Newly Synthesized mRNA Is Translated during the Initial Imbibition Phase of Germinating Maize Embryo

Abstract

RNA synthesis is activated in the cells of the plant embryo very soon after the start of seed imbibition. We previously reported that mainly heterogeneous nuclear RNA is synthesized in the radicle of Zea mays embryo during the first hours of germination. The present study was undertaken in order to detect the time of appearance of the newly synthesized messenger RNA in the polysomes of germinating maize axes.Free polysomes were prepared from embryonic axes rehydrated for 2 hours in the presence of radioactively labeled uridine. These polysomes were shown to be labeled and to contain labeled particles sedimenting, after dissociation with EDTA, in the 1OS to 40S region of a sucrose gradient. The labeled polysomal RNA migrates heterogeneously in a gel with a mean size corresponding to about 16S, and 60% of these molecules are polyadenylated.The data indicate that the newly synthesized RNA associated with the polysomes after 2 h of germination consists of messenger RNA molecules. Analysis of the polysomes prepared 0.5 and 1 h after the start of imbibition suggests that translation of the newly synthesized messenger RNA probably occurs within the 1st hour of imbibition of the isolated axis, thus well before the completion of the initial water uptake.Rehydration of a quiescent embryo under appropriate conditions induces the rapid reactivation of protein synthesis (20). Several studies showed that this phenomenon is not affected by the presence of transcription inhibitors (4,7,11,26,29). These results suggest that, during early germination, protein synthesis is not dependent on de novo mRNA synthesis, but rather is the consequence of polysome formation from preformed mRNA. This interpretation is supported by the fact that mRNA extracted from quiescent seeds is translatable in vitro (9).The effect oftranscription inhibitors on the synthesis ofprotein was examined at different times after the start of imbibition. The results suggest that translation of newly synthesized mRNA occurs after a delay of several hours (4,5,7,11 molecules (27). In this material, the initial water uptake is completed after 30 min (19). Similar observations were made in cotton embryos and radish axes labeled at a more advanced stage of germination (8, 11). It is surprising that these observations were not extended to the period of initial water uptake because they suggest that, in addition to the translation of preformed mRNA, new transcripts assume a functional role in protein synthesis soon after the start ofimbibition. Recently, the possible significance of the newly synthesized mRNA was pointed out by quantitative determination ofthe mRNA content in germinating tissues. It was shown that the synthesis ofgluconogenic organelles in germinating castor bean endosperm is accompanied by a large increase in translatable mRNA (25). A similar increase was also described in cotyledons ofgerminating cucumber and it was also shown, in the same tissue, that the rise in activity of isocitrate lyase and malate synthase is preceded by an increase in ...

Regulation of Gene Expression in Developing Zea mays Embryos

“…The presence of newly synthesized mRNA associated with polysomes in maize embryos is already detected after 1 or 2 h of germination (10). Nevertheless, in our case the synthesis of most of the ES0 polypeptides during the first 2 h of imbibition of dry embryos suggests a significant role for stored mRNA shortly after the onset ofimbibition.…”

“…In this respect, research in maize seeds has been focused on the period of rehydration of quiescent embryos in an attempt to clarify the role of stored mRNA in the seed embryos (10,24). However, no comparable data are available for maize embryogenesis and early germination.…”

Long-lived Messenger RNA and its Relationship to Protein Synthesis During Germination of Pea (Pisum sativum L.) Seeds

“…ribosomal, transfer and messenger. In this respect pea resembles rye, wheat, oil-seed rape, radish and lentil (Sen, Payne and Osborne, 1975;Delseney et al, 1977a;Payne, 1977;Brooker, Tamaszewski and Marcus, 1978;Payne et al, 1978;Van de Valle et al, 1983) and contrasts with maize, in which early RNA synthesis appears to involve only mRNA (Dommes and Van de Walle, 1983). Interestingly however, in pea the proportion of label in mRNA was very much higher in the early stages of germination than later (Tables 2 and 3) giving rise to the possibility that there is some preferential synthesis of mRNA in these early stages.…”

“…772 Greenway et al-Long-lived Messenger RNA In addition to the stored or long-lived tnRNA, there is also evidence from a few species that synthesis of mRNA is renewed very early in germination (Spiegel, Obendorf and Marcus, 1975;Delseney et al, 1977a;Payne, 1977;Dommes and Van de Walle, 1983;Van de Walle, Deltour and Forgeur, 1983) and by measuring the time taken for mRNA labelled in early germination to reach polysomes, Dommes and Van de Walle (1983) estimated that newly synthesized mRNA may support protein synthesis in vivo within 2 h of the onset of imbibition in maize embryos. However, the demonstration that protein synthesis occurs much sooner than this in several species (Walbot 1971;Maherchandani and Naylor, 1972;Bray and Dasgupta, 1976;Delseney et al, 19776), suggests that at least a propertion of the mRNA present in the dry seed is involved in protein synthesis during the early stages of germination.…”

On the Role of Stored mRNA in Protein Synthesis in Embryonic Axes of Germinating Vigna unguiculata Seeds

“…4). The stored mRNA found in radish embryonic axes (7), wheat embryos (4,24), maize embryos (8), and castor bean seeds (17,21) Recently, Sanchez de Jimenez and Aguilar (22) reported that, in the axes ofgerminating maize embryos, the changes in proteins observed between the periods of 0 to 6 h and 18 to 24 h post imbibition seem to be due to newly synthesized mRNA as well as to delayed translation of stored mRNA species. These observations agree with those reported in this paper.…”