Auxin-orientation effects on somatic embryogenesis from immature soybean cotyledons

Abstract

Development of somatic embryos of soybeanGlycine max (L.) Merr. has been studied using cultivars J103 and McCall and five auxin-sucrose treatment: naphthalene acetic acid at 10 mg/liter with 1.5% sucrose (N10); 2,4-dichlorophenoxyacetic acid (2,4-D) at 0.25, 0.5 or 1.0 mg/liter with 1.5% sucrose (D.25, D.5 D1); and 2,4-D at 25 mg/liter with 3% sucrose (D25). Cotyledons were excised aseptically from immature embryos 3 to 5 mm in length, and placed on the media with either the adaxial or abaxial side down (adaxial or abaxial orientiation, respectively). After 30 d, numbers of normal or total somatic embryos, or both, were counted. For J103 explants on D25 or N10 media, the greatest number of embryos was produced from the central region of abaxially oriented explants on D25, but only 5% of these embryos were normal. The greatest number of normal embryos was produced from the marginal region of adaxially oriented explants on D25. For McCall explants on D.25, D.5, D1, or N10 media, the greatest numbers of normal embryos were produced from the marginal regions of abaxially oriented explants on D1 or N10. Embryo induction was examined histologically for the N10, D25, and D.5 treatment, using serial section of paraffin-embedded cotyledons stained with hematoxylin and safranin. On N10 medium, embryos were produced unicellularly, or more frauently, multicellularly, from homogeneous embryogenic tissue arising from epidermal and subepidermal cells at the distal periphery of the cotyledon. On D25 medium, embryos were produced multicellularly from a geterogeneous embryogenic tissue formed in the central region of the cytoledon, and consisting of embryogenic cells interspersed with large, vacuolate cells. The D.5 treatment showed an intermediate response. Embryo initiation in this soybean system is predominantly multicellular.