The Bsister MADS gene FST determines ovule patterning and development of the zygotic embryo and endosperm.

Many homeotic MADS-box genes have been identified as controllers of the floral transition and floral development. However, information regarding Bsister (Bs)-function genes in monocots is still limited. Here, we describe the functional characterization of a Bs-group MADS-box gene FEMALE-STERILE (FST), whose frame-shift mutation (fst) results in abnormal ovules and the complete abortion of zygotic embryos and endosperms in rice. Anatomical analysis showed that the defective development in the fst mutant exclusively occurred in sporophytic tissues including integuments, fertilized proembryos and endosperms. Analyses of the spatio-temporal expression pattern revealed that the prominent FST gene products accumulated in the inner integument, nucellar cell of the micropylar side, apical and base of the proembryos and free endosperm nuclei. Microarray and gene ontology analysis unraveled substantial changes in the expression level of many genes in the fst mutant ovules and seeds, with a subset of genes involved in several developmental and hormonal pathways appearing to be down-regulated. Using both forward and reverse genetics approaches, we demonstrated that rice FST plays indispensable roles and multiple functions during ovule and early seed development. These findings support a novel function for the Bs-group MADS-box genes in plants.

Identification of active transposon dTok, a member of the hAT family, in rice.

Recent completion of the sequencing of the rice genome has revealed that it contains >40% repetitive sequences, most of which are related to inactive transposable elements. During the molecular analysis of the floral organ number 1/multiple pistil 2 (fon1/mp2) mutant, we identified an active transposable element dTok0 that was inserted at the kinase domain of FON1, a homolog of CLAVATA1. Insertion of the element into FON1 generated an 8 bp duplication of its target sites, which is one of the major characteristics of the hAT family of transposons. The dTok0 element was actively transposed out of the FON1 gene, leaving 5-8 bp footprints. Reinsertion into a new location was observed at a low frequency. Analysis of the genome sequence showed that the rice cultivar 'Nipponbare' contains 25 copies of dTok elements; similar numbers were present in all the Oryza species examined. Because dTok0 does not encode a transposase, enzyme activity should be provided in trans. We identified a putative autonomous transposon, Tok1 that contains an intact open reading frame of the Ac-like transposase.

Genetic characterization and fine mapping of a novel thermo-sensitive genic male-sterile gene tms6 in rice (Oryza sativa L.).

The application of genetic male sterility in hybrid rice production has great potential to revolutionize hybrid seed production methodology. The two-line breeding system by using thermo-sensitive genic male sterility (TGMS) has been discovered and successfully developed as a breeding strategy in rice. One TGMS gene was investigated by a spontaneous rice mutant line, Sokcho-MS, originated from a Korean japonica variety. It was shown that Sokcho-MS is completely sterile at a temperature higher than 27 degrees C and/or lower than 25 degrees C during the development of spikelets, but fertile at the temperature ranging from 25 to 27 degrees C regardless of the levels of day-length. Genetic analysis and molecular mapping based on SSR, STS and EST markers revealed that a single recessive gene locus involved the control of genic male sterility in Sokcho-MS. By using an F2 mapping population derived from a cross between Sokcho-MS and a fertile indica variety Neda, the new TGMS gene, designated as tms6, was mapped primarily to the long arm of chromosome 5 of Oryza sativa at the interval between markers E60663 (2.0 cM) and RM440 (5.8 cM). Subsequently, tms6 was fine mapped to the interval between markers RM3351 (0.1 cM) and E60663 (1.9 cM). As tms6 appeared to be independent of other mapped TGMS genes in rice, the genetic basis of Sokcho-MS was further discussed.

Characterization of a new member of the glutathione peroxidase gene family in Oryza sativa.

A cDNA encoding an Oryza sativa glutathione peroxidase, OsGPX1, was isolated and characterized. OsGPX1 encodes a protein of 168 amino acids with a predicted molecular mass of approximately 18.5 kDa. The protein has 92% identity to a GPX of Zea mays, but only 65% identity to rice PHGPX. The deduced amino acid sequence of OsGPX1 contains two GPX active site domains and one WNF(S/T)KF domain. There is no plastid transit peptide sequence, suggesting that OSGPX1 may function in the cytoplasm. OsGPX1 is located slightly over 85.5 cM from the end of the short-arm of chromosome 4. The OsGPX1 transcripts were abundant only in the leaves of mature plants, and were barely detectable in the leaves of seedlings. However, the transcription of OsGPX1 gene was induced in the seedlings within an hour of exposure to salt stress and was also gradually increased by cold and drought stress. These results indicate that OsGPX1 is a stress-inducible gene of the rice glutathione peroxidase family that protects cells against both metabolic and environmental oxidative stresses.

Gene flow from cultivated rice (Oryza sativa) to its weedy and wild relatives.

BACKGROUND AND AIMS: Transgene escape through gene flow from genetically modified (GM) crops to their wild relative species may potentially cause environmental biosafety problems. The aim of this study was to assess the extent of gene flow between cultivated rice and two of its close relatives under field conditions. METHODS: Experiments were conducted at two sites in Korea and China to determine gene flow from cultivated rice (Oryza sativa L.) to weedy rice (O. sativa f. spontanea) and common wild rice (O. rufipogon Griff.), respectively, under special field conditions mimicking the natural occurrence of the wild relatives in Asia. Herbicide resistance (bar) and SSR molecular finger printing were used as markers to accurately determine gene flow frequencies from cultivated rice varieties to their wild relatives. KEY RESULTS: Gene flow frequency from cultivated rice was detected as between approx. 0.011 and 0.046 % to weedy rice and between approx. 1.21 and 2.19 % to wild rice under the field conditions. CONCLUSIONS: Gene flow occurs with a noticeable frequency from cultivated rice to its weedy and wild relatives, and this might cause potential ecological consequences. It is recommended that isolation zones should be established with sufficient distances between GM rice varieties and wild rice populations to avoid potential outcrosses. Also, GM rice should not be released when it has inserted genes that can significantly enhance the ecological fitness of weedy rice in regions where weedy rice is already abundant and causing great problems.

Male-sterility of thermosensitive genic male-sterile rice is associated with premature programmed cell death of the tapetum.

The tapetum plays a crucial role in pollen development. This secretory tissue produces numerous nutritive proteins necessary for pollen maturation. The tapetum, whose cells undergo programmed cell death (PCD), is completely diminished by the time the pollen is fully mature. Our previous studies on a thermosensitive genic male-sterile (TGMS) rice (Oryza sativa L.) suggested that male-sterility was due to failure in pollen development. In this paper we describe how further analysis of the TGMS rice revealed that male-sterility is associated with premature PCD of the tapetum. Cytological observations of TGMS rice anthers at various developmental stages indicated that PCD initiates at an early stage of pollen development and continues until the tapetal cells are completely degraded, resulting in pollen collapse. Transmission electron microscopy showed the morphologically distinct hallmarks of apoptosis, including cytoplasmic shrinkage, membrane blebbing, and vacuolation. Identification of DNA fragmentation using the TUNEL assay supports the hypothesis that premature PCD is associated with male-sterility in the rice. The tissue-specific feature of the thermosensitive genic male-sterile phenotype is discussed with regard to PCD during anther development.