Abstracts now available online - Euro Fed Lipid

Castor Oil Biosynthesis and Expression Profiles of Lipid Genes in
Developing Seed of Ricinus communis L. (Castor bean).
Grace Q. Chen, Charlotta Turner, Xiaohua He, Tasha Nguyen, Thomas A. McKeon.
U.S. Department of Agriculture, Agricultural Research Service, Western Regional
Research Center, 800 Buchanan Street, Albany, CA 94710, U.S.A.
Castor oil contains 90% ricinoleate, an unusual fatty acid with numerous industrial
applications. Although the biochemical pathways of ricinoleate biosynthesis have been
well studied, the mechanism of such high accumulation of ricinoleate in castor seed
remains unknown. In order to better understand the key steps of this biosynthetic
process, we have conducted a series of seed development studies including
morphogenesis, fatty acids accumulation and expression profiles of genes involved. We
have identified two visual markers, seed coat color and endosperm volume, and defined
three phases that encompass the course of castor seed development. Castor seed
matured at about 60 days after pollination (DAP), and the endosperm tissue underwent
rapid expansion during the mid-phase (26-40 DAP). Ricinoleate was not detectable
before the endosperm emerged but rose quickly to 77% of its final content when seeds
developed into 26 DAP. The ricinoleate reached to a dominant level up to 86% at 33
DAP, and increased to top 90% at 40 DAP and later.
In theory, the efficient accumulation of the ricinoleate should involve transcriptional
activation of genes associated with the ricinoleate and oil synthesis. We have designed
quantitative RT-PCR analysis and investigated the transcript levels of 14 lipid genes at
each stage of seed development. All of the transcripts were induced to higher levels
when seed developed into 26 DAP, coinciding with the onset of endosperm expansion
and ricinoleate/oil synthesis. The transcripts reached to their peak levels at later
different stages, displaying various temporal patterns. Furthermore, the difference of
maximum induction varies dramatically among genes, ranging from 5 folds to >200,000
folds. By comparing these genes in the fatty acid biosynthesis pathway in plastid and
the Kennedy pathway in the endoplasmic reticulum, we propose several possible
limiting steps or control points in castor ricinoleate/oil synthesis. The genes involved the
Kennedy pathway seem to have stronger control over this process.