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a USDA-ARS, Corn Insects and Crop Genetics Research Unit, and Dep. of Agronomy, Iowa State Univ., Ames, IA 50010
b Retired from Soybean Research Foundation
c Dep. of Entomology, Soils and Plant Sciences and Dep. of Genetics, Biochemistry and Life Sciences Studies, Clemson Univ., Clemson, SC 29634
* Corresponding author (rpalmer{at}iastate.edu)
Genetic Type T368 is an apetalous, male-sterile line of soybean [Glycine max (L.) Merr.], Reg. no. GS-37, PI 633541, found by the Soybean Research Foundation as a single plant in the F12 generation in a cross of ‘SRF 200’ x (‘SRF 300’ x ‘Tracy’) (Hartwig, 1974). SRF 200 is a selection from ‘Amsoy 71’ x [‘Amsoy’ (5) x ‘SRF 350’] (Probst et al., 1972; Weber, 1966). SRF 350 is a selection from ‘Wayne’ (3) x D61-5141 (Bernard, 1966). D61-5141 is a narrow leaf selection from ‘Dorman’ (5) x PI 181537 (Weiss and Stevenson, 1955). SRF 300 is a selection from Wayne (6) x D61-5141. Genetic studies indicated that a single recessive nuclear gene was responsible for this apetalous, highly male-sterile trait in soybean (Skorupska et al., 1993). The morphological features were the lack of standard petal, lateral wings, keel petals, and the appearance of an elongated sepaloid calyx. Gynoecia were characterized by enlarged unfused ovaries and exposed ovules. The mutant line was added to the Soybean Genetic Type Collection in 2003 and is maintained as the heterozygote (T368H). The mutant line has purple flowers, gray pubescence, erect and normal pubescence, brown pod, yellow seed coat, buff hila, and is maturity group I.
Different types of malformations were observed in androecium development in mutant plants. Mutant flowers had only two to four stamens, which were unable to form a normal staminal column. A full complement of stamens was observed in only about 1% of the mutant flowers. Male sterility was attributed to tapetal malfunction. The plants, however, produced a few selfed pods (Skorupska et al., 1993). Also, a few plants may produce outcrossed pods.
In segregating progenies, the apetalous trait and the male-sterile trait were inherited together. We think that this is a pleiotropic effect, instead of tight linkage of the two traits. Similar pleiotropic effects were reported in an apetalous mutant in oilseed rape (Brassica napus L.) by Jiang and Becker (2003).
The plant hormones indole-3-acetic acid (IAA) and abscisic acid (ABA) were quantified and compared in the normal (wild type) and apetally mutant (Skorupska et al., 1994). The mutant had lower endogenous amounts of IAA and ABA than the wild type, and the differences were more pronounced in plants grown in the glasshouse than in plants grown in the field.
Apetalous mutants have been studied extensively in oilseed rape (Jiang and Becker, 2003). Apetalous genotypes may be more efficient in postanthesis photosynthesis and reallocation of assimilates to the seed (Lu and Fu, 1990; Fray et al., 1996; Jiang and Becker, 2001). These groups reported that in certain genetic backgrounds, apetalous mutants showed larger leaf area index and heavier dry matter biomass in comparison to sibling lines with normal flowers. In the apetalous mutant reported by Jiang and Becker (2001), reduced pollen production was recorded, but there was normal seed set after self-pollination.
The soybean apetalous mutant might have utility as a female parent in hybrid seed production for plant breeding studies. The manual cross-pollination success rate with apetalous plants as female parent are comparable to cross-pollinations made with fertile siblings plants as female parent. Under field conditions, we expect insect-mediated cross-pollination to be higher under more humid conditions than under low humid conditions. The unprotected stigma of the apetalous mutant is more vulnerable to desiccation under low humidity. Petal color, size, and volatiles (floral scents) are important cues to attract pollinating insects (Palmer et al., 2001). We have not verified if the apetalous soybean mutant is less attractive to pollinating insects than petalous sibling plants.
Perhaps of greatest importance is that certain apetalous mutants in oilseed rape may avoid some diseases, especially stem rot [caused by Sclerotinia sclerotiorum (Lib.) de Bary] and downy mildew (caused by Peronospora parasitica Pers.:Fr.) (Jiang and Becker, 2001). Lu and Fu (1990) reported 28 and 12.4% field incidence of S. sclerotiorum for two cultivars while the field incidence for the apetalous breeding line was only 1.8%.
Sclerotinia stem rot (syn. white mold) in soybean is distributed in the USA, Canada, Argentina, Brazil, and China (Kim et al., 1999). Wrather et al. (1997) ranked white mold as the second most important soybean disease in the USA during 1994. Soybean plants are infected primarily by ascospores that land on flowers (Grau, 1988). The ascospores germinate, use the flower petals as a nutrient source, and the fungus eventually girdles the stem resulting in plant death. The apetalous male-sterile soybean mutant may avoid this disease because it lacks petals.
A sample of 50 seeds will be available for at least 5 yr for research purposes from the corresponding author. Seed of T368H are available from the Curator, Soybean Germplasm Collection, USDA, ARS, Dep. of Crop Sciences, 1102 S. Goodwin Ave., University of Illinois, Urbana IL 61801.
NOTES
Joint contribution of the USDA-ARS CICGR Unit, and the Iowa Agric. and Home Econ. Exp. Stn. Ames, Project 3769 and supported by CSRESS/USDA, under project number SCD1917. SC Agric. Exp. Stn. Technical contribution no: 4973. Registration by CSSA.
Accepted for publication April 30, 2004.
REFERENCES
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