Characterization of a palmitoyl-acyl carrier protein thioesterase (FatB1) in cotton.

The relatively high level of palmitic acid (22 mol%) in cotton seeds may be due in part to a palmitoyl-acyl carrier protein (ACP) thioesterase (PATE), which prefers C16:0-ACP as its substrate. In embryo extracts, PATE activity was highest at the maximum rate of reserve accumulation (oil and protein), occurring about 30-35 d post anthesis. Thioesterase activity toward oleoyl-ACP was relatively similar at all developmental stages examined, but was considerably lower than the PATE activity. In developing seeds and in cotyledons and hypocotyls of seedlings, the PATE activity predominated. A cotton PATE cDNA clone isolated by screening a cDNA library with a heterologous Arabidopsis FatB1 probe has a 1.7-kb insert sequence with an open reading frame of 410 amino acids, lacking codons for the three N-terminal amino acids. The predicted amino acid sequence of the cotton PATE preprotein has a characteristic stromal-targeting domain and a 63% identity to the Arabidopsis long-chain acyl ACP-thioesterase FatB1 sequence. Alkaline blot hybridization of cotton genomic DNA with the Arabidopsis FatB1 probe suggested the presence of at least two FatB1 thioesterase genes in cotton. Relative cotton FatB1 transcript abundance was compared by RT-PCR and slot blot analysis in total RNA extracts from embryos, seedlings and leaves of mature plants. The cotton FatB1 mRNA apparently was expressed in all tissues but paralleled the profiles of PATE enzyme activity and seed oil accumulation in embryos.