The genetic control of ribosomal RNA accumulation in flax genotrophs

Abstract

The relationship between rRNA gene dosage, rRNA synthesis and various phenotypic characters was studied in environmentally induced phenotypically and genetically distinct flax lines. Large (L) and small (S) types had 2210 and 1110 rRNA genes respectively, assessed by rRNA/DNA hybridization. The mean fresh weight of mature L and S plants was 165 g and 98 g, respectively. Root growth rates were linear during the first five days after germination, showing a mean value of 1·35 cm/day for L and 0·77 cm/day for S. Accumulation of RNA showed a parallel difference with 2·46 µg/day in L and 1·48 µg/day in S. At two-three days after germination the rate of synthesis of RNA (assessed by either (32P)-orthophosphate incorporation or by (3H)-adenosine incorporation and measurement of the specific activity of the ATP pools by the luciferin/luciferase assay) was not significantly different between L and S types. However Polyacrylamide gel electrophoretic analysis of the (32P)-rRNA indicated a 25 per cent greater proportion of RNA synthesis devoted to production of pre-rRNA in L compared with S types. In addition there is evidence of more efficient processing of the pre-rRNA in S types but greater stability of the mature rRNAs of L lines. These post-transcriptional controls appear to be mainly responsible for the greater accumulation of rRNA in phenotypically large types. The rRNA gene number changes and corresponding changes in transcription of pre-rRNA appear to be relatively unimportant in regulating the accumulation of rRNA.