SeeDev-binary@ldeleger:SeeDev-binary-21635767-3 / 2765-2797
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While the entire coexpression network is useful for network topology analysis, isolation
of a subnetwork (or cluster) makes it more accessible to biologists [40,58]. More importantly, a subnetwork in the large coexpression network is often more biologically
relevant in a pathway context. Hence, we extracted subnetworks from this gene coexpression
network for genes relevant to the accumulation of seed storage reserves (Figure 4). Of the 48 genes known to encode enzymes involved in FA biosynthesis [17,59], we identified 44 (or ~92%) genes represented on the ATH1 array, and all of them
were found in one subnetwork (Figure 4A). This subnetwork cluster consists of 1854 genes (Additional File 1), which is in general agreement with an interactive correlation network generated
genome-wide in Arabidopsis using a heuristic clustering algorithm [41]. Such a gene list can be used to identify interactors of genes in FA synthesis in
developing seeds. Consistent with the coexpression subnetwork analysis, the majority
of genes involved in FA biosynthesis were associated with Cluster 1 (Figure 3). Their expression levels increased steadily from the globular embryo stage, generally
reached the peak at the expanded cotyledon stage, and dramatically declined subsequently
throughout late seed maturation (Figure 4B). Such a unified expression pattern for most FA biosynthetic genes supports earlier
studies showing that FA supply can be a limiting factor for triacylglycerol (TAG)
accumulation in developing embryos of Brassica napus [60], olive (Olea europaea L.) and oil palm (Elaeis guineensis Jacq.)[61], as well as cuphea lanceolata and other oil species [62]. Recent studies of metabolic flux in developing embryos of B. napus, however, indicated that TAG assembly was more limiting than FA biosynthesis in regulating
the flow of carbon into TAG [63]. The majority of genes encoding oilbody oleosins and SSPs were found in another subnetwork
with a distinct expression pattern (Figure 4C). The subnetwork encompassing genes encoding oleosins and SSPs is comprised of 1392
genes (Additional File 2). Genes encoding oleosins and SSPs were in Cluster 2 (Figure 3), and their expression profiles were strikingly similar. These genes were virtually
unexpressed at the globular stage, increased rapidly (>1000-fold in many cases) from
the globular stage to the bilaternal stage, and remained at the elevated expression
level throughout the remaining stages of seed maturation (Figure 4D). Transcripts for OLEOSIN and SSP genes are most abundant in the seed transcriptome late during seed development. In
contrast, most genes in the TAG assembly pathway were found in different subnetworks,
exhibiting various expression profiles during seed development (Figure 5). DIACYLGLYCEROL ACYLTRANSFERASE 1 (DGAT1), FATTY ACID DESATURASE 2 (FAD2), FATTY ACID ELONGASE 1 (FAE1) and STEAROYL DESATURASE (SAD) genes were identified in this subnetwork, albeit expressed at substantially lower
levels compared to genes encoding oleosins and SSPs (Additional File 3). DGAT catalyzes the acyl-CoA-dependent acylation of sn-1,2-diacylglycerol to produce TAG and CoA [64]. FAD2 catalyzes the introduction of a second double bond into acyl groups in phospholipid
whereas SAD catalyzes the formation of monounsaturated FA in the plastid [65]. FAE1 catalyzes the elongation oleoyl-CoA in the endoplasmic reticulum [65]. Our analysis determined that AT1G48300, which was named DGAT3, is the putative gene encoding a cytosolic DGAT in Arabidopsis. The amino acid sequence
of AT1G48300 has a significantly high degree of similarity (expect value < 1 Ã 10-21) to the soluble DGAT in peanut (Arachis hypogaea), where the cytosolic DGAT gene in plants was first discovered [66]. Notably, DGAT3 exhibited a similar expression pattern with DGAT1, but expressed higher during late seed maturation. In earlier studies, quantification
of DGAT activity during seed maturation in B. napus indicated that enzyme activity was maximal during the rapid phase of oil accumulation
with a substantial decrease in activity occurring as oil levels reached a plateau
[67,68]. Assuming DGAT activity shows a similar profile during seed development in Arabidopsis,
this suggests that DGAT may be down-regulated post-transcriptionally and/or post-translationally
during the latter stages of seed development.
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