Genetic Variation Revealed in the Chloroplast-Encoded RNA Polymerase β' Subunit of Downy Mildew-Resistant Genotype of Opium Poppy

Two accessions of opium poppy, Pps-1 (dark green leaves, highly resistant to downy mildew [DM]) and H-9 (yellowish green leaves, susceptible to DM), which originated from common progenitor SPS49 were selected, and their F₁ and F₂ progenies showed that leaf color trait was governed by single recessive nuclear gene, whereas DM resistance appeared to be the interaction between cytoplasmic and nuclear genes. Chloroplast DNA (cpDNA) analysis of these 2 accessions through arbitrarily-primed polymerase chain reaction generated a unique fragment in Pps-1. Subsequent sequence analysis upon cloning of this cpDNA fragment revealed its similarity with the plastid-encoded RNA polymerase β' subunit (rpoCI). Full-length rpoCI DNA was therefore isolated from both the genotypes that was 2707 bp long with a 658-bp intron (436-1093) and a 2049-bp open reading frame encoding 682 amino acid long polypeptide. Comparative sequence analysis of the rpoC1 gene from both the genotypes, revealed 4 single-nucleotide substitutions at 4 positions that caused 3 amino acid changes in the protein sequence--1) A to C transversion at position 825 (Glu275Asp), 2) A to G transition at position 1203 (Ile401Met), and 3) T to C transition at position 1422 and G to A transition at position 1423 both in same codon of the reading frame (Ala475Thr). This investigation is the first report indicating base substitution changes in the plastid-encoded rpoCI gene in DM-resistant genotypes of opium poppy. This finding may lead to implication of possible role of RNA polymerase β' subunit in resistance to DM caused by Peronospora arborescens.