RNA editing changes posttranscriptionally single nucleotides in chloroplast-encoded transcripts. Although much work has been done on mechanistic and functional aspects of plastid editing, little is known about evolutionary aspects of this RNA processing step. To gain a better understanding of the evolution of RNA editing in plastids, we have investigated the editing patterns in ndhB and rbcL transcripts from various species comprising all major groups of land plants. Our results indicate that RNA editing occurs in plastids of bryophytes, fern allies, true ferns, gymnosperms, and angiosperms. Both editing frequencies and editing patterns show a remarkable degree of interspecies variation. Furthermore, we have found that neither plastid editing frequencies nor the editing pattern of a specific transcript correlate with the phylogenetic tree of the plant kingdom. The poor evolutionary conservation of editing sites among closely related species as well as the occurrence of single speciesspecific editing sites suggest that the differences in the editing patterns and editing frequencies are probably due both to independent loss and to gain of editing sites. In addition, our results indicate that RNA editing is a relatively ancient process that probably predates the evolution of land plants. This supposition is in good agreement with the phylogenetic data obtained for plant mitochondrial RNA editing, thus providing additional evidence for common evolutionary roots of the two plant organellar editing systems.

Four real-time polymerase chain reaction systems aiming at the specific detection and quantification of maize DNA are described. They have been developed in four independent laboratories targeting different maize sequences, i.e., alcohol dehydrogenase (Adh1), high mobility group protein (hmga), invertase A (ivr1), and zein, respectively. They were all fully specific, showing a very similar quantification accuracy along a number of distantly related maize cultivars and being either single or low copy number genes. They were highly sensitive and exhibited limits of quantification below 100 maize genomic copies. In consequence, they are considered suitable for use as maize specific endogenous reference genes in DNA analyses, including GMO quantitative tests.

The psbB operon contained in the plastomes of higher plants consists of the genes psbB, psbH, petB and petD. The primary transcript of this operon is subject to a series of processing steps which include cleavages resulting in four monocistronic mRNAs and splicing of the petB and petD transcripts. A search for editing sites within the two latter transcripts from maize led us to the detection of one editing site within the petB coding region which is conserved at the DNA level in other graminean species and in tobacco. This shows that editing must be considered as an additional processing step of the psbB operon encoded primary transcript. As is evident from cDNA sequences derived from the dicistronic and/or unspliced petB/D transcripts which are completely edited, editing is an early step of mRNA processing which precedes both splicing and cleavage to the monocistronic mRNAs and which must, therefore, be independent of the latter two steps. This conclusion is confirmed by a similar observation with the editing site of the rpl2 transcript which is contained in the polycistronic transcript of the rpoA operon, although here only partial editing is observed for the unspliced dicistronic rpl23/rpl2 transcript.