Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 136

Vegetable oils are predominantly composed of triacylglycerols (TAGs), which consist of glycerol esterified with three fatty acids (FAs). The diverse uses of vegetable oils primarily depend on the FA composition of TAG. Dussert et al. (2013) analysed three TAG accumulating tissues of oil palm and showed that transcriptional regulation plays a key role in the considerable differences in oil content and FA composition that exist between these tissues. Fine-tuned modulation of EgFAD2 and EgFAD3 transcription appeared to control tissue variation for polyunsaturated fatty acids (PUFA) levels. Medium chain fatty acids (MCFA) accumulation in the endosperm of oil palm, and most likely of many species of the tribe Cocoseae, seems to rely on tissue-specific upregulation of neofunctionalized paralogues such as EgFatB3 and transcriptional shutdown of PC-DAG interconversion. The authors also evidenced tissue transcriptional specialization of WRI1 paralogues in the oil palm. Finally, considering the crucial role played by acyl-ACP (acyl carrier protein) thioesterases, the identification of factors that control FatA and FatB transcription is one of most exciting challenges for future research in oil palm (Dussert et al., 2013).

To gain insights into the mechanisms that govern oil synthesis and FA composition in the oil palm fruit, Guerin et al. (2016) used a multilevel approach combining gene coexpression analysis, quantification of allele-specific expression and joint multivariate analysis of transcriptomic and lipid data, in an interspecific backcross population between the African oil palm, E. guineensis, and the American oil palm, E. oleifera, which display contrasting oil contents and FA compositions. The gene coexpression network produced through this work revealed tight transcriptional coordination of fatty acid synthesis (FAS) in the plastid with sugar sensing, plastidial glycolysis, transient starch storage and carbon recapture pathways. It also revealed a concerted regulation, along with FAS, of both the transfer of nascent FA to the endoplasmic reticulum, where TAG assembly occurs, and the production of glycerol-3-phosphate, which provides the backbone of TAGs. Plastid biogenesis and auxin transport were the two other biological processes most tightly connected to FAS in the network. In addition to WRINKLED1, a transcription factor (TF) known to activate FAS genes (Ma et al., 2013), two novel TFs, termed NF-YB-1 and ZFP-1, were found at the core of the FAS module. The saturated FA content of palm oil appeared to vary in relation to the level of transcripts of the gene coding for β-ketoacyl-ACP synthase II. Such findings should facilitate the development of breeding and engineering strategies in the oil palm.

Stearoyl-ACP desaturase (SAD) is an important enzyme of fatty acid synthetic metabolism in higher plants. Located in plastid stroma, SAD catalyses the desaturation of stearoyl-ACP to oleoyl-ACP. SAD plays a key role in determining the ratio of saturated fatty acids to unsaturated fatty acids. Krutkaew et al. (2013) cloned and thoroughly characterized the SAD gene in oil palm. Mesocarp tissue of oil palm fruit at different developmental stages was used for expression pattern analysis. Results showed that the SAD gene was highly expressed at 90 DAP. The SAD gene is 1179 base pairs in length, comprised of three exons and two introns, and translated into 393 amino acids. The phylogenetic analysis was constructed using nucleotide sequences, showing that it is closely related to SAD gene from E. oleifera.

The molecular mechanisms of hybrid vigour (or heterosis) between duras, pisiferas and their hybrid progeny tenera are not well understood. Jin et al. (2017) compared the temporal and spatial compositions of lipids and transcriptomes for two oil-yielding organs, the mesocarp and endosperm from dura, pisifera and tenera. The authors found that multiple lipid biosynthesis pathways were highly enriched in a non-additive expression pattern in the endosperm, while cytokinin biosynthesis and cell cycle pathways are highly enriched both in endosperm and mesocarp. Compared with parental palms, the high oil content in tenera was associated with much higher transcript levels of EgWRI1, a homologue of A. thaliana WRINKLED1. Among 338 identified genes involved in lipid synthesis, 207 (61%) have been identified that contain the WRI1-specific binding AW motif. Functional identification of EgWRI1-1, one of three EgWRI1 orthologues, was done by genetic complementation of the Arabidopsis wri1 mutant. Ectopic expression of EgWRI1-1 in plants produced dramatically increased seed mass and oil content, with oil profile changed. Such findings provide an explanation for EgWRI1 as an important gene contributing hybrid vigour in lipid biosynthesis in oil palm.