Reduced expression of the proteolytically inactive FtsH members has impacts on the Darwinian fitness of Arabidopsis thaliana

FtsH (filamentation-temperature-sensitive protein H) proteases are a family of membrane-bound enzymes present in eubacteria, animals, and plants. Besides the 12 genes encoding proteolytically active members of the FtsH family in the genome of Arabidopsis, there are five genes coding for members that are assumed to be proteolytically inactive due to mutations in the protease domain; these are termed FtsHi (i for inactive). Despite their lack of proteolytic activity, these FtsHi members seem to be important for chloroplast and plant development as four out of five homozygous knockout-mutants of FtsHis are embryo-lethal. Here, we analysed the Darwinian fitness of weak homozygous (ftshi1,3,4) and heterozygous (ftshi/FTSHi2,4,5) mutants. We compared the growth and development of these mutants to their respective wild-type Arabidopsis plants under controlled laboratory conditions and in the field, and we also evaluated the photosynthetic efficiency by pulse-amplitude modulation fluorescence. Homologous genotypes were subjected to various stress conditions in a greenhouse and gene co-expression as well as phylogenetic analyses were performed. Analysis of the gene-expression network of the five FTSHi genes indicated common clusters with genes encoding FtsH12, OTP51, and methylase. Phylogenetic analyses pointed to a common evolution (and common disappearance in grasses and gymnosperms) of FtsH12 and multiple presumably proteolytically inactive FtsHi enzymes. Our data show that the FtsHi enzymes are highly important during the seedling stage and for Darwinian fitness analyses in semi-natural conditions.