THE EXPRESSION OF NUCLEAR GENES ENCODING PLASTID RIBOSOMAL-PROTEINS PRECEDES THE EXPRESSION OF CHLOROPLAST GENES DURING EARLY PHASES OF CHLOROPLAST DEVELOPMENT

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 (rps1, rps22, rpl21, and rpl40) 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 rpl21 gene starts on the first imbibition day. Inhibition 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 rpl21 is independent of a plastid signal.