Search

IR65598-112 and the two sister lines IR65600-42 and IR65600-96 are promising new plant type (NPT) rice lines with high yield potential. However, these lines are susceptible to bacterial blight caused by Xanthomonas oryzae pr. oryzae (Xoo). To improve the resistance of the three NPT lines to Xoo, three bacterial blight (BB) resistance genes, xa5, xa13, and Xa21, were successfully transferred to the NPT lines via a marker-aided backcrossing procedure. Sequence tagged site (STS) markers for the two resistance genes were developed based on DNA sequences of their linked restriction fragment length polymorphism (RFLP) markers (RG556 and RG207 for xa5 and RG136 for xa13). Marker polymorphism for xa5 was detected after digestion of RG556 polymerase chain reaction (PCR) products with MaeII enzyme and digestion of RG136 PCR product with HinfI enzyme for xa13. The STS marker for Xa21 was designed previously from the sequence of a genomic clone RAPD248. Fifty-nine [BC.sub.3][F.sub.2] near-isogenic lines (NILs) in the three NPT backgrounds containing one to three BB resistance genes in various combinations were developed through marker-assisted selection (MAS) for the resistance genes and phenotypic selection for the NPT. The [BC.sub.3][F.sub.3] NILs having more than one BB resistance gene showed a wider resistance spectrum and manifested increased levels of resistance to the Xuo races, as compared with those having a single BB resistance gene. Results for two [F.sub.2] populations and the progeny testing of their [F.sub.3] lines showed that MAS reached an accuracy of 95 and 96% of identifying homozygous resistant plants for xa5 and xa13, respectively. These NPT NILs for BB resistance genes provided valuable materials for breeding and genetic studies of single-gene effects and interaction of several resistance genes. The results demonstrate the usefulness of MAS in gene pyramiding for BB resistance, particularly for recessive genes, such as xa5 and xa13, that are difficult to select through conventional breeding in the presence of a dominant gene such as Xa21., THE ADVENT of high-density molecular maps coupled with the rapid development of PCR-based techniques has created opportunities for the application of DNA markers both in genetic and breeding research. DNA [...]

Genetic improvement in grain yield has been intensively studied in wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), maize (Zea mays L.), and soybean [Glycine max (L.) Merr.]. Such information is limited in rice (Oryza sativa L.). The objective of this study was to determine the trend in the yield of rice cultivars-lines developed since 1966. Twelve cultivars-lines were grown at the International Rice Research Institute (IRRI) farm and the Philippine Rice Research Institute farm during the dry season of 1996. Seven cultivars-lines were grown at IRRI farm in the dry season of 1998. Growth analyses were performed at key growth stages, and yield and yield components were determined at physiological maturity. Regression analysis of yield versus year of release indicated an annual gain in rice yield of 75 to 81 kg [ha.sup.-1], equivalent to 1% per year. The highest yields obtained with the most recently released cultivars was 9 to 10 Mg [ha.sup.-1], which is equivalent to reported yields of IR8 and other early IRRI cultivars obtained in the late 1960s and early 1970s at these same sites. Therefore, the 1% annual increase in yield may not represent genetic gain in yield potential. The increasing trend in yield of cultivars released before 1980 was mainly due to the improvement in harvest index (HI), while an increase in total biomass was associated with yield trends for cultivars-lines developed after 1980. Results suggest that further increases in rice yield potential will likely occur through increasing biomass production rather than increasing HI., DURING THE PAST 30 YEARS, rice production in Asia more than doubled as a result of the adoption of modern cultivars, increased investments in irrigation, greater use of fertilizer, and [...]

Since the release of IR8 in 1966, 42 additional indica rice (Oryza sativa L.) cultivars developed by the International Rice Research Institute (IRRI) for the irrigated and favorable rainfed lowlands have been released in the Philippines. The maximum yield of IR8 has been reduced by about 2 Mg [ha.sup.-1] during the past 30 yr. Empirical breeding for population improvement within the indica germplasm has resulted in the maintenance of rice yield potential in the tropics of about 10 Mg [ha.sup.-1]. To break the yield barrier, several approaches are being explored. These include development of a new plant type (NPT) with low tillering capacity and large panicles from tropical japonica germplasm and exploitation of heterosis through intervarietal and intersubspecific hybrids. Hybrid rice between indicas increased yield potential by about 9% under the tropical conditions. The higher yield potential of indica/indica hybrids compared with indica inbred cultivars was attributed to the greater biomass production rather than harvest index. New plant type breeding has not yet improved yield potential due to poor grain filling and low biomass production. Factors that cause poor grain filling and Iow biomass production of the NPT lines have been identified. Selecting parents with good grain filling traits, introduction of indica genes into NPT's tropical japonica background, and a refinement of the original NPT design are expected to improve the performance of the NPT lines. Further enhancement in yield potential may be possible from use of intersubspecific heterosis between indica and NPT lines., GLOBAL RICE PRODUCTION must reach 800 of Tg paddy rice to meet projected demand in 2025, which is 266 Tg more than rice production in 1995 (Table 1). This additional [...]