[Structure and function of the genetic information in plastids : V. The absence of ribosomal rna from the plastids of the plastom-mutant 'mrs. parker' of Pelargonium zonale]

The discovery of plastid-to-nucleus retrograde signaling-a personal perspective

DNA and machinery for gene expression have been discovered in chloroplasts during the 1960s. It was soon evident that the chloroplast genome is relatively small, that most genes for chloroplast-localized proteins reside in the nucleus and that chloroplast membranes, ribosomes, and protein complexes are composed of proteins encoded in both the chloroplast and the nuclear genome. This situation has made the existence of mechanisms highly probable that coordinate the gene expression in plastids and nucleus. In the 1970s, the first evidence for plastid signals controlling nuclear gene expression was provided by studies on plastid ribosome deficient mutants with reduced amounts and/or activities of nuclear-encoded chloroplast proteins including the small subunit of Rubisco, ferredoxin NADP+ reductase, and enzymes of the Calvin cycle. This review describes first models of plastid-to-nucleus signaling and their discovery. Today, many plastid signals are known. They do not only balance gene expression in chloroplasts and nucleus during developmental processes but are also generated in response to environmental changes sensed by the organelles.

The isolation of apparently homoplastidic mutants induced by a nuclear recessive gene in Arabidopsis thaliana

The nuclear recessive gene, chm1, of Arabidopsis thaliana is a imitator that induces a variety of plastid alterations giving rise to mixed cells and variegated leaves. The variegation is maternally transmitted but chm1 is transmitted in a Mendelian fashion (Rédei 1973; Rédei and Plurad 1973). In order to characterize the different types of plastid alterations induced by chm1, isolating homoplastidic lines, each apparently containing one type of mutant plastid in its cells, was essential since such characterization cannot be carried out on mixed cells. We have used two genetic approaches to isolate several apparently homoplastidic mutant lines by the removal of the mutator from the genetic background, and the maternal transmission of the mutant plastids. The rapidity of obtaining homoplastidic lines in the absence of chm1 indicated a non-stochastic sorting-out of plastids in mixed cells. That each of the chm1-free homoplastidic mutant lines was apparently homoplastidic for one type of mutant plastids was confirmed by electron microscopic observations. Here we report, for the first time, the production of different homoplastidic lines in the absence of the nuclear-mutator gene. Such genetically-stable homogeneous material should be a useful tool for studying the molecular mechanism(s) by which chm1 induces a variety of heritable plastid alterations.

The deficiency of soluble proteins and plastid ribosomal RNA in the albino pollen plantlets of rice

The components of soluble protein and ribosomal RNA in green and albino pollen plantlets of rice were studied by means of polyacrylamide gel electrophoresis. The results were as follows: (1) Soluble protein: the soluble proteins in green pollen plantlets may be fractionated into 15 bands of varying intensities in which the highest content and the most prominent one is band 3 (fraction I protein). Band 3, however, is nearly absent in albino pollen plantlets. (2) Ribosomal RNA (rRNA): rRNA of high molecular weight in green pollen plantlets may be fractionated into 4 bands, i.e. 25S RNA and 18S RNA in cytoplasmic ribosomes, and 23S RNA and 16S RNA in plastid ribosomes. Little or no 23S RNA and 16S RNA, however, is found in albino pollen plantlets. Together with the evidence obtained by other workers, it is suggested that albino forms of pollen plantlets is caused by the impairment of DNA.