Inheritance of resistance to soybean mosaic virus(SMV) C-strain in soybeans.

Abstract

Seventy five cultivars were evaluated for their resistance to a virulent strain of soybean mosaic virus (SMV-C) spreading in the central part of Japan. Reactions of trifoliolate leaves of the cultivars to the C strain were classified into the following three types ; no symptoms (resistant : R), necrotic spot symptoms (NS : Fig. 3) and typical mosaic sylriptoms (M). Three resistant cultivars (R), two cultivars with necrotic spots (NS) and seven susceptible ones (M) were used to study the mode of inheritance of the resistance by artificial inoculation tests conducted in the greenhouse. F1 plants of crosses between three resistant and susceptible cultivars (R-S) showed necrotic spots or did not exhibit symptoms, and the F2 plants were classified into three types in the same way as the cultivars (Table 1). If it is considered that the NS plants have the same type of resistance as the resistant plants, all the crosses fit to the ratio 3 resistant : 1 mosaic symptoms, indicating that the resistance is controlled by a single dominant gene. However in the case of the two crosses using “Tousan 140” as resistant cultivar, F2 plants fitted well to the ratio 1 no symptoms : 2 NS : 1 mosaic symptoms. All the F1 plants of these crosses corresponded to the NS group, and the F3 plants which were derived from F2 plants with necrotic spot symptoms segregated into the ratio 3 resistant : 1 mosaic symptoms. The backcross populations between susceptible cultivars and F1 plants segregated into the ratio 1 necrotic symptoms : 1 mosaic symptoms. These results suggest that the plants with the NS symptoms show on heteroallelic phenotypes in the case of “Tousan 140”. Thus the resistant cultivars are considered to have one totally or partially dominant resistance gene. Among the crosses between resistant cultivars (R-R), in two crosses the F2 populations segregated into the ratio 15 resistant : 1 mosaic symptoms while in another cross the populations did not exhibit symptoms (table 2). These data indicate that there are two kinds of independent resistance genes. On the other hand, the R-NS crosses segregsted into the ratio 15 resistant : 1 mosaic symptoms, and the S-NS crosses segregated into the ratio 3 resistant : 1 mosaic symptoms (table 3). These data indicate that the cultivars with necrotic spot symptoms (NS) have a dominant resistance gene which is inherited independently of the two kinds of resistance genes mentioned above. “Raiden” which was bred in Japan is considered to be the source of the resistance gene Rsv2 (BUZZEL and TU 1984), but is susceptible to the C strain. So it is assumed that the three resistant cultivars may have different gene from Rsv2. The relationship between the genes of these cultivars and that of “PI 96983” (gene symbol Rsv : KIIHL and HARTWIG 1979) and “PI 486.355” which was reported by LIM (1985) could not be verified in thls study. Since the reaction of “Ogden” was the same as that of “Tousan 122” and “Dorchsoy 31”, cultivars with NS symptoms such as “Tousan 122” may have the same resistance gene symbolyzed by rsvt (KIIHL and HARTWIG 1979). It is concluded that there are at least three kinds of independent resistance genes to the C strain.