The Mitochondrial Influence on Nuclear Gene Expression in

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 flowe