We detected a QTL for single seed weight in soybean that was stable across multiple environments and genetic backgrounds with the use of two recombinant inbred line populations. Single seed weight (SSW) in soybean is a key determinant of both seed yield and the quality of soy food products, and it exhibits wide variation. SSW is under genetic control, but the molecular mechanisms of such control remain unclear. We have now investigated quantitative trait loci (QTLs) for SSW in soybean and have identified such a QTL that is stable across multiple environments and genetic backgrounds. Two populations of 225 and 250 recombinant inbred lines were developed from crosses between Japanese and US cultivars of soybean that differ in SSW by a factor of ~2, and these populations were grown in at least three different environments. A whole-genome panel comprising 304 simple sequence repeat (SSR) loci was applied to mapping in each population. We identified 15 significant QTLs for SSW dispersed among 11 chromosomes in the two populations. One QTL located between Sat_284 and Sat_292 on chromosome 17 was detected (3.6 < LOD < 14.1) in both populations grown in all environments. This QTL, tentatively designated qSw17-1, accounted for 9.4-20.9 % of phenotypic variation in SSW, with a dominant allele being associated with increased SSW. Given its substantial effect on SSW, qSw17-1 is an attractive target for positional cloning, and SSR markers closely associated with this locus may prove useful for marker-assisted selection for SSW control in soybean.

Most commercial soybean varieties have yellow seeds due to loss of pigmentation in the seed coat. The I gene inhibits pigmentation over the entire seed coat, resulting in a uniform yellow color of mature harvested seeds. We previously demonstrated that the inhibition of seed coat pigmentation by the I gene results from post-transcriptional gene silencing (PTGS) of chalcone synthase (CHS) genes. Little is known about the structure of the I gene and the mechanism by which it induces PTGS of CHS genes. Here, we report a candidate of the I gene, GmIRCHS, which consists of a 5'-portion of a DnaJ-like gene containing a promoter region and a perfect inverted repeat (IR) of 1.1-kb truncated CHS3 sequences (5'-DeltaCHS3 and 3'-DeltaCHS3). RT-PCRs and RNase protection assay indicated the existence of the read-through product from 5'-DeltaCHS3 to 3'-DeltaCHS3 and the dsRNA region of DeltaCHS3, suggesting that dsRNA of DeltaCHS3 could be transcribed from GmIRCHS and could induce PTGS of CHS genes. Moreover, the IR structure of DeltaCHS3 in GmIRCHS was lost in the soybean mutants in which I was changed to i, supporting the conclusion that GmIRCHS is the I gene.

Seed coat color in soybeans is determined by the I ( Inhibitor ) locus. The dominant I allele inhibits seed coat pigmentation, and it has been suggested that there is a correlation between the inhibition of pigmentation by the I allele and chalcone synthase ( CHS ) gene silencing in the seed coat. Analysis of spontaneous mutations from I to i has shown that these mutations are closely related to the deletion of one of the CHS genes (designated ICHS1 ). In soybeans with the I/I genotype (cv. Miyagi shirome), a truncated form of the CHS gene ( CHS3 ) is located in an inverse orientation 680 bp upstream of ICHS1 , and it was previously suggested that the truncated CHS3-ICHS1 cluster might be involved in CHS gene silencing in the seed coat. In the current study, the truncated CHS3-ICHS1 cluster was compared with the corresponding region of pigmented seed coat mutants in which I had changed to i in Miyagi shirome and in the strain Karikei 584. In the Karikei 584 mutant, the truncated CHS3-ICHS1 cluster was retained and the sequence diverged at a point immediately upstream (32 bp) of this cluster. The sequences upstream of the points of divergence in both mutants almost perfectly matched a part of the registered sequence in a soybean BAC clone containing the soybean cyst nematode resistance-associated gene, and inspection of the sequences suggested that the sequence divergence of the CHS gene in the Karikei 584 and Miyagi shirome mutants was due to an unequal crossing-over via 4-bp or 5-bp short repeats, respectively.

Northern Japan, Hokkaido has cold weather damage in agriculture almost every four years. Cold weather damage to soybeans [Gfycine max (L.) MerrJ during flowering is especially severe and is caused by both low temperature and insufficient sunlight. Therefore, the damage should be analyzed from both aspects. We analyzed the effects of low temperature and shading during the flowering season on seed yield and yield components in two varieties of soybeans: cv. Hayahikari, an excellent cold weather tolerant variety, and cv. Toyomusume, a cold weather sensitive variety.The soybean plants were exposed to a low temperature of 18'C day/13'C night, shaded (50%) without low temperature treatment, or shaded at a low temperature, during the four-week flowering season. The control plants were kept under normal conditions. The results indicated that cold weather damage is mainly caused by the low temperature, which severely reduced the number of pods per plant, in Toyomusume. However, shading also reduced the number of pods per plant in both varieties. All of the yield components examined were reduced by cold weather more severely in Toyomusume than in Hayahikari. Furthermore, shading combined with low temperature treatment caused greater damage in both Hayahikari and Toyomusume than either a low temperature or shading treatment alone.Key words: Cold weather damage, Flowering stage, Low temperature, Pod, Seed yield, Shading, Soybean, Varietal difference.Hokkaido, which is the largest soybean production area in Japan, is located in northern Japan. The Tokachi district is the main soybean producing region in Hokkaido, but its climate is not always appropriate for soybean production.Soybeans currently grown in Hokkaido often suffer from low-temperature damage. There have been 11 years of cold weather damage in the past 44 years in the Tokachi district, and thus cold weather damage occurs at an average of once every four years (Tanaka, 1997). Cold weather damage is caused mainly by three factors (Yamamoto and Narikawa, 1966) : 1) Poor growth due to low temperatures in the early stages of growth, 2) flower abscission and pod setting failure due to low temperatures before and during the flowering stage, 3) insufficient grain filling due to low temperatures in the pod filling stage. In only three of the 11 years with cold weather damage, soybean was damaged by one of these factors alone. In the other eight years of damage, the damage was caused by a combination of two or three of the factors mentioned above. The frequency of the years with damage by poor growth was the highest, but the damage caused by flower abscission and pod-setting failure was severe. In 1993, when the mean temperature during flowering stage was around 15'C and soybean yield in the Tokachi district was 87% below the average yield, flower abscission and pod-setting failure were remarkable (Matsukawa, 1994).The cold weather damage in the Tokachi district was caused by both low temperature and insufficient sunlight. Numerous publications documented that ...