Severe reduction of plastid DNA is characteristic of w2 mutant --Chang-deok Han and Edward H. Coe, Jr. The copy number of plastid genomes per cell increases dramatically during leaf formation and maturation of the leaf tissues (Lawrence and Possingham, Plant Physiol. 81:708, 1983; Miyamura et al., Protoplasma 133:66, 1986). It is still puzzling what is the significance and role of the multiplicity of plastid genomes in the expression and regulation of the plastid. A possible explanation for the multiplicity of the plastid genome has been proposed in terms of cellular demand for plastid ribosomes during leaf development (Bendich, BioEssays 6:279-282, 1987).

During studies on 'class II' albino mutants (Robertson, J. Hered. 1978), on which very little work has been done beyond the early studies by Lindstrom (1921) and Demerec (1924), we found evidence that the albinism of w2 plants might be due to severe reduction of the plastid DNAs. The relative content of the plastid DNA was measured by Southern blot analysis (Figure). Total cellular DNAs were extracted from the seedlings grown under very dim white light. BamHI-digested total cellular DNAs were probed at the same time with the nuclear rDNA, pGmr1, and 1.2 kb plastid BamHI fragment containing psaB. The signal intensities of the autoradiogram, exposed without an intensifying screen, were measured with a densitometer. After normalization with the hybridized signals of the nuclear rDNA, we found that the relative content of the plastid DNAs of the mutant decreases 20 fold. The general patterns of the transcripts of the plastid genes in the w2 + and w2 l mutants are relatively unaltered and the levels of the transcripts are uniformly reduced 10 - 20 fold. Since there is a good correlation between the levels of the transcripts and the relative content of plastid DNA, the limited expression of the w2-affected plastids might be due to the shortage of templates of the plastid genes. We propose that maintenance of a minimum num-ber of plastid genomes is necessary or required for the expression of plastids. Current studies are directed to ask whether the severe reduction of the plastid DNA in the mutant is tissue-dependent or is due to failure of the plastid division. Also it is worthwhile examining whether w2 also affects the level of mitochondrial DNA.

Figure. Southern analysis of the nuclear rDNAs and 1.2 kb plastid DNA containing psaB in w2 + and w2 l seedlings. The seedlings were exposed to very dim light (0.01 umol/m2.sec) for 1 day after 5 days darkness. Total cellular DNAs were extracted from the mutants (w2 + and w2 l). 5 ug of the DNAs were loaded in each lane. '+' indicates the normal siblings. The probes for the nuclear rDNA and plastid DNA were hybridized at the same time.


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