Chloroplast development affects expression of phage‐type RNA polymerases in barley leaves

Summary We have identified the barley gene and cDNA encoding the plastid phage‐type RNA polymerase (RNAP), nuclear‐encoded plastid RNAP (RpoTp), and the nearly full‐length cDNA of the mitochondrial RNAP, nuclear‐encoded mitochondrial RNAP (RpoTm). RpoTp spans more than 9000 nt, consists of 19 exons and 18 introns, gives rise to a 3632‐nt mRNA and is localized to the long arm of chromosome 1 (7H). The length of the deduced polypeptide is 948 residues. The mRNA levels of RpoTp and RpoTm were determined in roots and primary leaf sections of 7‐day‐old barley seedlings of the albostrians mutant, which were either phenotypically normal and exhibited a gradient of chloroplast development, or contained ribosome‐deficient undifferentiated plastids. Transcript levels of RpoTp and RpoTm in almost all sections reached higher concentrations in plastid ribosome‐deficient leaves than in the wild‐type material, except in the most basal part of the leaf. These data indicate a role of plastid‐to‐nucleus signalling in the expression of the two RpoT genes. The mRNA levels of the plastid genes, β‐subunit of plastid‐encoded RNAP (rpoB), proteolytic subunit of the Clp protease (clpP) and ribosomal protein Rpl2 (rpl2), all known to be transcribed by the nuclear‐encoded RNAP (NEP), followed closely the pattern of RpoTp mRNA accumulation, strongly suggesting that RpoTp and NEP are identical. Transcripts of RpoTm and RpoTm‐transcribed mitochondrial genes cytochrome oxidase subunit 2 (coxII) and ATPase subunit 9 (atp9) accumulated to the highest levels in the most basal parts of the leaf and declined considerably towards the leaf tip with a pronounced reduction in green versus white leaves. Our data revealed a marked influence of the developmental stage of the plastid on the expression and activity of organellar phage‐type RNAPs and their target genes. Thus, interorganellar cross‐talk in the regulated expression of nuclear‐encoded plastid and mitochondrial RNAP genes might be a key element governing the concerted expression of genes located within plastids, mitochondria and the nucleus of the plant cell.