in rice using drought susceptible but agronomically superior lines IR 20 and CO 43, and drought tolerant genotypes PL and PMK 3 and their F₁ hybrids. The parents and hybrids were subjected to two moisture regimes viz., one under drought condition and another under control condition. The cytosine methylation polymorphism in genomic DNA was quantified under both the conditions at the reproductive stage of the plant using the Methylation Sensitive Amplified Polymorphism (MSAP) technique devised by Xiong et al. (261:439–446, 1999). The results depicted that under drought condition, hyper-methylation was predominant in the drought susceptible genotypes while drought tolerant genotypes presented hypo-methylation behavior. While imposing drought, spikelet sterility per cent was positively correlated to percentage of methylation whereas, panicle length, number of seed per panicle, panicle weight, 100 seed weight, and yield/plant were negatively correlated indicating the role of epigenetic regulation in yield attributing traits in response to drought. Thus, methylation can be considered as an important epigenetic regulatory mechanism in rice plants to adapt drought situation. From this study, we speculate that the hyper- methylation may be an indicator of drought susceptibility and the hypo-methylation for drought tolerance and this methylation polymorphism can be effectively used in drought screening program.     

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.     

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.     

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.     

Response of excised embryos of rice (Oryza sativa L.) to X-rays

Summary The response of rice (Oryza sativa L.) embryos to X-rays (M₁ to M₃) was studied. By means of irradiating excised embryos, both chlorophyll and macromutation were successfully induced in three genotypes of rice. However, differential responses in terms of mutation frequency, mutation spectrum and optimal levels of X-rays required for induction of mutation (chlorophyll as well as morphological) were found to exist between cultivars. In Satika and Ashkhata, LD₅₀ values and maximum induced seed sterility are concomitant to optimum level of radiation required for triggering chlorophyll mutation. However, optimum dose for induction of macromutation in Satika and Kerangserang is independent of either LD₅₀ and/or induced seed sterility.Chances of obtaining both dominant and locus specific recessive mutations in the immediate X-ray treated generation (M₁) are large. This indicates the very high degree of effectiveness of the excised embryo irradiation technique with rice.     

Water-use efficiency in rice (Oryza sativa L.) in relation to resistance to salinity

Abstract The effects of sodium chloride on water-use efficiency of a number of varieties of rice have been investigated. Sensitivity to salinity in rice arises in large part from excessive rates of sodium ion transport to the shoot consequent upon high rates of transpirational water loss. Varietal differences in water-use efficiency were found: these were greater when measured for whole shoots over a period of one week, than when made as instantaneous measurements on individual leaves. Salinity had rather little effect on water use efficiency but overall resistance of seedlings to salt was, in general, greater the greater the water-use efficiency. The reasons for the differences between varieties are discussed in terms of differences in growth pattern: among the seven varieties investigated, water-use efficiency and salt-resistance were lower in dwarfed as opposed to non-dwarfed varieties.     

Plant regeneration from indica rice (Oryza sativa L.) protoplasts

Rice (Oryza sativa L.) plants of the indica cultivar IR54 were regenerated from protoplasts. Conditions were developed for isolating and purifying protoplasts from suspension cultures with protoplast yields ranging from 1·10⁶ to 15·10⁶ viable protoplasts/1 g fresh weight. Protoplast viability after purification was generally over 90%. Protoplasts were cultured in a slightly modified Kao medium in a Petri plate by placing them onto a Millipore filter positioned on top of a feeder (nurse) culture containing cells from a suspension culture of the japonica rice, Calrose 76. Plating efficiencies of protoplasts ranged from 0.5 to 3.0%; it was zero in the absence of the nurse culture. Protoplast preparations usually contained no contaminating cells, and when present, the number of cells never exceeded 0.1% of the protoplasts. After three weeks the Millipore filter with callus colonies were transferred off feeder cells and onto a Linsmaier and Skoog-type medium for an additional three weeks. Selected callus colonies that had embryo-like structures were then transferred to regeneration medium containing cytokinins, and regeneration frequencies up to 80% were obtained. Small shoots emerged and were transferred to jars for root development prior to transferring to pots of soil and growing the plants to maturity in growth chambers. Of the cytokinins evaluated, N⁶-benzylaminopurine was the most effective in promoting shoot formation; however, kinetin was also somewhat effective. Regeneration medium could be either an N₆ or Murashige and Skoog basal medium. Of 76 plants grown to maturity, 62 were fertile, and the plant heights averaged about three-fourths the height of seed-grown plants.Two other suspension cultures of IR54, one developed from the protoplast callus of the initial IR54 line, and the other developed from callus produced by mature seeds, have yielded protoplasts capable of regenerating plants when using cells of the Calrose 76 suspension as a nurse culture. In addition, protoplasts obtained from three-week-old primary callus of immature embryos of IR54 were capable of regenerating plants when using the same culture conditions.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - pcy packed cell volume - BAP N⁶-benzylaminopurine - FDA fluorescein diacetate - FW fresh weight - IAA indole-3-acetic acid Media AA Muller and Grafe (1978) - CPW Frearson et al. (1973) - Kao^* Kao (1977) - LS Linsmaier and Skoog (1965) - MS Murashige and Skoog (1962) - N₆ Chu et al. (1975) - PCM Ludwig et al. (1985)     

Long-term effects of exogenous silicon on cadmium translocation and toxicity in rice (Oryza sativa L.)

