RNA editing in plant organelles: a fertile field.

In 1989, three groups simultaneously and independently discovered RNA editing in the mitochondria of angiosperms (flowering plants; refs. 1-3). Two years later, H. Kossel and colleagues reported the same type of editing in angiosperm chloroplasts (4). In both cases, multiple site-specific changes from C to U occur in RNAs transcribed from the organellar DNA. These changes take place predominantly at first or second positions of codons, so that edited triplets specify a different amino acid than their unedited (genome-encoded) counterparts (reviewed in refs. 5-15). In contrast, sequence comparisons suggest that no such editing is required for gene expression in either mitochondria or chloroplasts of the liverwort, Marchantia polymorpha (a bryophyte; see refs. 16 and 17). Initially, therefore, it seemed that C-to-U RNA editing might be restricted to angiosperms (18), or at least to more recently evolved lineages within the plant kingdom (19). However, in more recent work, A. Brennicke and colleagues (20, 21) have shown that editing of mitochondrial transcripts is widespread within the land plants, occurring in all major groups, including the Bryophyta. In this issue of the Proceedings, Wakasugi et al. (22) present evidence that C-to-U editing of chloroplast transcripts is not restricted to angiosperms, but also occurs in a gymnosperm, Pinus thunbergii (black pine). Together with a recent report (23) of extensive RNA editing in the chloroplasts of the hornwort, Anthocerosformosae (another bryophyte), this work considerably extends the known distribution of plant chloroplast RNA editing, the phylogenetic range of which now parallels that of plant mitochondrial editing. Aside from their evolutionary significance, these new data implicate RNA editing in novel aspects of chloroplast gene expression. Moreover, they emphasize possible differences in the mechanism of editing and the determinants of its specificity in the two organelles and among different plant species.