Abstract
In angiosperm seeds, the endosperm develops to varying degrees and accumulates different types of storage compounds remobilized by the seedling during early post-germinative growth. Whereas the molecular mechanisms controlling the metabolism of starch and seed-storage proteins in the endosperm of cereal grains are relatively well characterized, the regulation of oil metabolism in the endosperm of developing and germinating oilseeds has received particular attention only more recently, thanks to the emergence and continuous improvement of analytical techniques allowing the evaluation, within a spatial context, of gene activity on one side, and lipid metabolism on the other side. These studies represent a fundamental step toward the elucidation of the molecular mechanisms governing oil metabolism in this particular tissue. In particular, they highlight the importance of endosperm-specific transcriptional controls for determining original oil compositions usually observed in this tissue. In the light of this research, the biological functions of oils stored in the endosperm of seeds then appear to be more diverse than simply constituting a source of carbon made available for the germinating seedling.
Highlights
Seeds originate from the fertilized ovule, with double fertilization of the embryo sac initiating the development of the embryo and the endosperm, which grow embedded in maternal tissues called seed coat [1,2]
EgDGAT1-1 is induced in the endosperm of E. guineensis seeds and encodes a diacylglycerol acyltransferases (DGATs) that preferentially uses medium-chain fatty acids for triacylglycerol assembly when expressed in heterologous systems such as Yarrowia lipolytica [101] or leaves of N. benthamiana [102]
In agreement with this model, a transcriptomic approach aimed at studying the expression of genes in the endosperm of germinating A. thaliana seeds revealed that all the major characterized fatty-acid ß-oxidation transcripts were highly expressed in this tissue shortly after radicle emergence, just as transcripts encoding enzymes involved in gluconeogenesis and sugar transporters [136]
Summary
Seeds originate from the fertilized ovule, with double fertilization of the embryo sac initiating the development of the embryo and the endosperm, which grow embedded in maternal tissues called seed coat [1,2]. Endosperm constitutes most of the seed volume in the caryopsis of cereals and consists of abundant dead storage tissues called starchy endosperm that are surrounded by the living aleurone layer [16]. The starchy endosperm of cereal grains is subjected to programmed cell death during seed maturation, while the outer aleurone layer remains viable in the mature seed [22]. Triacylglycerols are accumulated in the endosperm of dicot oilseeds, regardless of the relative proportion of this endosperm in the seed, as in the single-layer aleurone-like endosperm of Brassica napus (rapeseed) or in the enlarged endosperm of Ricinus communis (castor) [25]. While starch represents the main form of carbon storage in the starchy endosperm of most cereal grains, triacyglycerols appear to be preponderant in the aleurone layer of the very same grains [28]. While the regulation of starch metabolism in the endosperm of cereal grains has been thoroughly characterized [29,30], the study of lipid metabolism in endosperm tissues has only emerged more recently
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