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.     

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.     

Gene cloning and expression of cytosolic glutathione reductase in rice (Oryza sativa L.)

We have isolated a cDNA (RGRC2) encoding glutathione reductase (GR) from rice (Oryza sativa L.). The comparison of deduced amino acid sequences from RGRC2 and other plant GR cDNAs indicated that RGRC2 encodes a putative cytosolic isoform. The recombinant RGRC2 protein had enzymatic properties comparable to those of GR from rice embryo. Subcellular fractionation showed that the RGRC2 protein is localized primarily in cytosol. mRNA and protein of RGRC2 were observed mainly in roots and calli but little in leaf tissues. Southern blot analysis showed that the RGRC2 gene exists as a single copy gene. Here, we have also isolated a genomic clone completely corresponding to RGRC2. The RGRC2 gene is split into 16 exons spread about 7.4 kb of chromosomal DNA, with coding sequence beginning in the 2nd exon and ending in the 16th exon. From the presence of two ABA-responsive elements in the 5'-flanking region of RGRC2, we examined the expression in rice seedlings treated with ABA and the ABA-related environmental stresses, chilling, drought and salinity. The expression of RGRC2 was strongly induced by all these treatments. We suggest that the expression of the rice cytosolic GR gene is regulated via ABA-mediated signal transduction pathway under environmental stresses.     

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)     

RFLP analysis of rice (Oryza sativa L.) introgression lines

Summary Fifty-two introgression lines (BC₂F₈) from crosses between two Oryza sativa parents and five accessions of O. officinalis were analyzed for the introgression of O. officinalis chromosome segments. DNA from the parents and introgression lines was analyzed with 177 RFLP markers located at approximately 10-cM intervals over the rice chromosomes. Most probe/enzyme combinations detected RFLPs between the parents. Of the 174 informative markers, 28 identified putative O. officinalis introgressed chromosome segments in 1 or more of the introgression lines. Introgressed segments were found on 11 of the 12 rice chromosomes. In most cases of introgression, O. sativa RFLP alleles were replaced by O. officinalis alleles. Introgressed segments were very small in size and similar in plants derived from early and later generations. Some nonconventional recombination mechanism may be involved in the transfer of such small chromosomal segments from O. officinalis chromosomes to those of O. sativa. Some of the introgressed segments show association with genes for brown planthopper (BPH) resistance in some introgressed lines, but not in others. Thus, none of the RFLP markers could be unambiguously associated with BPH resistance.     

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.     

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     

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.     

Genetic analysis of plant regeneration in rice (Oryza sativa L.)

Summary A diallel study involving reciprocal crosses of four genotypes (IR8, 36, 54, and 64) was carried out to understand the genetic mechanism of plant regeneration from immature embryo-calli in rice. Regeneration frequency (% of calli that produced plants) varied from a high of 86% for IR54 to a low of 0% for IR36, while regenerated plants per embryo numbered from 0 to 7 when these same IR lines and the F₁ hybrids were compared. Combining ability analysis revealed that both nuclear (with both additive and dominant effects) and cytoplasmic genes are important in controlling regeneration in rice. Parental lines and F1 hybrids with high ability to regenerate were identified.     

Genetic analysis of salinity tolerance in rice (Oryza sativa L.)

Summary The genetics of salinity tolerance in rice was investigated by a nine-parent complete diallel including reciprocals. Test materials involved susceptible (IR28, IR29, and MI-48), moderately tolerant (IR4595-4-1-13, IR9884-54-3-1E-P1, and IR10206-29-2-1), and tolerant (Nona Bokra, Pokkali, and SR26B) parents. Twoweek-old seedlings were grown in a salinized (EC = 12 dS/m) culture solution for 19 days under controlled conditions in the IRRI phytotron. Typical characteristics of salinity tolerance in rice were found to be Na⁺ exclusion and an increased absorption of K⁺ to maintain a good Na-K balance in the shoot. Genetic component analysis (GCA) revealed that a low Na-K ratio is governed by both additive and dominance gene effects. The trait exhibited overdominance, and two groups of genes were detected. Environmental effects were large, and the heritability of the trait was low. Our findings suggest that when breeding for salt tolerance, selection must be done in a later generation and under controlled conditions in order to minimize environmental effects. Modified bulk and single-seed descent would be the suitable breeding methods. Combining ability analysis revealed that both GCA and specific combining ability (SCA) effects were important in the genetics of salt tolerance. Moderately tolerant parents — e.g., IR4595-4-1-13 and IR9884-54-3-1E-P1 — were the best general combiners. Most of the best combinations had susceptible parents crossed either to moderate or tolerant parents. The presence of reciprocal effects among crosses necessitates the use of susceptible parents as males in hybridization programs. Large heterotic effects suggest the potential of hybrid rice for salt-affected lands.     

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.     

Nitrate reductase of rice (Oryza sativa L.) under foot-rot disease

Summary Studies reported here reveal a low nitrate reductase activity in the shoots of MTU 9 rice plants while in roots high enzyme activity has been recorded.Under pathogenesis, a low nitrate reductase activity in the roots and a high enzyme activity in the shoots have been recorded in susceptible rice plants. In the resistant rice plants (GEB 24) no such marked difference in the enzyme activity has been observed.The effect of fusaric and gibberellic acid on this enzyme activity has been studied. In the case of fusaric acid, the nitrate reductase activity in the roots is inversely proportional to the concentration whereas in gibberellic acid, it is directly proportional to the concentration in the shoots.     

Genetic studies of rice (Oryza sativa L.) anther culture response

Anthers of two rice (Oryza sativa L.) varieties, Lunhui 422 (P₁) and Jinzao 5 (P₂), their F₁, F₂ and first backcross generation-F₁ x Lunhui 422 (B₁), and F₁ x Jinzao 5 (B₂)-were cultured in L8 medium to study the inheritance of rice anther culturability using generation mean analysis. Significant effects of generation were observed for the four traits measured: anther response (%), frequency of callus induction (%), frequency of callus differentiation (%) and culture efficiency (%). Variation among the generations was similar for all traits: significant differences were found among six generations and the means of the P₂ and B₂ were significantly lower than those of the other generations. The frequency of callus differentiation showed a nonsignificant difference among the P₁, F₁, F₂ and B₁ generations which had slightly high values than the P₂ and B₂. Additive genetic variance (V_A) was higher than non-additive genetic variance (V_D) for anther response and frequency of callus induction. However, A_V was lower than V_D for frequence of callus differentiation and culture efficiency, V_A was significant for all traits except for the culture efficiency, and V_D was nonsignificant for all traits except for the frequency of callus differentiation. On the other hand, environmental variation (V_E) was significant for the 4 traits. Narrow-sense heritability estimates were 95.52%, 82.19% and 13.54% for anther response, frequency of callus induction and culture efficiency, respectively.Abbreviations 2,4-d 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid     

DNA-identification of rice varieties (Oryza sativa L.) of Ukrainian selection

Genetic diversity of 11 rice varieties of the Institute of rice UAAS was assessed using a set of 25 SSR loci. Based on analyses of 12 polymorphic microsatillite loci the procedures for composing genetic formulas of varieties and their identification have been elaborated. UPGMA-cluster-analysis based on genetic distance coefficients clearly separated all the varieties into two groups, and showed that the Ukrainian rice varieties are closely related. Although the genetic diversity was low, SSRs proved to be an efficient tool in assessing the genetic diversity of rice genotypes.