Functional characterization of the two ferrochelatases in Arabidopsis thaliana

The enzyme ferrochelatase catalyses the formation of protoheme by inserting Fe2+ into protoporphyrin IX. Although most organisms express only one ferrochelatase, all land plants analysed so far possess at least two ferrochelatase proteins. Analysis of publicly available expression data suggests that the two Arabidopsis thaliana ferrochelatases, FC1 and FC2, serve different functions, corroborating previous assumptions. Co-expression analysis of FC1 and FC2, together with microarray analyses, implies that fc1 and fc2 trigger different modes of plastid signalling in roots and leaves, respectively, and indicates that FC2 might be involved in stress responses. Thus, loss of FC2 increases resistance to salt and flagellin treatment. Whereas fc1 plants showed no obvious mutant phenotype, fc2 mutants formed abnormally small, pale green rosette leaves; were low in chlorophylls, carotenoids and several photosynthetic proteins; and their photosynthetic performance was impaired. These phenotypes are attenuated by growth in continuous light, in agreement with the finding that fc2 plants accumulate protochlorophyllide and display a fluorescent (flu) phenotype in the dark. In consequence we show that, contrary to earlier suggestions, FC2 produces heme not only for photosynthetic cytochromes, but also for proteins involved in stress responses, whereas the impairment of FC1 apparently interferes only marginally with stress responses. While most organisms express only one ferrochelatase (FC) protein, land plants possess at least two. We therefore set out to obtain further insights into the functions of FC1 and FC2 in Arabidopsis. In silico analyses, microarray data and phenotypic characterization of the knockdown mutants fc1-1, fc2-1 and fc1-1 fc2-1 point to a more prominent role for FC2 than for FC1 in responses to abiotic and biotic stresses. Furthermore, in fc2-1 knockdown plants, photosynthetic activity is disrupted and the photosensitizing pigment protochlorophyllide (Pchlide) accumulates in the dark, providing evidence for a flu (fluorescent) phenotype of the fc2 mutant.