Distribution of Cytosolic mRNAs Between Polysomal and Ribonucleoprotein Complex Fractions in Alfalfa Embryos

Abstract

Cell-free translational and northern blot analyses were used to examine the distribution of storage protein messages in the cytoplasmic polysomal and mRNA-protein complex (mRNP) fractions during development of somatic and zygotic embryos of alfalfa . No special array of messages was identified in the mRNP fraction; however, some messages were selectively enriched in either the polysome or mRNP fractions, and their distribution pattern varied quantitatively during development of the embryos. During the earliest stages of somatic embryo development, storage protein messages already were present, but there was no detectable accumulation of the proteins. Selective enrichment of messages for the 11S, 7S, and 2S storage proteins occurred in the mRNP fraction during the globular, heart, and torpedo stages of somatic embryogenesis, but the distribution pattern was shifted toward the polysomal fraction at the beginning of cotyledon development. Thus, there was translational repression of storage protein synthesis at the early stage of somatic embryo development that was relieved later. During the cotyledonary development stages in the somatic and zygotic embryos, storage protein synthesis and distribution of the messages were similar in that these specific messages were predominantly in the polysomal fraction.immediately after fertilization, which may result from the unmasking of messages stored in repressed mRNPs (9). Stagespecific activation of different mRNA species is also common during early embryogenic development in the surf clam (22). The presence and importance of mRNPs in plants has not been subjected to intensive study, although there is ample evidence for the presence of 'stored' mRNAs in RNPs in the mature dry seed (10,18,20,24). Their presence during embryogenesis is uncertain, although it has been reported that the mRNA for the ,B subunit of 13-conglycinin increases several days before the accumulation of the protein (14). It is suggested in this case, however, that failure of the protein to accumulate is caused by its initial instability (23).There are marked quantitative and qualitative differences in storage protein accumulation between zygotic and somatic embryos, including those of alfalfa (Medicago sativa cv Rangelander RL-34) (13). To understand the differential regulation of protein synthesis in these two embryo types, we have followed the events that take place during development. Here we relate the extent of storage protein synthesis to the distribution of their specific mRNAs in the polysomal and mRNP, nonpolysomal fractions of the cells of developing embryos. Evidence for a translational control of protein synthesis during the early stages of somatic embryogenesis is presented. MATERIALS AND METHODS Somatic Embryo ProductionA highly embryogenic clone of alfalfa (Medicago sativa cv Rangelander RL-34) isolated by Meijer and Brown (16)

Synthesis of ribosomal proteins from stored mRNAs early in seed germination

“…The characterization of the stored mRNA has remained elusive for a long time, and just recently some of these mRNAs have been identified [1,22]. In the present research we identified RPs among the early products of c~-amanitin-treated germinating axes, which proved the presence of RP-mRNAs within the population of stored mRNAs.…”

Somatic Embryogenesis: A Model for Early Development in Higher Plants

“…Stored mRNAs appear to support embryo development until the eight-cell stage in the fern Marsilea vestita (Kuligowski et al, 1991), and maternal mRNAs are activated in early zygote development in Fucus (Masters et al, 1992). In addition, Pramanik et al (1992) demonstrated that storage protein synthesis in alfalfa somatic embryos is translationally regulated; the mRNA is present but restricted to the nontranslated mRNP pool early in embryogenesis (globular-tetorpedo stages) and is then shifted to the polysome pool at the cotyledon stage. Moreover, posttranscriptional regulation, including differential transcript stability, has been suggested to be an important component of embryo mRNA accumulation in zygotic embryos (Walling et al, 1986).…”

Somatic Embryogenesis in Grapevine

“…Development of somatic embryogenesis has been described to follow patterns similar to the ones of zygotic embryogenesis (Konar et ai., 1972;Altamura et ai., 1992); however, in studies conducted on model plants, it proved to be more variable in early division patterns (McWilliam et ai., 1974;Dos Santos, 1983) and further morphological development (Toonen et ai., 1994;Mordhorst et al, 1997), cellular organell composition (Taylor and Vasil, 1996), and gene expression as well (Pramanik and Krochko, 1992; In model plants, somatic embryogenesis is the most utilized model system for plant cell totipotency and developmental studies, while it is one of the most powerful techniques offered by tissue culture for genetic improvement of species important to agriculture.…”