A non-photosynthetic ferredoxin gene is induced by ethylene in Citrus organs

The sequence and expression of mRNA homologous to a cDNA encoding a non-photosynthetic ferredoxin (Fd1) from Citrus fruit was investigated. The non-photosynthetic nature of this ferredoxin was deduced from: (1) amino acid sequence alignments showing better scores with non-photosynthetic than with photosynthetic ferredoxins, (2) higher expression in tissues containing plastids other than chloroplast such as petals, young fruits, roots and peel of fully coloured fruits, and (3) the absence of light-dark regulation characteristic of photosynthetic ferredoxins. In a phylogenetic tree constructed with higher-plant ferredoxins, Citrus fruit ferredoxin clustered together with root ferredoxins and separated from the photosynthetic ferredoxins. Non photosynthetic (root and fruit) ferredoxins, but not the photosynthetic ferredoxins, have their closest homologs in cyanobacteria. Analysis of ferredoxin genomic organization suggested that non-photosynthetic ferredoxins exist in Citrus as a small gene family. Expression of Fd1 is developmentally regulated during flower opening and fruit maturation, both processes may be mediated by ethylene in Citrus. Exogenous ethylene application also induced the expression of Fd1 both in flavedo and leaves. The induction of non-photosynthetic ferredoxins could be related with the demand for reducing power in non-green, but biosynthetically active, tissues.