Temporal and transient expression of stearoyl-ACP carrier protein desaturase gene during olive fruit development

Abstract

To study the regulation of gene expression during olive (Olea europaea) fruit development, a full length cDNA encoding the stearoyl-ACP desaturase (EC 1.14.99.6), which catalyses the conversion of saturated stearic acid to mono-unsaturated oleic acid was isolated. The primary structure of the protein as deduced from the nucleotide sequence of the cDNA includes a putative 27-amino acid transit peptide. Amino acid sequence comparison showed substantial similarity to Δ9-desat-urases isolated from other plant species. The two conserved motifs D/EEXXH, involved in the catalytic reaction are localized in similar positions to the other desaturases. Δ9-desaturase gene expression is developmentally regulated. Comparative Northern blot analysis exhibited distinct patterns of temporal gene regulation and mRNA accumulation in the three tissues of the developing olive fruit. In embryos and in the corresponding endosperms, Δ9-desaturase expression starts early during heart-stage, while in the mesocarp it starts later. Maximum expression and transcript accumulation in embryos and endosperms is observed at 13-14 WAF (early torpedo stage), whereas at 22 WAF (late torpedo stage), the Δ9-desaturase message was almost undetectable. In the mesocarp, gene expression and mRNA accumulation is active upto 28 WAF. This accumulation during fruit growth parallels the synthesis of oil in this oleogenic tissue. ABA modulates transiently Δ9-desaturase gene expression in immature olive embryos. Short exposures to ABA positively influenced gene expression whereas longer exposures decreased mRNA accumulation. Fluridone, an inhibitor of ABA synthesis and accumulation, decreased mRNA levels while ABA added to fluridone supplemented media restored Δ9-desaturase transcript accumulation. These results suggest that Δ9-desaturase gene expression in olives is temporally and developmentally regulated in olive fruit.