Raboy, 1997). Trace levels (Ͻ5% of total Ins P) of "lower" Ins polyphosphates (Ins bis-, tris-, tetrakis-, and Phytic acid (myo-inositol 1,2,3,4,5,6 hexakisphosphate) is the most pentakisphosphates) are also often observed in mature, abundant form of phosphorus (P) in seeds and is virtually indigestible by humans or non-ruminant livestock. It was hypothesized that one wild-type seeds. Normally, inorganic P (P i ) typically repclass of maize (Zea mays L.) and barley (Hordeum vulgare L.) low resents about 5% (Ϯ3%) of seed total P and all other phytic acid mutations, designated lpa1, interrupt myo-inositol supply forms of organic P (DNA, RNA, free nucleotides, phosduring seed development and may be mutations of the myo-inositol pholipids, sugar phosphates, etc.), referred to here as 1-phosphate synthase (MIPS) gene. This study describes the isolation, cellular P, represent about 10 to 20% of seed total P. inheritance, and genetic mapping of the first rice lpa1 mutation and Substantial variation in seed total P of a given line or reexamines the MIPS/lpa1 candidate gene hypothesis in rice. Grain genotype can result from environmental or genotypic from 3632 rice M2 lines, derived from gamma-irradiated seed, was factors that alter the supply of P to the developing seed. screened for the lpa phenotype. Two mutations, one lethal and oneIn wild-type plants, this variation is mostly due to varianon-lethal, were identified. The non-lethal mutation is phenotypically tion in seed phytic acid P, while the P i and cellular P similar to maize and barley lpa1 mutants and was designated rice lpa1-1. Homozygosity for rice lpa1-1 reduces the phytic acid portion fractions of seed total P tend to remain constant (reof seed P from 71 to 39% and increases the inorganic portion of seed viewed in Raboy, 1997). P from 5 to 32%, with little effect on total seed P. This rice lpa1Chemically induced, non-lethal recessive mutants that mutation was mapped to a 2.2-cM interval on chromosome 2L. A decrease seed phytic acid content have been isolated single-copy rice MIPS gene was mapped to a locus on rice chromosome and genetically mapped in maize (Zea mays L.; Raboy 3 that is orthologous to MIPS loci on maize chromosome 1S (near and Gerbasi, 1996; Raboy et al., 2000) and barley maize lpa1 ) and barley chromosome 4H. Unlike maize lpa1, the rice (Hordeum vulgare L.; Larson et al., 1998; Rasmussen and barley lpa1 mutations loci are clearly distinguishable from this and Hatzak, 1998). These low phytic acid (lpa) mutacanonical MIPS gene. No relationship can be inferred between the tions have the potential to alleviate the environmental maize, barley, and rice lpa1 loci. Although this canonical MIPS gene and nutritional problems associated with phytic acid in may be an appropriate target for controlling seed phytic acid synthesis, modifications of other genes (e.g., maize lpa2, barley lpa1, barley

Red rice has long been a troublesome, conspecific weed of cultivated rice. Rice varieties carrying certain herbicide-resistant traits acquired through genetic modification (herbicide-resistant varieties) now offer new options for red rice control. In concert with this innovation is the risk of gene flow, which can result in the transfer of that specific herbicide resistance to red rice and thus render this weed control measure ineffective. Gene flow in concept is simple, however, the parameters that determine the establishment of a new trait in a weed population are complex. Cross-pollination to make hybrid seed and the subsequent fate of those hybrid families in the general weed population are some of the biological factors that influence gene flow between red rice and cultivated rice. Natural outcrossing among rice plants is generally low. Most of the pollen dispersal studies published to date indicated that rice × rice outcrossing rates were less than 1.0%. Numerous reports summarized in this study suggest that outcrossing rates between rice and red rice can be highly variable but usually are similar to or lower than this level. However, once hybrids form, they may introgress into a red rice population within only a few generations. If hybrid seed families are to persist and establish herbicide-resistant red rice populations, they must successfully compete in the crop–weed complex. The ability to survive a herbicide applied to a herbicide-resistant rice variety would be a strong selective advantage for these hybrid families. Thus, the well-established principles of weed resistance management appear to be relevant for herbicide-resistant crop systems and should be used in combination with practices to minimize coincident flowering to mitigate the potential impact of gene flow from herbicide-resistant rice into red rice. For the rice–red rice crop–weed complex, there are both biological factors and agricultural practices that can work together to preserve these new weed control options.

