Characterization of a ferritin mRNA from Arabidopsis thaliana accumulated in response to iron through an oxidative pathway independent of abscisic acid.

A ferritin cDNA, AtFer1, from seedlings of Arabidopsis thaliana has been characterized. The deduced amino acid sequence of the AtFer1 protein indicates that A. thaliana ferritin shares the same characteristics as the plant ferritin already characterized from the Leguminosae and Graminacea families: (i) it contains an additional sequence in its N-terminal part composed of two domains: a transit peptide responsible for plastid targeting and an extension peptide; (ii) amino acids that form the ferroxidase centre of H-type animal ferritin, as well as Glu residues characteristic of L-type animal ferritin, are conserved in AtFer1; (iii) the C-terminal part of the A. thaliana ferritin subunit defining the E-helix is divergent from its animal counterpart, and confirms that 4-fold-symmetry axis channels are hydrophilic in plant ferritin. Southern blot experiments indicate that AtFer1 is likely to be encoded by a unique gene in the A. thaliana genome, although a search in the NCBI dbEST database indicates that other ferritin genes, divergent from AtFer1, may exist. Iron loading of A. thaliana plantlets increased ferritin mRNA and protein abundance. In contrast to maize, the transcript abundance of a gene responding to abscisic acid (RAB18) did not increase in response to iron loading treatment, and A. thaliana ferritin mRNA abundance is not accumulated in response to a treatment with exogenous abscisic acid, at least in the culture system used in this study. In addition, iron-induced increases in ferritin mRNA abundance were the same as wild-type plants in abi1 and abi2 mutants of A. thaliana, both affected in the abscisic acid response in vegetative tissues. Increased AtFer1 transcript abundance in response to iron is inhibited by the antioxidant N-acetylcysteine. These results indicate that an oxidative pathway, independent of abscisic acid, could be responsible for the iron induction of ferritin synthesis in A. thaliana.