The mitochondrial influence on nuclear gene expression in cytoplasmic male-sterile Brassica napus

Phenotypic, genetic and molecular studies were made on Brassica napus (Arabidopsis thaliana) lines carrying mitochondria with DNA from both B. napus and A. thaliana. The lines were isogenic regarding the nuclear and chloroplast genomes, which consisted of B. napus DNA. Most lines contained large but variable portions of A. thaliana mitochondrial DNA detected by a dense set of A. thaliana specific markers. Several of the A. thaliana sequences gave rise to novel transcripts. The lines were divided into three classes; male-sterile, semi-sterile and fertile according to their stamen morphology. The flowers of the male-sterile lines were characterized by replacement of stamens with carpeloid organs, which resemble the apetala3 (ap3) and pistillata (pi) mutants found in A. thaliana. The AP3 and PI gene expression were lower in the male-sterile lines. The AP3 expression was down-regulated in the stamens shortly before these organs developed carpeloid characteristics. Repression of PI succeeded that of AP3 and might be a consequence of loss of AP3 activity. Low levels of AP3 and PI proteins were found in a male-sterile line. These results suggest that AP3 expression in stamens depends on proper mitochondrial function and correct nuclear–mitochondrial interactions. To study the nuclear gene expression profiles of flower buds in the lines, two different types of microarrays were used. Gene expression profiling revealed that a large number of genes differed in expression between the lines. These results show that the mitochondrial genome strongly influences nuclear gene expression and reveal the importance of retrograde signalling between the mitochondria and the nucleus. In addition, the mitochondria directly or indirectly influence several different aspects of plant development and metabolism, since not only flower morphology but also growth rate, flowering time and adenylate content were affected. An additional CMS-system, B. napus (Ogu-INRA) was also studied. The conclusion is that the two CMS-systems differ in expression at stage 8, likely due to timing. The nuclear gene expression of the B. napus (A. thaliana) CMS-line was altered already at stages 0-5 while the expression of the Ogu-INRA CMS-line was altered at stage 8. This was reflected in the phenotypes of the two different CMS-lines