The development of different plant organs (root, hypocotyl, and cotyledons) during seed germination is connected with the transformation of proplastids, which are found in embryonic and meristematic tissues, into amyloplasts in root tissues and into chloroplasts in cotyledons. We have analyzed the expression of nuclear and plastid genes coding for the plastid translational apparatus during the first 7 d of Spinacia oleracea development. Results show that the nuclear genes (rpsl, rps22, rp121, and rp140) are expressed from the 1st d of seed imbibition and precede transcription of the chloroplast-encoded genes (photosynthetic and nonphotosynthetic), which starts the 3rd d after the beginning of imbibition. Transcription from the leaf-/cotyledon-specific P1 promoter of the rp121 gene starts on the first imbibition day. lnhibition of chloroplast biogenesis by bleaching in the presence of norflurazon has no influence on the expression from this P1 promoter, suggesting that the onset of transcription of nuclear gene 41/21 is independent of a plastid signal.Plant plastids probably represent the most metamorphic organelles of the living kingdom. Different plastid forms are characterized by tissue-or organ-specific functions such as photosynthesis in chloroplasts of leaves and cotyledons, synthesis and/or storage of starch in amyloplasts of roots and seeds, and carotenoid accumulation in fruit chromoplasts. AI1 plastid types arise during plant development from proplastids, which are small, undifferentiated plastids found in meristematic tissues. They can also originate from the interconversion of plastid types (Schnepf, 1980).In the present paper, we focus on early phases of chloroplast biogenesis during seed imbibition, germination, and seedling growth. Late phases of chloroplast development in leaves have been analyzed at the molecular level (Gruissem, 1989). In particular, the expression of nuclearand chloroplast-encoded photosynthetic genes has been studied during dark to light transition. In contrast, less is known about the molecular level of early phases of chloroplast development. The transcriptionl translation apparatus is synthesized during the early phases of development, as suggested by the presence of numerous ribosomes in etioplasts of dark-grown leaves. Also, it has been shown * Corresponding author; e-mail rmache@bio.grenet.fr; fax 33-76-51-4336, that the conversion of proplastids into chloroplasts is accompanied by high transcription levels of chloroplast genes encoding the transcription/translation apparatus. In contrast, the chloroplast photosynthetic genes are highly expressed only later in development (Bisanz-Seyer et al., 1989;Baumgartner et al., 1993).The nuclear and chloroplast genetic systems participate in a coordinate manner in the synthesis of protein complexes present within plastids. Plastid biogenesis and interconversion are tightly coupled with temporal and spatia1 stages of plant development and are thought to be under the control of both nuclear and plastid genes (Taylor, 1989;Bogorad, 1...

The expression of components of the 70S plastid ribosome has been determined during the first 13 days of spinach plant development. Total cellular RNA and proteins were used to determine the relative steady-state levels of mRNA for ribosomal proteins (r-proteins) by dot blot hybridization and the relative amounts of proteins by immunodetection with specific antibodies. The 16S rRNA as well as mRNAs for 9 out of 11 proteins studied, including those for the 32 kDa polypeptide of photosystem II and the large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase (Rubisco) show a marked increase at the beginning of the germination (day 5). At this time the plastid DNA content increases from 4% to 6% of total DNA content and so the plastome copy number can only in part account for the important increase in mRNA steady-state levels. Interestingly the transcripts of the rpl23 and rps19 genes show a different accumulation pattern, indicating either a differential gene transcription and/or an increased stability of the transcripts. In the western blot analysis a group of r-proteins can be detected in dry seeds or after 24 hours of imbibition while a second group of proteins accumulates after 3 to 5 days of development. The differential accumulation pattern of r-proteins and mRNA for r-proteins indicates that post-transcriptional control plays an important role in plastid r-protein synthesis.

A full size cDNA clone encoding the chloroplast ribosomal protein L21 from spinach is presented. The identity of the clone and the location of the transit peptide processing site were determined by comparison with the N-terminal amino acid sequence of the spinach chloroplast protein CS-L7 previously identified. L21 r-protein sequences from spinach, Marchantia polymorpha and Escherichia coli are compared. Quite surprisingly, the data do not suggest that the rpl21 nuclear gene from spinach was derived through intracellular gene transfer from the chloroplast genome. The possibility of a mitochondrial origin for rpl21 gene of spinach is discussed.

We have isolated and analysed a 2 kb region of the mitochondrial genome of Arabidopsis thaliana (Columbia) showing a high level of nucleotide identity with the mitochondrial (mt) rps14 small-subunit ribosomal protein gene from Oenothera berteriana and Vicia faba, as well as with an open reading frame (ORF) located upstream of the nad3 locus in O. berteriana. The rps14 locus is present as a single copy in the A. thaliana mt genome and has a translational stop codon located near the initiation codon, as well as a deletion of one nucleotide that disturbs the coding sequence. The cloning and sequencing of nine amplified mt rps14 cDNAs clearly demonstrated that this gene is transcribed and that the mRNA precursors are edited at three positions, all involving C-to-U conversions. No editing events changing the stop codon and restoring the correct coding sequence were witnessed within the 9 individual cDNA clones. Therefore, we conclude that the single rps14 sequence of the mitochondrial genome from A. thaliana is in fact a pseudogene that is transcribed and edited but not translated.