During July to August in 1992, a unusual foliar blight disease was observed on soybean plants intercropped between rows of winter wheat in an upland field converted from a paddy field at Tohoku National Agricultural Experiment Station in Morioka, northern part of Japan. The symptoms appeared as primary lesions consisting of small, circular necrotic spots, 1-mm or less than 1-mm in diameter, followed by secondary lesions showing circular to irregularly-shaped and large-sized areas of necrosis around the primary lesions under humid conditions. All the isolates of Rhizoctonia solani Kuhn consistently recovered from leaves with the primary and secondary lesions (hereinafter referred to as leaf spot isolates) formed anastomoses in a high frequency (>75%) with the tester isolates of the anastomosis subgroup AG-2-1 but in a low frequency (<16%) with those of the AG-2-2 IIIB and IV and anastomosis group AG-BI. Among the 66 leaf spot isolates, 64 were auxotrophic for thiamine, whereas the isolates of the AG-2-1 were autotrophic for thiamine. The remaining 2 isolates could not grow even in the presence of thiamine. Culture appearance and optimum growth temperature of the leaf spot isolates were similar to those of the AG-2-1 rather than to those of the AG-2-2 IIIB and IV subgroup. Inoculation tests revealed that the leaf spot isolates were highly pathogenic to soybean, adzuki bean and kidney bean and caused severe pre-emergence and post-emergence damping-off, but were not pathogenic to rape and radish. The isolates caused foliar blight on soybean. These results indicated that most of the leaf spot isolates of AG-2 from soybean did not fit to either the AG-2-1 or AG-2-2 subgroup. Hence, we assigned these isolates to a new subgroup 3 in AG-2 (designated as AG-2-3).

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Somatic embryos of Glycine max (L.) Merrill cultivar 'Jack' were co-transformed with coat protein (CP) gene of attenuated isolates of soybean mosaic virus (SMV) and hygromycin phosphotransferase (hpt) gene by means of microprojectile bombardment. These transformed embryogenic tissues were selected in hygromycincontaining liquid medium. The hygromycin-resistant embryogenic tissues obtained through the selection were regenerated, and CP gene was detected in the 11 transgenic plants out of them. In order to assess their resistance to SMV, mechanical inoculation was performed in T 1 generation. The disease symptom was examined visually and confirmed by the enzyme-linked immunosorbent assay (ELISA). Finally we obtained three independent lines highly resistant to SMV. This is the first report of the soybean plants that were conferred a high resistance to SMV by the transformation with CP gene of the SMV attenuated isolates. In these three lines, the presence of transgene transcript was confirmed by Northern blot analysis, and the transgene product was detected in two of them by Western blot analysis.

This report describes Rhizoctonia solani anastomosis group (AG)-2-3, a newly recognized pathogen of foliar blight of soybean. This group was separated from other subgroups of AG-2 analyzing the internal transcribed spacers (ITS) of nuclear ribosomal DNA repetitive units. The ITS regions of 35 isolates of AG-2, containing 17 isolates of AG-2-3, were amplified by the polymerase chain reaction and analyzed by digestion with 4 restriction enzymes (EcoRI, HaeIII, MspI and TagI). Clear restriction fragment length polymorphisms were found among AG-2 subgroups. The polymorphisms suggest that the group AG-2-3 is a genetically distinct subgroup of AG-2.

Somatic embryos of Glycine max (L.) Merrill cultivar 'Jack' were co-transformed with the coat protein (CP) gene of attenuated isolates of Soybean mosaic virus (SMV) and hygromycin phosphotransferase gene by microprojectile bombardment. CP gene was detected in eleven transgenic plants, and three independent lines highly resistant to SMV were obtained in a previous study. One of these lines, line No. 55, which was assumed to have acquired RNA-mediated resistance, was selected for further gene expression analysis of T 4 and T 5 plants in relation to their viral resistance. The resistant plants contained a lower level of transgenederived RNA than the susceptible ones. On the other hand, based on RNA analysis after mechanical inoculation, SMV-specific RNAs were detected faintly in the resistant plants, while large amounts of RNAs were found in the susceptible ones. During the development of the resistant lines, SMV CP sequence-specific small interfering RNAs (siRNAs) appeared initially in the leaves of the first leaf stage but not in those of the primary stage, and were detected thereafter in the leaves of later stages constantly. The presence of the siRNAs before SMV inoculation was strongly correlated with the resistant phenotype of the lines tested.