ALTERED GENE-EXPRESSION DURING SOMATIC EMBRYOGENESIS IN NICOTIANA-PLUMBAGINIFOLIA AND DIGITALIS-LANATA

Somatic embryogenesis in Nicotiana plumbaginifolia was elicited by protoplast release from leaf mesophyll cells. In the presence of 2,4-dichlorophenoxyacetic acid (2,4-D), clusters of loosely attached cells designated proembryogenic masses (PEMs) appeared. In the absence of 2,4-D, globular and heart-like embryos developed successively from which viable plantlets emerged. Gene expression during somatic embryogenesis was analyzed by in vivo and in vitro protein synthesis. As demonstrated by Coomassie staining and autoradiography of S-35-methionine-labeled polypeptides, three main protein classes could be distinguished: (i) embryo-specific proteins, (ii) PEM-specific proteins and (iii) constitutive polypeptides. Different subclasses of PEM- and embryo-specific proteins were defined, whose members changed in their rate of synthesis coordinately. The differential induction and decay kinetics of embryo- and PEM-specific proteins, respectively, correlated with corresponding alterations at the mRNA level suggesting a transcriptional mode of control. A detailed analysis between the in vivo rate of synthesis and the translatable mRNA level was made for those polypeptides which had a counterpart among the in vitro translation products. From the comparison, it became evident that an additional post-transcriptional component is probably involved in regulating the synthesis of these polypeptides. Embryo-specific mRNAs were traced by Northern blot hybridization of RNA with a synthetic late embryogenesis abundant (Lea) gene-specific oligonucleotide probe [1]. Two transcript species were detected at a T(m)-15-degrees-C criterion. They were present in low levels in PEMs, increased in abundance upon formation of globular embryos and disappeared in heart-like stages. A very similar expression pattern of Lea transcripts and of certain in vitro-translatable mRNAs was found for an embryogenic cell line of Digitalis lanata, strain VII [2], suggesting that common embryogenesis-related gene expression programs were realized in both plant species.