Inheritance of tolerance to rice tungro bacilliform virus (RTBV) in rice (Oryza sativa L.)

Summary From a large number of rice varieties tested, no variety was identified as resistant to tungro bacilliform virus (RTBV). Only in Utri Merah was the RTBV multiplication restrictive, whereas other varieties such as Kataribhog and Pankhari 203 were identified as tolerant. These varieties were crossed with a susceptible variety. TN1, to study the inheritance of restrictive multiplication and tolerance to RTBV. After 3 weeks of inoculation with RTBV, F₁; F₂, and F₃ progenies were assessed by enzyme-linked immunosorbent assay (ELISA). The RTBV concentration in all F₁ populations was intermediate between parents. The frequency distribution of F₂ seedlings with various levels of RTBV concentration indicated that the RTBV tolerance is controlled by multiple genes. The RTBV concentrations in F₁ and F₂ progenies from the Utri Merah x TN1 cross revealed that restrictive multiplication of RTBV in Utri Merah is a polygenic character. The continuous variation observed in F₂ populations from crosses between tolerant varieties and Utri merah indicated no allelic relationships between tolerant and restrictive multiplication traits.Part of PhD thesis submitted by senior author to University of Kebangsaan Malaysia, Bangi     

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.     

The extent of linkage disequilibrium in rice (Oryza sativa L.)

Despite its status as one of the world's major crops, linkage disequilibrium (LD) patterns have not been systematically characterized across the genome of Asian rice (Oryza sativa). Such information is critical to fully exploit the genome sequence for mapping complex traits using association techniques. Here we characterize LD in five 500-kb regions of the rice genome in three major cultivated rice varieties (indica, tropical japonica, and temperate japonica) and in the wild ancestor of Asian rice, Oryza rufipogon. Using unlinked SNPs to determine the amount of background linkage disequilibrium in each population, we find that the extent of LD is greatest in temperate japonica (probably >500 kb), followed by tropical japonica (approximately 150 kb) and indica (approximately 75 kb). LD extends over a shorter distance in O. rufipogon (<40 kb) than in any of the O. sativa groups assayed here. The differences in the extent of LD among these groups are consistent with differences in outcrossing and recombination rate estimates. As well as heterogeneity between groups, our results suggest variation in LD patterns among genomic regions. We demonstrate the feasibility of genomewide association mapping in cultivated Asian rice using a modest number of SNPs.     

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.     

Characterization of factors affecting plantlet regeneration from rice (Oryza sativa L.) callus

Various components of culture media were tested to characterize factors affecting plantlet regeneration from rice (Oryza sativa L.) callus. It was found that plantlet regeneration from rice callus was affected by concentrations of gelling agents, osmoticum, and the combination of hormones in the regeneration medium. High concentrations (4–6 g/l gellan gum, 10–16 g/l agar) of gelling agents promoted regeneration frequency. However, the total number of plantlets decreased with gellan gum concentrations above 4 g/l. Addition of sorbitol (15–75 g/l) promoted plantlet regeneration. However, the addition of mannitol was inhibitory and no regeneration was observed at concentrations above 30 g/l. This difference in the effects on regeneration suggests that sorbitol had another function besides as a osmoticum. High regeneration frequency was obtained with combinations of NAA (0.05–0.5 g/l) and kinetin (0.5–2 mg/l). However, higher concentrations (2 mg/l) of NAA are preferred to increase the total number of regenerated plantlets.     

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     

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.     

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.     

水稻花药特异启动子Osg6B的序列及功能分析

对水稻花药特异表达基因Ｏｓｇ６启动子简称Ｏｓｇ６Ｂ的全序列进行了测定,结果表明,Ｏｓｇ６Ｂ含有１．７３ｋｂ个核苷酸,与文献报道的该启动子相差仅８个核苷酸,两者核苷酸同源性为９９．５％。将Ｏｓｇ６Ｂ同ＧＵＳ基因编码区相连,将构建成的融合基因用基因抢轰击烟草的花药和幼叶,荧光分析结果为含Ｏｓｇ６Ｂ的ＧＵＳ融合基因在大于２ｍｍ烟草花药中的表达量比对照和幼叶均高出８倍,在小于或等于２ｍｍ的花药中,则高达到     

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.     

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.     

Isozymic variability of traditional rice (Oryza sativa L.) in Africa

Summary Eight hundred and thirtyone traditional varieties of rice, Oryza sativa L., were collected in Africa and analysed for their isozymic variability on 15 enzymatic systems, representing 37 presumed loci. There appears to be a correlation between the type of rice growing and the two groups Indica and Japonica. The degree of genetic diversity is nearly equal in African rice and the Asian one. Alleles due to introgression or mutational events were identified. The results suggest that the evolution of O. sativa is continuous in Africa by means of inter-subspecific or inter-specific crosses.     

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.