Knockout of frataxin gene causes embryo lethality in Arabidopsis

Friedreich ataxia (FRDA) is an autosomal recessive neurological disease caused by a deficiency in frataxin, a mitochondrial targeted protein. Different roles proposed for frataxin include involvement in the process of iron homeostasis and storage, stimulation of oxidative phosphorylation, as well as iron-chaperone activity. Recently, a frataxin homolog has been identified in Arabidopsis, named AtFH. We applied the in situ hybridisation tecnique on Arabidopsis flowers prior to fertilization, as well as on embryos at different Days After Pollination (DAP): AtFH is expressed in ovule primordia as well as in stamen prior to fertilization. Also, AtFH transcript accumulates in embryos at different stages up to mature seeds. We analysed two independent Arabidopsis insertional mutant lines with T-DNA inserted in the coding region of AtFH gene: the genotypic class atfh/atfh (homozygous for the T-DNA insertion) is missing; also, the segregation ratio of the expected remaining genotypic classes AtFH/AtFH:AtFH/atfh is 1:2. The heterozygous AtFH/atfh plants show no obvious phenotype; however microscopy inspection of siliques produced from AtFH/atfh plants, at different DAP, reveals that indeed around ¼ of the seeds in the siliques of AtFH/atfh plants are dramatically reduced in size and do not reach maturity. Analysis by Nomarski optics of seeds from immature siliques of AtFH/atfh plants reveals that embryos in defective seeds arrest at globular stage. All these evidences suggest that such defective embryos truly correspond to the missing genotypic class atfh/atfh. Treatment with 100 or 500 microM Fe-citrate, able to induce the accumulation of the Arabidopsis AtFer1 ferritin isoform in liquid cell cultures, does not affect frataxin transcript levels. To explore the relationship between plant frataxin and ferritin, crosses between heterozygous AtFH/atfh and Arabidopsis lines overexpressing ferritin have been performed and progeny analysed.