Abstract
In plants, the conversion of stearoyl-ACP to oleoyol-ACP is catalyzed by a plastid-localized soluble stearoyl-acyl carrier protein (ACP) desaturase (SAD). The activity of SAD significantly impacts the ratio of saturated and unsaturated fatty acids, and is thus a major determinant of fatty acid composition. The cacao genome contains eight putative SAD isoforms with high amino acid sequence similarities and functional domain conservation with SAD genes from other species. Sequence variation in known functional domains between different SAD family members suggested that these eight SAD isoforms might have distinct functions in plant development, a hypothesis supported by their diverse expression patterns in various cacao tissues. Notably, TcSAD1 is universally expressed across all the tissues, and its expression pattern in seeds is highly correlated with the dramatic change in fatty acid composition during seed maturation. Interestingly, TcSAD3 and TcSAD4 appear to be exclusively and highly expressed in flowers, functions of which remain unknown. To test the function of TcSAD1 in vivo, transgenic complementation of the Arabidopsis ssi2 mutant was performed, demonstrating that TcSAD1 successfully rescued all AtSSI2 related phenotypes further supporting the functional orthology between these two genes. The identification of the major SAD gene responsible for cocoa butter biosynthesis provides new strategies for screening for novel genotypes with desirable fatty acid compositions, and for use in breeding programs to help pyramid genes for quality and other traits such as disease resistance.
Highlights
Theobroma cacao L. is an understory tropical tree domesticated in the Amazon basin and today widely cultivated in West Africa, Central and South America, and Southeast Asia (Argout et al, 2011)
Multiple amino acid sequence comparison and phylogenetic analysis including stearoyl-acyl carrier protein (ACP) desaturase (SAD) isoforms from cacao, castor, and Arabidopsis revealed that Tc04g017510 is closely clustered with RcSAD1 and AtSSI2 (Figure 1), both of which have significant impacts on oleic acid contents and fatty acid profiles in Arabidopsis (Lightner et al, 1994) and castor (Lindqvist et al, 1996), respectively, suggesting a potential significant role of TcSAD1 in cacao seed oil biosynthesis
The eight TcSAD isoforms in cacao genome share high amino acid sequence conservation but they have specific differences in key determinative amino acid residues and distinct tissue specific expression patterns
Summary
Theobroma cacao L. (cacao) is an understory tropical tree domesticated in the Amazon basin and today widely cultivated in West Africa, Central and South America, and Southeast Asia (Argout et al, 2011). As the dominant form of monounsaturated fatty acid exported from the plastid, 18:1n-9 can be further desaturated into polyunsaturated fatty acid derivatives, such as18:2n-6 and 18:3n-3, both of which serve as major components of cell membrane systems in the form of phospholipids (Ohlrogge and Browse, 1995) In this respect, given the fact that most plants lack other desaturases that utilize 18:0 as substrate, the activity of SAD is of particular interest because of its significant effects on the ratio of saturated fatty acids to unsaturated fatty acids (Lindqvist et al, 1996), and the effects on the fluidity and rigidity of membrane system and the relationship of this to the adaption of plants to various environmental conditions. Sunflower, flax, and castor grown in lower temperature conditions contained higher proportions of polyunsaturated fatty acid compared to those in high temperature growing regions (Green, 1986; Garces et al, 1992)
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