Comparative protein accumulation patterns in soybean somatic and zygotic embryos

Abstract

The relative maturity and competence of somatic embryos is often estimated on the basis of their morphologic similarity to various stages of immature zygotic embryo development. Morphologic abnormalities noted in soybean [Glycine max (L.) Merr.] somatic embryos are similar to those observed in zygotic embryos maturing in vitro and may reflect common interruptions of normal developmental processes. We provide here a more objective means of assessing the point(s) at which cultured embryos deviate from the normal embryogenical pathway by comparing the accumulation of the embryo-specific marker proteins (11S and 7S storage globulins, soybean agglutinin, and seed lipoxygenase) between somatic and immature zygotic embryos maturing in culture to zygotic embryos maturingin planta. Immature (heart-stage) soybean (cv. ‘McCall’) zygotic embryos were removed from the testa and cultured for 5, 15, or 45 days in nien modified Linsmaer-Skoog salts, 5% sucrose liquid medium. Somatic embryos were induced from immature cotyledon explants on a medium containing either naphthalene acetic acid or 2,4 dichlorophenoxyacetic acid (10 mg·liter−1). The measured level of the marker proteins present in cultured embryos never exceeded those observed in mature soybean seeds. During the culture period, immature zygotic embryos accumulated significant levels of all marker proteins except a 29 kDa soybean agglutinin associated with the final stages of seed maturationin planta. Somatic embryos of all morphologic classes exhibited similar levels of the marker proteins suggesting that morphology may not accurately represent the developmental state of the culture-derived embryos. Somatic embryos induced on naphthalene acetic acid-containing medium accumulated detectable levels of all maturation-specific marker proteins except the 7S β and 29-kD soybean agglutinin antigen and seemed similar in most respects to the cultured zygotic embryos. Embryos induced on 2,4-dichlorophenoxyacetic acid accumulated none of the mature 7S or 11S storage globulin subunits nor any soybean agglutinin antigen, and yet the synthesis of 7S and 11S precursor polypeptides was similar in both naphthalene acetic acid-and 2,4-dichlorophenoxyacetic acid-induced somatic embryos. These observations are consistent with the view that embryos induced on high 2,4-dichlorophenoxyacetic are arrested at a relatively earlier developmental stage than naphthalene acetic acid-induced embryos of similar morphology and may indicate that some external signal (e.g., abscisic acid or desiccation or both) is necessary for the transition to the late maturation stage of seed ontogeny.