A NONPHOTOSYNTHETIC FERREDOXIN GENE IS INDUCED BY ETHYLENE IN CITRUS ORGANS

The sequence and expression of mRNA homologous to a cDNA encoding a no n-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-photosy nthetic than with photosynthetic ferredoxins, (2) higher expression in tissues containing plastids other than chloroplast such as petals, yo ung fruits, roots and peel of fully coloured fruits, and (3) the absen ce of light-dark regulation characteristic of photosynthetic ferredoxi ns. In a phylogenetic tree constructed with higher-plant ferredoxins, Citrus fruit ferredoxin clustered together with root ferredoxins and s eparated from the photosynthetic ferredoxins. Non photosynthetic (root and fruit) ferredoxins, but not the photosynthetic ferredoxins, have their closest homologs in cyanobacteria. Analysis of ferredoxin genomi c organization suggested that non-photosynthetic ferredoxins exist in Citrus as a small gene family. Expression of Fd1 is developmentally re gulated during flower opening and fruit maturation, both processes may be mediated by ethylene in Citrus. Exogenous ethylene application als o induced the expression of Fd1 both in flavedo and leaves. The induct ion of non-photosynthetic ferredoxins could be related with the demand for reducing power in non-green, but biosynthetically active, tissues .