Plastid 23S rRNA is present in ij seedlings, not in maternal exception seedlings

To understand the behavior and genetics of plastids subject to the genetic lesion of the nuclear gene ij, we reexamined whether 23S plastid rRNA in white seedlings of ij and of maternal exceptions could be detected by using RNA blotting techniques. Because 23S rDNA has been reported to contain plastid-specific sequences (D.B. Stern and D.M. Lonsdale, Nature, 1982) and the plastid genome has coding capacity for some of the subunits of RNA polymerase, measuring the transcripts of 23S rDNA will be informative in understanding the capability of the transcription and translation apparatus of plastids in ij and maternal-exception seedlings. To minimize photodamage that usually occurs in pigment-deticient mutants, the seedlings were grown in darkness for 3 days and then in dim light (0.01mmol/sec cm-2, 28C) for 3 days. Total cellular RNAs were examined for this study. Samples from ij seedlings were selected that were pure white to the eye.

The DNA probe for the 23S rRNA was kindly supplied by Dr. K. Oishi (University of Arizona). As shown in the Figure, yellow (ij l) and white (ij +) seedlings contain 23S rRNAs, but maternal-exception (+ /ij) whites and yellows do not contain any detectable level of rRNAs. We also examined the transcripts in 6-day-old white maternal seedlings that were grown and harvested in darkness. There was no detectable signal even after long exposure of the blot. Also we found that transcripts of 6-day high light (97 mmol/sec cm-2) ij yellow seedlings are present in as high amount as those grown in dim light. Our data extend the previous report by V. Walbot and E.H. Coe (PNAS, 1978) by showing that ribosomal RNA is actually present in white or yellow tissue of ij plants.

We have shown genetic evidence for reversibility of defective plastids in ij plants (Genetics Abstract, 1987). Once the ij gene back-mutates in some tissues of ij, ij-affected plastids in the clone are converted to normal green plastids. In contrast, ij-like patterns have never been observed in maternal-exception seedlings.

Is the detectable amount of 23S plastid rRNA in the ij seedlings due to low activity of expression of the plastid genome, or just due to green plastids in heteroplastidic cells of ij-affected tissues (Thompson et al., Amer. J. Bot. 1983)? Since there is no detectable level of plastid rRNA in maternal -exception seedlings, if two kinds of plastids (functional green and nonfunctional white or yellow plastids) are involved in organelle transmission from the ij female parent, maternal-exception seedlings should result from complete sorting-out of one type of plastids from the other. But distribution of maternal -exception seedlings in ear maps shows polarity (Coe et al., Stadler Symp., 1982) rather than following the lineage of ij-affected white tissues. Such arguments raise the question whether ij-affected plastids may fail to follow properly the developmental mechanism imposing positional distribution of plastids during embryogenesis. We will be examining those possibilities.

In any event, our data on maternal-exception seedlings, which show no plastid rRNA and no different morphology of leaves from green siblings, indicate that the plastid genomic compartment alone takes little, if any, part in leaf ontogeny and development. (The RNA work in this study has been done in Dr. Mary Polacco's lab. Expression of nuclear genes, responding to ij-affected plastids, is being examined in cooperation with Drs. Mary Polacco and Bill Patrie.)

Chang-deok Han and Ed Coe


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