This study was undertaken to investigate the population structure of U.S. rice (Oryza sativa L.). A total of 115 U.S. rice cultivars and 30 ancestral accessions introduced from Asia were genotyped by means of 169 simple sequence repeat (SSR) markers that are well distributed throughout the rice genome. SSR‐based clustering analysis identified three groups of U.S. rice cultivars that were recognizable as temperate japonica with short to medium grains, tropical japonica with medium grains, and tropical japonica with long grains. Indica cultivars were represented among ancestral accessions, but always clustered independently. Indica germplasm has been used for cultivar improvement, but never directly in U.S. rice production. Cluster analysis of cultivars based on four time periods representing their first release date or introduction (1900–1929, 1930–1959, 1960–1979, and 1980–2000) resulted in the identification of the same three groups. This suggests that the population structure in U.S. rice was established before 1930 and remains essentially intact today, despite a large amount of controlled crossing and artificial selection as a part of the breeding process. Fifty‐seven percent of U.S. rice cultivars were developed from intragroup crosses, indicating the availability of substantial genetic variability within each group. Similar results were obtained using genetic distance‐based and model‐based clustering methods. Information about population structure and associated phenotypic characteristics recognized by geneticists and breeders paves the way for coordinated association mapping studies in the future.

A valuable core collection that is a subset of a whole germplasm collection should capture most of the variation present in the whole collection, while allowing for more efficient evaluation and management due to smaller size. The United States Department of Agriculture (USDA) rice (Oryza sativa L.) core subset (RCS), assembled by stratified random sampling, consists of 1790 entries from 114 countries and represents approximately 10% of the 18412 accessions in the rice whole collection (RWC). Data for this study were obtained from the USDA germplasm system at http://www.ars-grin.gov for the RWC and from an evaluation conducted in 2002 for the RCS. Comparative analysis for frequency distributions of 14 descriptors demonstrated that the RCS was highly correlated with the RWC (r = 0.94, P < 0.0001). Thus, information drawn from the RCS could be effectively used to assess the RWC with 88% certainty. Correlation coefficients between the RCS and the RWC for eight descriptors were ≥ 0.9, indicating that the RCS was highly representative of the RWC. Correlation coefficients for the other six descriptors were lower (0.65–0.88), but still significant.

The exact cause of straighthead is unknown, but independent studies have shown that straighthead is in-Straighthead disease is a physiological disorder of rice (Oryza sacreased by consistent flood (Wells and Gilmour, 1977; tiva L.) characterized by sterility of the florets leading to reduced Wilson et al., 2001), sandy to silt loam textured soils grain yield. Knowledge of the straighthead response of new cultivars (Collier, 1912;Adair et al., 1973), low soil pH and low is important for producer control of this disease, and identification of resistant germplasm is essential for breeding improved cultivars. free iron (Baba and Harada, 1954), rich organic matter The objectives of this study were to characterize U.S. cultivar reactions in soil (Jones et al., 1938, p. 28), and high arsenic (As) to straighthead and search for resistant germplasm. Twelve lines, inlevel in the soil (Wells and Gilmour, 1977; Horton et al., cluding 10 U.S. cultivars, a Chinese and Japanese cultivar, were tested 1983). Straighthead has been frequently observed when for straighthead reaction induced by monosodium methanearsonate rice is grown on land where As has accumulated from (MSMA) at 0, 6.7, and 9.0 kg ha Ϫ1 under nitrogen fertilizer of 0, 67, previous applications of herbicides with an As base such 134, and 269 kg N ha Ϫ1 in 1999 and 2000. Straighthead delayed heading as monosodium methanearsonate (MSMA) (Gilmour date, shortened plant height, and dramatically reduced grain yield. and Wells, 1980). Cocodrie, Mars, Kaybonnet, and Bengal were highly susceptible to MSMA is a popular herbicide in cotton (Gossypium straighthead with ratings from 7.2 to 8.0 and grain yield reductions spp.) production in the USA; therefore, rice fields with from 80 to 96%. Wells, LaGrue, Drew, Cypress, and Japan 92.09.31 were a cotton growing history usually have residual As (Gilsusceptible with ratings from 5.9 to 6.7 and yield reductions from 49 mour and Wells, 1980). Residual As chemicals in the soil to 73%. Priscilla and Jefferson were tolerant with ratings of 4.9 and have been shown to cause injuries in rice that are similar 5.3, and yield reductions of 24 and 36%, respectively. The Chinese indica cultivar Zhe 733 was essentially immune to straighthead, show-to straighthead (Baker et al., 1976; Schweizer, 1967; Wells ing neither symptoms nor detectable yield reduction. A total of 124 and Gilmour, 1977; Gilmour and Wells, 1980). Wells and Chinese cultivars including 109 indica and 15 japonica were evaluated Gilmour (1977) noted that cultivars showing tolerance for straighthead resistance in 2001. Nineteen cultivars, 18 indica and to MSMA also appeared to be resistant to straighthead. 1 japonica, were identified as straighthead resistant. Grain yields of On the basis of this observation, a straighthead testing the resistant cultivars were not significantly reduced by their straightarea based on the application of MSMA has been estabhead ratings of 1 to 3. Variation in yield, plant height, maturity, and lished at the University of Arkansas, Rice Research...