The tobacco N and Arabidopsis RPS2 genes, among several recently cloned disease-resistance genes, share a highly conserved structure, a nucleotide-binding site (NBS). Using degenerate oligonucleotide primers for the NBS region of N and RPS2, we have amplified and cloned the NBS sequences from soybean. Each of these PCR-derived NBS clones detected low-or moderate-copy soybean DNA sequences and belongs to 1 of 11 different classes. Sequence analysis showed that all PCR clones encode three motifs (P-loop, kinase-2, and kinase-3a) of NBS nearly identical to those in N and RPS2. The intervening region between P-loop and kinase-3a of the 11 classes has high (26% average) amino acid sequence similarity to the N gene although not as high (19% average) to RPS2. These 11 classes represent a superfamily of NBS-containing soybean genes that are homologous to N and RPS2. Each class or subfamily was assessed for its positional association with known soybean disease-resistance genes through near-isogenic line assays, followed by linkage analysis in F2 populations using restriction fragment length polymorphisms. Five of the 11 subfamilies have thus far been mapped to the vicinity of known soybean genes for resistance to potyviruses (Rsvl and Rpv), Phytophthora root rot (Rpsl, Rps2, and Rps3), and powdery mildew (rmd). The conserved N-or RPS2-homologous NBS sequences and their positional associations with mapped soybean-resistance genes suggest that a number of the soybean disease-resistance genes may belong to this superfamily. The candidate subfamilies of NBScontaining genes identified by genetic mapping should greatly facilitate the molecular cloning of disease-resistance genes.Over the past few years, we have witnessed a breakthrough in the molecular cloning of disease-resistance genes (for review, see ref.

Photoperiod-sensitive genie male sterile (PS-GMS) rice has a number of desirable characteristics for hybrid rice production. In this study we made use of a published rice genetic linkage map to determine the locations ofPSGMS genes and we have characterized the effects of these genes on sterility by using molecular markers. A two-step approach was designed for maing the genes: (i) Identifying possible PSGMS gene-containig chromosome regions with bulked DNA from extreme fertile and extreme sterile plants of a very large F2 population and (i) determining the map locations of the genes in extreme sterile individuals. We show that this mapping method is much more cost effective and statistay efficient than using a random sample of an F2 population. We idented two chromosomal regions each containing a PSGMS locus, one designatedpmsl on chromosome 7 and one desinatedpms2 on chromosome 3. The existence of these two loci was confirmed by a large sample assay and with data on ratooning progenies of the F2 plants. A marker-based analysis shows that the effect of pms) is 2-3 times larger than that of pms2 and that dominance is almost complete at both loci. Implications in the breeding of PSGMS rice lines are discussed.A photoperiod-sensitive genic male sterile (PSGMS) rice was found in 1973 as a spontaneous mutant in ajaponica (Oryza sativa ssp. japonica) rice cultivar (Nongken 58) grown in Hubei Province, China (1). Large numbers of studies conducted in the last decade have established that this novel mutant (referred to as Nongken 58S) possesses a number of desirable characteristics that might be useful in hybrid rice (2): pollen fertility of Nongken 58S is regulated by photoperiod length (3); it is completely sterile when grown under long-day conditions, whereas pollen sterility varies when it is grown under short-day conditions; and the critical stage comes between secondary branch differentiation and microsporogenesis during panicle development (4). Thus, PS-GMS rice can be used to propagate itself under short-day conditions and also to produce hybrid seeds by interplanting it with normal fertile lines under long-day conditions. PSGMS rice may therefore provide opportunity to replace the widely used "three-line" (male sterile, maintainer,, and restorer) system with a "two-line" system that promises to greatly reduce costs in labor, time, and resources in hybrid rice production. PSGMS rice has a broad spectrum ofrestoration; almost all normal rice strains restore the fertility of the F1 hybrid. Deliberately bred restorer lines are consequently not required. Fertility is controlled by a relatively simple genetic system, usually one or two major Mendelian loci (1, 5). Thus it should be relatively easy to develop new PSGMS lines by transferring the PSGMS alleles from one genetic background to another, particularly if marker-aided systems of transfer can be developed. A further advantage is that the performance of PSGMS hybrids does not suffer from adverse effects of male sterile cytoplasm such as has commonly been...

Few quantitative trait loci (QTL) have been mapped for the expression of partial resistance to Phytophthora sojae in soybean and very little is known about the molecular mechanisms that contribute to this trait. Therefore, the objectives of this study were to identify additional QTL conferring resistance to P. sojae and to identify candidate genes that may contribute to this form of defense. QTL on chromosomes 12, 13, 14, 17, and 19, each explaining 4 to 7% of the phenotypic variation, were identifi ed using 186 RILs from a cross of the partially resistant cultivar 'Conrad' and susceptible cultivar 'Sloan' through composite interval mapping. Microarray analysis identifi ed genes with signifi cant differences in transcript abundances between Conrad and Sloan, both constitutively and following inoculation. Of these genes, 55 mapped to the fi ve QTL regions. Ten genes encoded proteins with unknown functions, while the others encode proteins related to defense or physiological traits. Seventeen genes within the genomic region that encompass the QTL were selected and their transcript abundance was confi rmed by quantitative reverse transcription polymerase chain reaction (qRT-PCR). These results suggest a complex QTL-mediated resistance network. This study will contribute to soybean resistance breeding by providing additional QTL for marker-assisted selection as well as a list of candidate genes which may be manipulated to confer resistance.

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reaction to SMV (Buzzell and Tu, 1989). Finally, an SMV resistance locus was reported by Ma et al. (1995). Soybean mosaic virus (SMV) is a prevalent viral pathogen of soy-This gene, referred to in this paper as Rsv4, confers bean [Glycine max (L.) Merr.] wherever it is grown. Several genes that confer resistance in soybean to soybean mosaic virus have been resistance to all known strains of SMV. The Rsv4 allele identified. One of these resistance loci, Rsv4, confers resistance to all reported by Ma et al. (1995) is derived from the line the known strain groups of SMV. This study was conducted to deter-PI486355, which was shown to contain two resistance mine the map position of the Rsv4 locus in the soybean genome. A loci, one which is allelic to Rsv1 and another (Rsv4) population of 255 F 2 individuals from the cross of the SMV resistant which is not allelic at either the Rsv1 or Rsv3 locus. It is line LR2 (Rsv4) by the susceptible line Lee68 (rsv4) was evaluated of interest to note that this resistance gene is completely in a mapping study. DNA from 12 to 15 F 2 individuals being either dominant, in contrast to Rsv1 alleles which show syshomozygous resistant or susceptible were pooled to produce bulk temic necrosis in the heterozygous state (Chen et al., resistant and bulk susceptible DNA samples. Parents and bulks were 1994). The Rsv4 locus from PI486355 shows resistance screened with 101 AFLP primer pairs and two linked polymorphisms without necrosis in both the heterozygous and homozywere identified. A putative linked marker, amplified by the primers Mse-AAA and Eco-AAG, was converted to a restriction fragment