Deletion of an organellar peptidasome PreP affects early development in Arabidopsis thaliana

A novel peptidasome PreP is responsible for degradation of targeting peptides and other unstructured peptides in mitochondria and chloroplasts. Arabidopsis thaliana contains two PreP isoforms, AtPreP1, and AtPreP2. Here we have characterized single and double prep knockout mutants. Immunoblot analysis of atprep1 and atprep2 mutants showed that both isoforms are expressed in all tissues with the highest expression in flowers and siliques; additionally, AtPreP1 accumulated to a much higher level in comparison to AtPreP2. The atprep2 mutant behaved like wild type, whereas deletion of AtPreP1 resulted in slightly pale-green seedlings. Analysis of the atprep1 atprep2 double mutant revealed a chlorotic phenotype in true leaves with diminished chlorophyll a and b content, but unchanged Chl a/b ratio indicating a proportional decrease of both PSI and PSII complexes. Mitochondrial respiratory rates (state 3) were lower and the mitochondria were partially uncoupled. EM pictures on cross sections of the first true leaves showed aberrant chloroplasts, including less grana stacking and less starch granules. Mitochondria with extremely variable size could also be observed. At later developmental stages the plants appeared almost normal. However, all through the development there was a statistically significant decrease of ~40% in the accumulated biomass in the double mutant plants in comparison to wild type. In mitochondria, deletion of AtPreP was not compensated by activation of any peptidolytic activity, whereas chloroplast membranes contained a minor peptidolytic activity not related to AtPreP. In summary, the AtPreP peptidasome is required for efficient plant growth and organelle function particularly during early development.