Most nutrient solution studies on the interactions between silicon (Si) and cadmium (Cd) are short term. Here we reported a long-term experiment in which rice (Oryza sativa L.) was cultured for 105 days and harvested at four different growth stages to measure biomass accumulation and Cd uptake and distribution in shoots and roots. Exogenous Si increased shoot biomass by 61–238% and root biomass by 48–173% when the culture solution was free of Cd. When 2 μmol L^?1 Cd was added, Si supply increased shoot and root biomass by 125–171% and by 100–106% compared to the zero-Si treatment. Increasing the Cd concentration to 4 μmol L^?1 decreased the beneficial effects of Si on root and shoot biomass. Silicon supply decreased shoot Cd concentrations by 30–50% and Cd distribution ratio in shoot by 25.3–46%, compared to the treatment without Si supply. Additionally, lower Si supply or more serious Cd stress would lead to roots with bigger biomass and higher Si concentration. Energy-dispersive X-ray microanalysis showed that both Si and Cd accumulated synchronously in the border and middle of phytoliths of the shoots. We conclude that Si enhances plant growth and decreases Cd accumulation in shoots and thereby helps to lower the potential risks of food contamination.     

Diallel analysis of heading date in rice (Oryza sativa L.)

Summary The genetics of heading date was investigated in an 8×8 diallel set of crosses involving diverse rice cultivars. Wr, Vr graph analysis revealed the presence of a complementary type of non-allelic interaction which apparently affected the position and slope of the regression line such as if there were overdominance. Omission of two interacting parents resulted in a 6×6 subset of diallel crosses from which, as observed in the Wr, Vr graph, the non-allelic interaction had disappeared and the regression line exhibited partial dominance. Estimates of the genetic components of variation were in close conformity with the results obtained from the Wr, Vr graph: the average degree of dominance, as measured by (H₁/D)^1/2, was in overdominance range in the interacting 8×8 set of diallel crosses whereas it was reduced to partial dominance in the non-interacting 6×6 set of crosses. Further analysis by a standardized deviations graph indicated that earliness was controlled, on the average, by an excess of dominant alleles.     

Zinc-copper antagonism in the nutrition of rice (Oryza sativa L.)

Summary The effect of various doses of copper and zinc on their uptake and on the yield of rice were studied. Copper applications increased copper contents in the plants without effecting the zinc contents. However, zinc applications though increased zinc contents but markedly decreased the copper contents in the plants. This antagonistic effect of zinc on copper suggests that zinc applications can reduce rice yield if available copper is marginal in the soils. re]19720628     

Chronopathological aspects of disease incidence in rice (Oryza sativa L.)

Rice seedlings maintained under uncontrolled glasshouse conditions were inoculated with conidial suspensions of a fungal pathogen, Helminthosporium oryzae, at various times during the 24 h. Significant increase in the percent germination and germ tube length of conidia were observed in the rice samples inoculated at 02:00 and 06:00h. The 24 h temporal variation in leaf temperature was positively correlated with variation in stomatal movements. The results indicate a 24 h rhythm in the behavior of the fungal pathogen on the host in relation to the conditions of the growing environment. In all the inoculated seedlings, the appearance of a large number of brown leaf spots was confined to the light span. Among the plants inoculated, earlier initiation of brown leaf spot appearance, maximum number of leaf spots, and highest disease severity were observed when plants were inoculated at 02:00h. There was a positive correlation between disease severity of the host and in vivo values of percent germination of conidia and germ tube length of the pathogen in plants inoculated between 02:00 and 06:00h. The findings of this study implicate that light intensity and temperature could play a predominant role in controlling disease susceptibility rhythms in plants.     

Genetic analysis of waxy locus in rice (Oryza sativa L.)

Summary Inheritance of waxy locus was studied in crosses of a waxy variety with four non-waxy parents having high-, intermediate-, low- or very low-amylose content. The analysis for amylose content was done on a single grain basis in parents, F₁, F₂, B₁F₁, and B₂F₁ seeds. The waxy parent lacking synthesis of amylose content was found to differ from the ones having high-, intermediate-, low- or very low-amylose content by one gene with major effect. Dosage effects for amylose content were observed to have great influence on segregation pattern and efficiency of selection. Selection efficiency for amylose content can be enhanced by selecting for endosperm appearance in early segregating generations.