Secrets of palm oil biosynthesis revealed.

G lobally, vegetable oils are harvested from a handful of oil crops at an annual rate exceeding 100 million tons (1). Most oils are used for human or animal consumption, although a minor fraction is derivatized to oleochemicals. More recently, an increasing amount of vegetable oil is being diverted to the production of biodiesel [i.e., fatty acid (FA) methylesters], and is an attractive feedstock for the so-called “drop-in” biofuels of the future, further increasing the demand on this commodity (2). Because of the commercial importance of vegetable oils, during the past 30 y, the biosynthesis of FAs and their derived acyl lipids, especially plant triacylglycerols (TAGs), has attracted a tremendous amount of scientific interest, resulting in the identification and characterization of the functions of most of the enzymes involved in their production and sequestration (3). In addition, researchers have attempted to understand the quantitative regulation of biosynthesis with the ultimate goal of increasing oil production in crops. Practically all investigations in this field have been conducted in developing embryos of oilseeds, most notably in Arabidopsis. As cDNA sequences of enzymes considered “rate-limiting” became available in the mid-1990s, increasing oil production in plants has become the “holy grail” of plant FA biochemistry (4, 5). However, even though there are more than a dozen reports of marginal increases of seed oil through genetic manipulation, channeling carbon intermediates to TAGs has had very limited success, and no high-oil engineered crop is on the market (6, 7). Only very recently a comprehensive analysis of oil palm mesoderm during fruit ripening was published by Tranbarger et al. (8). With this investigation, Tranberger et al. stepped “outside the box” (i.e., the seed) and delivered a first detailed insight into the compositional, hormonal, and transcriptional changes during the different phases of oil palm fruit development and compared their finding to the established knowledge of oilseed development. While comparing the transcriptomes and metabolomes of developing oil accumulating mesoderm of oil palm (Elaeis guineensis) and sugar accumulating mesoderm of date palm (Phoenix dactylifera), Bourgis et al. (9) follow a strategy that enables them to differentiate metabolic determinants of the respective tissues from other developmental phenomena. Not only is oil palm by far the most productive oil crop, with annual yields often exceeding 4 tons/ha, but during fruit maturation, its mesocarp cells efficiently convert photosynthate to TAGs, which subsequently accumulate to levels as high as exceeding 90% of the tissue dry weight at maturity. In contrast, in the mesocarp of date palm, no TAGs are found. Instead, large amounts of sugars accumulate during maturation to levels as high as 50% of dry weight (9). For their study in PNAS, Bourgis et al. (9) obtained several million ESTs from developing fruit tissues of the two palm species. After gene annotation using Arabidopsis as a reference genome (3), expression levels of orthologous transcripts encoding lipid biosynthesis enzymes found in oil palm and date palm were compared. Sequence identities between the two palm species were generally higher than 90%, as expected for members of the same plant family. In a second approach, the authors monitored the levels of lipid